Summary of the geology of Ardnamurchan

PROCEEDINGS OF THE

GEOI.JOGISTS' ASSOCIATION. SUMMARY

OF THE GEOLOGY ARDNAMURCHAN,

OF

Including the Report of the Summer Field Meeting to Ardnamurchan, Staffa, Iona and Oban; July 22nd to August 1st, 1932. By the Director,

J.

E.

RICHEY,

M.C., B.A., F.R.S.E., F.G.S.

[Received t.tth. October, 1932.]

THEofwestern portion of the Ardnamurchan peninsula is formed Tertiary vents and intrusions, which pierce Tertiary basalt lavas and underlying Mesozoic sediments and Moine Schists. The complex illustrates many of the features characteristic of Tertiary igneous activity in Britain. In particular, ring-dykes and cone-sheets are well developed, and are displayed in a manner easy to comprehend. They are arranged, together with vents filled with agglomerates, around three centres, and indicate deep-seated foci which can be shown to have functioned successively. The following account, in addition to supplying a report of the Field Meeting, summarises the main features of the geology as seen along the routes traversed by members of the Association, and, it is hoped, may be a useful guide to future visitors. The country is one of comparatively low relief; there is abundance of rock-exposure; and roads lead from Kilchoan to within reach of almost every place of interest. The members of the official party left London by train on the morning of July zrst, arriving at Oban in the early hours of the zznd, and then proceeded by steamer to Kilchoan, where they disembarked at Mingary Pier at 9.30 a.m. Headquarters were at the Kilchoan Hotel, but 22 members were quartered in private houses or camped out. The total number present during the Ardnamurchan tour was 43, and for the extension to Staffa, lona and Oban 23. At Oban members stayed at the Marine Hotel. During the first few days at Kilchoan Dr. J. Pringle assisted by demonstrating the Mesozoic succession. Weather conditions were favourable throughout the whole period, except on July 27th, when there was intermittent heavy rain. PROC. GEOL.

Assoc.,

VOL. XLIV., PART I, 1933.

J.

z

E. RICHEY,

On the day of arrival, after lunch, a brief statement of the geology of Ardnamurchan' was given by the Director and by Dr. Pringle. Ardnamurchan Sequence. The rocks composing the western extremity of the Ardnamurchan peninsula consist of Moine Schists, Mesozoic sediments (Trias, Lower, Middle and Upper Lias, Inferior Oolite, and Great Estuarine Series), and Tertiary igneous assemblages. The latter include: (r) the plateau basalt lavas; (2) intrusive ring-complexes consisting of volcanic vents and associated major intrusions, various sets of basic and composite cone-sheets, and two assemblages of ring-dykes, chiefly gabbro; and (3) basic and acid dykes, mainly running north-west. The Tertiary basalt lavas overlie with marked discordance the Mesozoic strata. Near Kilchoan all the Mesozoic subdivisions named above are represented. Eastwards, in the vicinity of Ben Hiant, the basalts commonly rest upon the Trias or the Broadford Beds (the lowest, calcareous portion of the Lower Lias); but in certain sections they repose directly On the Moine Schists. The Tertiary ring-complexes may be regarded, broadly, as comprising an immense volcanic vent intruded by two plugs (suites of ring-dykes) and fringed by inwardly-inclined minor. intrusions (cone-sheets). The vent-complex and the two ring-dyke assemblages, together with the cone-sheets, indicate three Centres of Intrusion. Successive events connected with the three centres are as follows (see Fig. r) : CENTRE I (earliest)

CENTRE

2.

CENTRE 3 (latest)

Volcanic vents, mainly filled with agglomerates, piercing the plateau basalt lavas. Set of massive cone-sheets, and major intrusions of Ben Hiant and Beinn an Leathaid, in part sheet-like. Other major intrusions, dyke-like and plug-like in form. Ring-dykes, the majority of which are earlier than an Inner Set of cone-sheets, while probably all are later than an encircling Outer Set of very numerous cone-sheets. Linear vent of Glas Eilean, later than Outer Cone-sheets. Ring-dykes, the outermost and oldest of which is cut by a few cone-sheets.

Friday, July 22nd. Shore Section, Mingary Pier to Kilchoan Bay. A few hours were spent during the afternoon in the examination of this section, special interest being focussed on the Outer Cone-sheets of Centre 2 and the elongate vent of Glas Eilean. x For information additional to that given in this C Summary' see" The Geology of Ardnamurchan, North-West Hull and Coll." Mem. Geol. Suru., Scotland, '930.

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4

J.

E. RICHEY,

Outer Cone-sheets of Centre 2: Shore Section near Mingary Pier.-The pre-Tertiary rocks here consist of: (1) Moine Schists, apparently rather lowly metamorphosed, mainly well-bedded altered sandstone with occasional pebbly beds and pelitic layers; (2) Trias cornstone, conglomerate, and reddish and greenish sandstone ; and (3) Lower Lias limestone and calcareous shale (Broadford Beds). Above the road, near the pier, a basal Trias cornstone rests upon steeply-dipping schists and penetrates down into the schists along their planes of bedding. Other Triassic deposits are seen on the shore just west of the pier, where they are succeeded by the Lower Lias shales and limestones. These contain, among other shells, " Cypricardia " porrecta Dum., which indicates the Planorbis zone, the lowest zone of the Lias. Slightly higher beds succeed along the coast, but within a short distance the schists reappear from beneath a basal Trias breccia. The coast-line cuts directly across the trend of the Outer Cone-sheets of Centre 2, and provides a continuous rock-section that includes about a third of this cone-sheet belt. The combined total thicknesses of the cone-sheets along the half-mile of coast shown on Fig. 2 is about 1,000 feet. In composition the sheets are predominantly quartz-dolerite of the Talaidh type characteristic of the Late Basic Cone-sheets of Mull, which form practically all the western half of Ben Talaidh. This peak, bearing the lower spur of Beinn Bheag upon its eastern flank, is well seen from Mingary Pier, looking southwards down the Sound of Mull. There are occasional composite cone-sheets and sills with margins of quartz-dolerite and more massive intermediate and acid central portions of craignurite, felsite or granophyre. An example, running as a sill along the bedding of the Lower Lias limestones, occurs a short way west of Mingary Pier, and clearly demonstrates that, as in so many composite sheets of the Hebrides, an acid magma followed quickly upon the intrusion of a basic magma. Only the lower half of the sill, however, is on view, the upper part being cut off by a massive basic sill, an offshoot or an aberrant portion of a normal cone-sheet. The best exposure of the composite sill is below and to the east of the basic sill, which is chilled against the acid interior portion. The composite intrusion consists of a basic margin, 1! to 2 feet thick, chilled externally against the country-rock, and internally in sharp or merging contact with a grey basified granophyre. This internal junction is quite unchilled. The granophyre contains abundant basic xenoliths evidently derived from the basic margin. The xenoliths are more or less rounded and often oval in shape. They become smaller and paler in colour, due to their progressive absorption and acidification, towards the more central part of the sill. Finally, for a foot or two beneath the chilled margin of the later sill, a normal white granophyre

PROG. GEOL. Assoc., VOL. XLIV. (1933).

A

B

P LATE 1.

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[P hoto by Afiss D , R eylloltls. CONg-SHlmTS ON S HORE SOUTH OF KI T.C H O A N , AR DNAMURCHAN.

J n " A," count ry rocks are Moin e Schists. In " B," count ry rocks are Lower Li as lim estone and shale ; the ha m mer lies along a cone-sheet fracture. (To face p. 4.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

5

is seen, which is practically free from basic inclusions. The basic marginal rock is a variolitic quartz-dolerite of Talaidh type. The interior intermediate portions are albite-granophyre basified by the resorption of earlier basic material; while the central portion is a granophyre rich in soda-felspar. Farther west, at the southern extremity of the headland on the north side of which the pier is built, another type of composite cone-sheet was examined. Marginal basic portions, weathering spheroidally, are succeeded internally by cross-jointed dolerite, and in the centre of the latter there is a layer zo yards in length crowded with xenoliths of Moine granulite. While there is external marginal chilling, no internal contacts can be ascertained, although the different portions are sharply marked off from one another. The basic portions are alike, being quartz-dolerite approximating to the Talaidh type. The central, xenolith-filled portion, however, is a much more acid quartzdolerite, and has become still further acidified by absorption of the siliceous granulite. Around individual xenoliths there is a reaction border of small augite crystals, and the quartz grains of the enclosures themselves bear fringes of tridymite that has reverted to quartz. In calcareous Liassic shales immediately overlying this curious composite sheet, Dr. H. H. Thomas has found that the secondary minerals developed by contact-action include garnet, monoclinic pyroxene, idocrase, tremolite and prehnite. As a rule, however, contact-alteration imposed upon country-rocks by individual cone-sheets is relatively slight. From this point on to Kilchoan Bay a magnificent crosssection of the basic cone-sheets is exposed (Fig. z). The majority of the sheets are comparatively thin, varying in width up to a maximum of 20 feet; but they occur in great numbers, with an almost uniform inclination of about 40 degrees inwards towards Centre 2. Sill-offshoots along the bedding of the Lias limestones and Moine Schists frequently occur. A clear example of continuity between sheet and sill is exposed at a pint 330 yards west of the pier (Plate I, A). Individual cone-sheets were intruded successively, for they bear well-chilled margins whether in contact with country-rock or with an earlier cone-sheet. Successive intrusion is also indicated by the fact that some cone-sheets may be cut by a northwest dyke which is in its turn cut by other later cone-sheets, The intrusion of each cone-sheet was preceded by the formation of a fracture, for a number of cone-sheet fractures, unfilled by magma, and running parallel to adjoining cone-sheets, may be seen crossing a low cliff of Lias limestone inshore of a patch of beach-shingle zoo yds. west of Mingary Pier (Plate I, B). At this point, on the shore below the cliff, there is an early dyke of big-felspar basalt, crowded with phenocrysts of labradorite

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FIG. 3.-SECTION ALONG LINE A-B OF FIG. 2.

(Reproduced, by permission, from Ardnamurclmn. Geol. Surv. Mem., 1930.)

8

J.

E. RICHEY,

a couple of inches long, and cut off by the cone-sheets, which are chilled against it. Similar early dykes occur sporadically throughout the eastern part of the Ardnamurchan district, and this remarkable type of rock also forms large blocks in the vent-agglomerates of Ben Hiant. In Mull, it may be recalled, a zone of lavas of similar type occurs at a high level in the plateau basalt group. Possibly the Ardnamurchan rocks may be of a like early date. Linear Vent of Glas Eilean.-This elongate vent is marked by brecciated country-rocks that are exposed in a narrow strip along the coast north-eastwards from Glas Eilean promontory, which forms the south-easterly extremity of Ki1choan Bay. There the vent is crossed by a north-east fault. The breccias continue from the coast north-eastwards in disconnected exposures towards the road leading from Kilchoan to Mingary Pier, and the total observed length of the vent is thus threequarters of a mile. Its breadth at its widest part is less than 100 yards. The vent runs parallel to adjoining Outer Cone-sheets of Centre Z, but is later than the sheets, fragments of which are involved in the vent-materials. Other ingredients are derived from the plateau basalt lavas and Moine Schists, rocks which respectively form the western and eastern walls of the vent. The latter appears, in fact, to be situated along a line of fault that cuts out all the Mesozoic beds and brings down the lavas against the schists. On Glas Eilean itself, however, numerous blocks of Jurassic limestone and shale are found. These may have been carried up from below by vent-explosions, or, more probably, have slipped down into the vent from its eastern side, where the blocks are especially large and numerous. No fragments indicative of the magma responsible for the vent are found. Occasional pieces of albite-granophyre may belong to some composite cone-sheet like that near Mingary Pier. But the existence of a magma below ground and its composition are clearly demonstrated by an acid tuff that locally fills in all the cracks and crevices of the breccias. On the eastern side of Glas Eilean in a low cliff near high-water mark, such apparently .. net-veined" rocks are seen; but under the microscope the clastic nature of the pale "veins" becomes evident. Scoriaceous fragments and wisps of acid glass show plainly the characteristic lunar and curvilinear form of glass fragments due to explosion. Near the Inshore end of the vent an irregularly-running dyke of well-fluxioned rhyolite cuts the ventbreccias, and may be another expression of the acid magma responsible for the explosions. At this point, on the foreshore, the normal and faulted margins of the vent are best studied. To the north-west, breccias formed of basalt lava gradually give place outwards to

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

9

solid basalt representing the vent-wall. No definite ventmargin can be ascertained. There is a complete passage from typical agglomerate into brecciated basalt and finally into the solid basalt. The south-east side of the vent is here sharply bounded by the fault-plane. A cone-sheet cutting and chilled against the schist-wall is seen to extend to the fault, where the sheet is deflected from its normal course and where a fractured portion of it appears as though extending along the fault-plane. Since it does not present a chilled margin against the fault, the latter is concluded to be of later date. The parallelism between vent and adjacent cone-sheets seems to point to the vent having been formed during the period when Centre 2 was functioning. Such a linear vent may be the superficial expression of a linear intrusion, below ground, which was guided by a ring-fissure with central downthrow towards Centre 2. The fault seen may represent only the final stage of an earlier movement. Saturday, July 23rd. Ben Hiant: Vents and Intrusions of Centre 1. Members proceeded by motor to the east side of Ben Hiant. From a point where streams descend to cross the road It miles southwards of Loch Mudle, the party climbed the steep eastern face of the hill to the north-east margin of the Ben Hiant ventcomplex. They continued around to the south-eastern slopes, keeping to a level of about 1,000 feet, and then descended obliquely down to the cliffs above Maclean's Nose, southwards of Stallachan Dubha. They ascended again across Stallachan Dubha to climb to the western face, and thereafter the central dolerite of Ben Hiant (the Ben Hiant Intrusion) was traversed. Members completed the circle of the hill, and arrived back at the road on the eastern side, where they were met by motors at the watershed a mile south of Loch Mudle. In the morning, before the party left the road, the geology of Ben Hiant was outlined by the Director. Outline of the Geology of Ben Hiant (See Figs. 4 and 5).Ben Hiant is the highest peak in Ardnamurchan, its summit being at 1,729 feet a.D. Rising with steep or precipitous slopes from sea-level, it is a mountain of rugged outlines, due in large measure to a central mass of dolerite intrusive into the volcanic vents (Plate z, A). There are two vents, whose margins transgress across horizontally-bedded basalt lavas and the underlying Mesozoic and schistose rocks. The vents are best seen on the extensive south-eastern slopes, facing Loch Sunart and the basalt plateau of Morvern. There is evidence here of a Northeast Vent which was succeeded by a still more extensive Southwest Vent.

J.

10

E. RICHEY,

Highly metamorphosed Moine Schists forming the lower slopes of the mountain are followed uphill by the Tertiary basalt lavas. In various stream-sections there intervenes a thin development of Mesozoic sediments, chiefly red Triassic sandstone. Yellow sandstone with jet seen in one stream is tentatively referred to the Tertiary period. At the base of the lavas there is a bed of red mudstone, usually containing wind-rounded quartz grains, which is found throughout Ardnamurchan and also in the adjoining plateau-country of Mull and Morvern. It is considered to represent a basaltic ash that has been subjected to subaerial weathering. The overlying lavas are mainly composed of olivine-dolerite or -basalt of the Mull plateau type. On Ben Hiant they weather spheroidally with the red rusty surfaces characteristic of basalts. Each flow consists of a solid lower portion, sometimes with a basal layer of slag, and a slaggy ~w

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upper portion. A thin bed of compact red bole often separates two successive flows, and contemporaneous subaerial weathering is also indicated by the reddening of the underlying slag. On Ben Hiant itself the basalt lavas are only a few flows thick and are of limited outcrop, but farther east, brought down by an extensive fault, lavas cover a wider area. The fault continues from the hollow east of Ben Hiant north-north-west by Loch Mudle across the whole peninsula. The vents are exposed chiefly about half-way up the mountain, but outcrops descend to sea-level around Maclean's Nose. The main infillings are agglomerates. Basalt fragments predominate over other rock-types, near the vent-walls, while in the interior the agglomerates are made up chiefly of rhyolite and trachyte, together with large scattered blocks of big-felspar basalt. The latter are especially conspicuous in the cliffs above

PROC. GEOL.

Assoc.,

VOL. XLIV.

(1933).

PLATE 2.

[Photo by H.M. Grot, SUfVC".

A.-VIEW

OF BEN HIANT FROM NEARKILCHOAN.

[Photo by H.M. Geol. Su,,'e!'. B.-AGGLOMERATE CLIFFS ABOVE MACLEAN'S NOSE, BEN HIANT.

(Reproduced, hV permission, from Artinunntrchu.n Geed,

SH YV.

Mem., 1930 . ) [To face

p.

10.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

II

Maclean's Nose, where they were noted in the first instance by Sir Archibald Geikie. The agglomerate is itself not stratified, but its bedded nature is disclosed by flat-lying beds of fine-grained tuff, a foot or more in thickness. Tuff-beds can be best studied in the cliffs of Maclean's Nose, where they recur at intervals of about 20 feet (Plate 2, B). They are either perfectly flat or are inclined slightly inwards, away from the vent-wall. In the South-west Vent there are also sheets of columnar pitchstone, flatly disposed like the tuffs, which are considered to be lavaflows within the crater. Individual sheets are structurally asymmetrical, consisting of slaggy or amygdaloidal tops and lower columnar portions arranged with upright columns below and irregular or flat-lying columns above. This three-tier structure is thus similar to that of the well-known columnar basalt lavas of Staffa and Western Mull and of the Giant's Causeway on the Antrim coast. The flatly-bedded nature of the materials infilling the vents becomes of special interest when the external shape of the vents is considered. The walls of the North-east Vent, exposed on the east face of Ben Hiant, are inclined inwards at angles of 35-50 degrees. At one contact of agglomerate and wall, a composite triple dyke with tholeiitic margins and an acid central portion of granophyre traverses the basalt lavas of the wall, but ends at the vent, where all three portions are cut across by the agglomerate. The transgressive nature of the vent-agglomerate is obvious. In the case of the South-west Vent, contacts of agglomerate and wall are vertical, as is seen, for example, where agglomerate crosses lavas at a level of 500 feet in a recess in the cliffs north-east of Maclean's Nose. At the headland itself, near sea-level, agglomerate is in contact with schists. The bedded agglomerates and tuffs of the cliffs were therefore laid down within a vent-cavity bounded by vertical or very steep walls. Since the bedded materials of the Southwest Vent occur at a height. above sea-level of 1,100 feet, the vent-cavity must have been enormous. Another important fact is the absence of country-rocks as fragments in the agglomerates except in the vicinity of a ventwall. Along vent-margins, whether against the lavas or in contact with schists at Maclean's Nose, basalt lava fragments abound. Thus we may picture Ben Hiant as having once been a great vent-cavity which became infilled by agglomerates, tuffs and flows of pitchstone. The agglomerate fragments were no doubt derived from the explosive brecciation of acid and trachytic rocks that issued as lavas from a central volcanic pipe; while basalt screes formed along the interior slopes of the crater. The origin of the blocks of big-felspar basalt is obscure. Perhaps, as in Mull, a zone of lavas of this type may have been interbedded at a high level in the plateau group of Ardnamurchan.

I2

J.

E. RICHEY,

Alternatively, like the more acid materials, these rocks may have come from the volcano itself. No vestige of a main central pipe or pipes is ascertainable, though one probable orifice is presented in a mass of porphyritic dolerite, intruded high up on the mountain into the agglomerates of the South-west Vent. This may be the infilling of a crater, or a neck. A pitchstone lava overlying the dolerite appears to be in its normal position. If so, the dolerite is merely an episode in the long volcanic history of Ben Hiant. The central part of Ben Hiant is formed of the great Ben Hiant Intrusion, consisting of quartz-dolerite. The rock differs from the type characteristic of the majority of the cone-sheets in having a well-marked variolitic mesostasis and ih containing olivine. In chemical composition, however, the two rock-types are practically identical. The intrusion rises as a broad dykelike mass from sea-level on the western side of the mountain. At a height of 400 feet on the north side this mass spreads outwards and over-rides flatly vent-agglomerates and older bedded rocks. On its south side a similar extension at a height of over 1,000 feet is indicated by an outlying mass of dolerite on Stallachan Dubha. When followed uphill along the centre of the dyke-like portion, the dolerite grades into variolite, which is thought to be a quickly-cooled upper surface of the intrusion. The western portion of the Ben Hiant dolerite is therefore like a mushroom in shape. The intrusion of the dolerite through the vent-agglomerates, its mushroom shape, and its termination upwards, render it likely that the mass fills the crater and throat of the Ben Hiant volcano. On the other hand, the north-eastern part of the intrusion follows a different plan. On the lower ground northeast of the summit, tongues from the dolerite extend out into the country-rocks. The tongues follow the direction and inclination of adjoining cone-sheets belonging to Centre 1. They are probably offshoots along cone-sheet fractures, and they would appear to link the emplacement of the Ben Hiant dolerite with the inception of the earliest set of cone-sheets. Details of Sections, Ben Hiant.-The exposures of chief interest examined by members in the order in which they were visited were as follows (see Fig. 5) : North-east Vent. (1) Steeply inclined junction of basaltic agglomerate with spheroidally-weathering plateau basalt lavas, in the middle stream of three shown on Fig. 5 on the east face of Ben Hiant. (2) Fairly steeply inclined junction of basaltic agglomerate with composite dyke cutting basalt lavas, in the most southerly stream of the three above-mentioned. (3) Slightly inclined bed of tuff in agglomerate of rhyolite, etc., on the east bank of a stream midway between Bourblaige

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J.

E. RICHEY,

and the summit of Ben Hiant, with agglomerate-scarps (indicating bedding) in view to north. The abrupt change from the agglomerate of the North-east Vent to pitchstone sheets of South-west Vent was here noted. South-west Vent. (r) Downstream from the last exposure, a few yards southwest of the stream-section showing basalt lavas, the junction of the agglomerate of the South-west Vent with the lavas was seen to extend uphill in alignment with the mutual boundary of the pitchstone sheets and the agglomerates to the north-east. (2) In the stream itself, a contact of two basalt flows was examined. In the north-east bank the lower flow with amygdaloidal, partly reddened top, is succeeded by a thin layer of red hole, overlain by slag with well-developed pipe-amygdales. The solid basal portion of the second flow succeeds. Spheroidal weathering characteristic of the unaltered basalts of Ben Hiant may be observed here. The lower flow overlies an outcrop of the basal red mudstone, which is resting directly upon the schists. (3) Two successive scarps of columnar pitchstone are considered to represent lava flows within the south-west crater, and to have terminated to the north-east against a vent-wall formed of the agglomerates of the North-east Vent. In the upper flow, on the south side, a lower set of upright columns is succeeded by more slender flatly-disposed columns, in the upper part of which amygdales are developed. Actual tops of the flows are not exposed. (4) The plug or crater-infilling of porphyritic dolerite is well exposed in the middle branch of the stream to the south of the pitchstone sheets, where a highly inclined contact of the spheroidally-weathering dolerite with slightly contact-hardened tuff was examined. The margin of the dolerite is unchilled. (5) The lowest columnar pitchstone sheet on Ben Hiant is exposed in and adjacent to the stream 400 yds. south of the preceding locality, beneath (south-east of) the capping of the Ben Hiant Intrusion forming Stallachan Dubha. Structurally it consists of three tiers. A lower tier of upright columns gives place abruptly to a middle tier of flat or somewhat irregularly arranged columns of much smaller diameter than those below. Towards the top of the middle tier amygdales make their appearance and become abundant in an upper tier of unjointed pitchstone. High up on the north-eastem bank of the stream the rock is practically a pitchstone slag, with abundant irregular vesicles. Lower down, amygdales are almond-shaped, and some on the south-west bank are as much as a foot in length. Amygdale infillings are chalcedony-lined with drusy quartz which, in the larger cavities, is centred by calcite. The base of the pitchstone sheet is exposed below the sill of a small water-

SUMMARY OF GEOLOGY OF ARDNAMURCHAX.

IS

fall, where it rests upon slightly hardened tuff. In the stream a crush crosses the pitchstone without causing appreciable displacement. The actual top of the sheet is not seen. On the north-east bank a few feet of unexposed ground, across a hillpath, separates the exposure of pitchstone slag from apparently overlying tuff and agglomerate. (6) Agglomerate cliffs (Plate 2, B), with an interstratified flatlying bed of tuff about a foot thick, were viewed from near the top of the cliffs, 600 yds. north-east of Maclean's Nose.

Ben Hiant Intrusion of Quartz-dolerite. (I) Close-set vertical jointing is well seen in the outlying capping on Stallachan Dubha, at the south-west end of which the dolerite is cut by a cone-sheet. Erosion hollows that extend across the capping show sections through the dolerite down to its flat base. Members climbed through one of these gaps to ascend the central part of Ben Hiant, which is formed of the main mass of the intrusion. Material was collected from the base of the central peak south-south-west of the summit. (2) The party then descended a few hundred feet along the outcrop of variolite to examine an important exposure by a stream 1,000 yds. west-south-west of the summit-cairn. A cliff forming the north bank, 20 feet in height, is composed of prismatically jointed variolite, with small slightly-tilted hexagonal columns. The variolite passes downhill into normal quartz-dolerite by a gradual increase in the number and size of plagioclase felspar laths and a decrease in the amount of the variolitic portion. In the normal quartz-dolerite there is a mesostasis of variolite which varies somewhat in amount from place to place. The transition, which is seen in the cliff section and also elsewhere, suggests that the variolite represents a more quickly cooled portion of the quartz-dolerite magma. The outcrop of variolite may be a remnant of a fine-grained upper portion of the intrusion consolidated under little or no cover of older vent-materials. A few amygdales occur near the top of the variolite exposed. (3) The party proceeded around the higher central part of the mountain to examine the tongues that continue north-northeastwards from the main mass. They are to be distinguished by their well-marked scarps with dips sloping away from the main mass of the intrusion. The most extensive tongue continues northwards to the south end of Loch Mudle, where its outcrop is shifted southwards by the Loch' Mudle Fault. The scarp formed by it consists mainly of cross-jointed solid dolerite, which is underlain by much weathered dolerite. The scarp was crossed at a point west of the most easterly stream that traverses the north side of Ben Hiant, some 300 yds. south of a cross-country track that here skirts the foot of the mountain. On the east bank of the stream, a section of the much weathered portion

~,£\ (,;~i\'"i)' ·jll! l\~\iit;i!:~¢ll.\;t·~ i;·;:.·:-::

-::.::... :-:.. -.:.:.::..: ,:-:<".;:-

,"

" : -F

,

'.. :

Ur ~

\

loor,~~,;, ,,s>

Rudh' a; J."\II h Lle Scale I

I

1"<':'; o,o", Boac h . . .••r Deposit.s ::0.':::( Ra is e d Be a ch : : : ; ::

Dep o s it s

19 0

i

I

!

290 YAROS

Ba Sic ,~ Dykes Sub -b asic ]l t C" a in nu" it e) -t S ill b Ba s ic Con e - sh e ets ]l & S ill s -<

>

H ~~,'i~ Shales Lime ston e

1.1 .

to

[l

1

& Ca lcareous , Sandstone

r

o

j ~ ~ ~

}i l[m~ Moine S chi sts

--

+

Hor izonta l) (/) Incl ined . ~ Gently ~ Incl ine d . 40,," Dip of Intru s ive Margins , - L - . _ Fault.s.cro ss ma r k on dowrrbh r-ow s ide .

30

I.C" lit

RWM: High Wa t e " Ma " k . L:\Yh r. wwWate r Ma r k .

FIG. 6.-MAP OF SHORE NEAR MINGARY CASTLE, EAST-SOUTH-EAST OF

(Reproduced, by permission, from Ardnamurchan. Geoi. Suru. Mem .., 1930.)

KILCHOAN,

SUMMARY OF GEOLOGY OF

ARD~A;\IURCHA:f.

17

is exposed. It discloses a perfectly developed set of prismatic joints enclosing six-sided columns a yard or more in diameter. The margins of the columns have proved more resistant to weathering than the softer interiors. They form slightly raised six-sided rims, each separated by a joint plane from the rims of adjoining columns. The hardening is presumably due to the action of vapours that escaped along the joints from laterconsolidated interior portions of the tongue. On the dip slope of the tongue, westwards of the stream, as well as elsewhere along its elongate outcrop, the dolerite contains abundant xenoliths of felspathic schist. Sunday, July 24th. Shore near Mingary Castle. Trias and Lower Lias cut by Outer Cone-sheets of Centre 2 and by North-West Basic Dykes. In the afternoon the shore section from the promontory of Rudh'a'Mhile to Mingary Castle was examined (Fig. 6). Of main interest is the Lower Lias sequence, which was demonstrated by Dr. Pringle. Trias.-On Rudh'a'Mnile, the Trias is less than 20 feet thick and consists of red sandstone and conglomerate together with concretionary deposits of cornstone. A basal breccia is well displayed resting upon the upturned edges of the Moine Schists along the eastern side of the promontory. The Mesozoic strata have a slight general dip towards the west, and the Lower Lias succeeds the Trias in this direction. The contact of the two sets of sediments, however, is best seen at the seaward end of the promontory near the base of a stack formed by a Tertiary sill. Triassic pebbly sandstone above a cornstone is overlain by white sandstone and sandy limestone, which are to be referred to the Lias. Lower Lias.-The limestones that succeed the Trias, forming the well-known Broadford Beds, are extensively exposed along the coast westwards to near Mingary Castle, where they are followed by calcareous shales. The sequence is frequently interrupted by Tertiary minor intrusions as well as by faulting. Three of the ammonite zones of the Broadford Beds have been definitely recognized from fossils collected for the Geological Survey by Mr. W. Manson and named by the late Mr. S. S. Buckman and the late Dr. G. W. Lee. The basal beds overlying Trias at the stack at Rudh'a'Mhile have yielded a few lamellibranchs, including Modiola hillana J. Sow., and a Cardinia and Pleuromya, but the first zone indicated is represented in limestones and shales which are faulted against Trias near high-water mark just west of the neck of the promontory. These beds contain a large-ribbed Ostrea, and a gryphseate Ostrea, PROC. GEOL.

Assoc.,

VOL.

XLIV.,

PART I, 1933.

2

18

J.

E. RICHEY,

and specimens have been found suggesting a young form of Lima gigantea. The fossils, though scanty, point to the beds being either in the Angulatus zone or slightly higher. Farther west, north-west of Port Dr, honeycomb-weathering limestones contain shaly layers with abundant Gryphaa arcuate Lam. No ammonites have been found here, but what appear to be stratigraphically higher beds belong to the top of the Bucklandi zone or to the "Semicostatus" zone. The beds concerned consist of shales in disconnected outcrops partly enveloped in a Tertiary sill-complex east of Port Dr. From here ammonites have been obtained, including several species of Arnioceras together with Asteroceras d. undaries (QuensL). The horizon may possibly be Dr. Spath's- "bed full of Arnioceras " (base of " Semicostatus " zone or top of Bucklandi zone), though the presence of Asteroceras suggested to Buckman a much later date. About 50 yds. east-south-east of the Castle, flaggy shales are faulted down against the Gryphsea Limestones and belong to the upper part of the " Semicostaius "zone. They contain, in addition to species of Agassiceras, numerous brachiopods, including Piarorhynchia and S'piriferiua walcotti (1. Sow.). The examination of slightly higher beds, below the sills on which the Castle is built, has yielded specimens of Arnioceras together with various brachiopods. From still higher beds, on the shore west of the Castle, an ammonite fragment was obtained by Dr. Scott during our visit, and this Dr. Pringle recognized as belonging to the genus A rietites. It seems probable that this is the bed from which Dr. Spath obtained Arietites. At any rate, his record at Mingary of the Obiusus zone is now confirmed. Tertiarv Minor Lntrusions.s-Yu« numerous minor intrusions that traverse the Mesozoic strata eastwards of Mingary Castle include cone-sheets and sills belonging to the Outer Set of Centre 2, and basic north-west dykes. On Rudh'a'Mhile, capping the prominent stack at the seaward end, a quartz-dolerite sill is intruded along the bedding of the Lias limestones. Northeastwards the sill is continuous with a cone-sheet that cuts through a north-west dyke. Near the fault east-south-east of the Castle, a somewhat thick cone-sheet also cuts an early basic dyke, but is itself cut by two later dykes. Mingary Castle stands upon a massive sill of pale-weathering craignurite, beneath which there is a lower darker-coloured sill of quartz-dolerite, probably a variolitic type. The craignurite is typical of its kind, with needles of augite, discernible under a lens. I L. F. Spath," On Lower Lias Ammonites from Skye," Geol. Mag. (l922), P.l7S.

SCMMARY OF GEOLOGY OF ARDNAMURCHAN.

19

Monday, July 25th. (a) Glebe Hill: Outer Margin of Hypersthene-gabbro Ringdyke and Sapphire-bearing Xenoliths. In the morning a visit was made to Glebe Hill. The route followed was by the road leading past the Free Church to the margin of the hypersthene-gabbro; thence across the outcrop of this rock to a roadside quarry south of the bridge where the Achnaha road crosses the Amhainn Chro Bheinn ; and from the quarry across country to the sapphire-xenolith locality. The exact position of the last-mentioned exposure is just above the south bank of the Amhainn Chro Bheinn, a quarter of a mile east of the road-bridge referred to. The party returned to the road, adjacent to which two acid dykes were noted. Raised Beaches.-Gravels of the loa-ft. late-Glacial raised beach border the road south-east of the Free Church Manse. An exposure of well-bedded gravel and sand in a pit west of the road was noted. No arctic shells have been found here, though these are frequently present in similar deposits in other parts of Scotland. Some 50 feet above the grassy flat formed by the late-Glacial beach, east of the road, a rock-notch extends along the hillside. A continuation of this platform at a height of about 145 feet O.D. is a conspicuous feature along the western side of Kilchoan Bay as far as the headland of Sron Bheag. At close quarters it is seen that the platform has suffered considerably from erosion subsequently to its formation, and it must long antedate the loose gravels of the roo-ft. late-Glacial beach, which occur in extensive flats on the slopes below it. In any case it cannot be ascribed to the late-Glacial sea, for it lies at too high a level, and further the late-Glacial sea did not effect any extensive coastal erosion. It finds its counterpart, however, in the preGlacial marine platform so magnificently developed in Western Mull and the Treshnish Isles (see p. 51). To the latter platform a pre-Glacial date has been assigned, since it bears glacial striae upon its surface, and at one place is seen to be overlain by boulder-clay.' Older Rocks outside the Hypersthene-gabbro Ring-dyke (Centre 2). A progressively higher degree of contact-metamorphism is apparent in the older rocks as the hypersthene-gabbro is approached. These rocks consist of Tertiary basalt lavas and the sandy micaceous Pabba Shales of the Lower Lias traversed by numerous cone-sheets belonging to the Outer Set. Signs of alteration are evident at a distance of over a quarter of a mile from the gabbro in basalt lavas adjacent to the road between Kilchoan Hotel and the turn to the Free Church, nearly opposite I .. The Tertiary and Post-Tertiary Geology of Mull," etc., Mem, Geol, Sur», (1924), pp, 387, 388, 39°·

20

J.

E. RICHEY,

the Established Church Manse. Amygdaloidal lavas, here exposed in a small quarry, weather as solid rock-faces with the grey tints characteristic of contact-altered basalts, in contrast to the rusty-red hues and spheroidal jointing prevalent amongst unaltered basalts. Cone-sheets cutting the lavas, however, still retain their dark chilled margins. Farther on, south-south-east of the Free Church Manse, at less than a quarter of a mile from the gabbro, margins of sheets are pale in colour and stony owing to the recrystallization of these fine-textured selvedges. Pabba Shales cut by the conesheets are considerably indurated. In a slight hollow at a bend in the road a short way north of the Free Church the line for the outer margin of the gabbro has been drawn, but the actual contact with the older rocks is not exposed. The older rocks at this point consist of granulitized (i.e., recrystallized) basalt lavas cut by granulitized cone-sheets of Centre 2, and also by unaltered porphyritic conesheets, whose chilled edges are quite fresh. The latter sheets presumably belong to a partially developed set recognized in this neighbourhood and are referable to Centre 3. In addition to recrystallization, a frequent contact-effect is the development of abundant crystals of biotite in the rock itself or, in the case of the lavas, in vesicles filled originally with chlorite, while limesoda-zeolites occurring in the amygdales are altered to plagioclase felspar, Under the microscope these and other changes may be observed. To quote Dr. H. H. Thomas' description: the original ophitic augite has been redistributed as small granules or has gone over to rhombic pyroxene [presumably accompanied by Al-diopside]. Crushing followed by recrystallization of the comminuted material is also seen in rock-slices. Hypersthene-gabbro Ring-dyke.-The marginal portion of this intrusion on Glebe Hill, and indeed at all other places around the periphery of the mass, consists of quartz-dolerite traversed by acid veins. The marginal quartz-dolerite is to be distinguished from the older baked rocks by its brown-weathering surfaces. It passes gradually into the normal coarse-textured gabbro internally, and there is no doubt that both rocks belong to the same magma-injection, though they are distinct as regards composition. Olivine is prevalent in the gabbro, but is absent from the quartz-dolerite. Hypersthene, however, occurs in both rocks, in addition to augite. Some 250 yds. within the gabbro, north-east of the road, there is a large xenolithic mass of granulitized fine-grained basic rock. Its origin is obscure, but it may have been originally part of the basalt lavas which appear to be disposed on Glebe Hill as a roof overlying the hypersthene-gabbro. The gabbro is well exposed in a roadside quarry 180 yds.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

21

south-south-east of the bridge over the Amhainn Chro Bheinn, in which there are present two distinct gabbro-types. A type largely composed of a somewhat greasy-looking basic plagioclase felspar resembles allivalite. More easily-weathering portions represent the normal hypersthene-gabbro rich in pyroxene. Acid veins are plentiful. Sapphire-bearing Xenoliths.-A quarter of a mile east of the bridge referred to above, the xenoliths occur as dead-black irregular-shaped or rounded masses enclosed in a grey, completely recrystallized basic rock. The whole exposure is only about 10 yds. in length, and lies between hypersthene-gabbro seen in the adjoining bank of the Amhainn Chro Bheinn and basalt lava that belongs to the Glebe Hill outcrop. The basic rock enclosing the xenoliths is a hypersthene-olivine-granulite, and is perhaps an early-consolidated portion of the hypersthenegabbro. The xenolithic masses are composed mainly of dark sage-green spinel, basic plagioclase and colourless or blue corundum (sapphire). In hand-specimens sapphire appears to be rather sporadic in its development, but its small blue crystals are locally quite easily seen. The xenoliths resemble in their mineralogical composition the sapphire-bearing xenoliths of Loch Scridain in Mull. An interesting point in common is the occurrence of abundant slender crystals of mullite (silicate of alumina) as enclosures in the basic' plagioclase. Acid Dykes.-A north-north-west acid dyke of quartzfelsite, 20 ft. in width, crosses the Amhainn Chro Bheinn 100 yds. south of the Achnaha road bridge, where it forms the sill of a waterfall. A chilled margin is seen on the upstream side. An adjoining acid dyke running east-north-east was also examined. It is exposed close to the northern side of the roadside quarry in the hypersthene-gabbro (p. 20), and consists of green banded spherulitic felsite or pitchstone. Acid dykes of both these types are met with fairly frequently in the area of the plutonic complex of Centre 3. (b)

Shore Section west of Kilchoan Bay: Jurassic Strata traversed by Outer Cone-sheets of Centre 2 and by Basic and Acid Dykes. In the afternoon the shore section fronting Ormsaigbeg was visited, from the Landing Jetty as far as the prominent headland of Sron Bheag (Fig. 7). It provides the most complete sequence of the Jurassic rocks in Ardnamurchan, and was demonstrated to members by Dr. Pringle. The party returned by the road that ends at the most westerly cottage of Ormsaigbeg, a short distance east of Sron Bheag. The sequence includes the upper part of the Lower Lias (Pabba Shales), the Middle Lias (Scalpa. Sandstone), Upper Lias

-?i~

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'd

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Present Beach Deposits

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VJI Beach Raisell Deposits

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Granophyre & Felsite -< Q.u artz-
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Dip of' Strata. Oip of' Cone- sheets.

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Filults, crossmark on downthrow side.

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<

Y4 i

300

I

Y2

I i i

600

i

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MILE

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900 YARDS

Upper Lias

.,

Lower Lias

FiG. 7.-MAP OF MESOZOIC STRATA AND TERTIARY BASALT LAVAS CUT BY TERTIARY MINOR INTRUSIONS, WEST OF

(Reproduced, by permission, from Ardnamurchun Geol. Surv. Mem., 1930.)

f 'l

Sandstone'. ' .: 5' . flags :.: .:..: Limestone on .;:-~._,: Calcareous Sst. ' { Black Shales :~1i~::; With Rassay ~O:=, Ironstone .-~--~~ { '".... "Z",· Sandstone . {SandY Shales ~g:'::i:; & Calcareous,:::: -:..=:: Sandstone -=-=:

L• { In,erlor Oolite

a: M 'ddl I. e :::l Lias

SCALE

a I i i a

}{

1,

ro

cliff',

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b

. dyke }.... cut by cone Sheets) : .30' . ,'" ". . & by a st,II la.ter '. " ···Grano Iw r e Composite non-porphyrItIc , \, ,<._.Quartz-dofer,te} Intrusion dyke \ , ' Granophyre Basalt Javas}\ S n. \ {ShaleS with ", 'Estheria murchisonill!: . on Tert.ar,y ) mudstone rn Bheag . ?Great Estuarine Series,

Besett: Lavas Basic Oykes

Kilchoaru .B a,v 'J

'6 3

' \ ".Composlte 511' ".Base of g 5 651imestone with Ammonites 6 above calcareous sandstone

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.

g2 ...{Pitchstone dyke

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.gs

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,

L/'""":~

>K:::

KILCHOAN BAY.

SUMMARY OF GEOLOGY OF ARDNAl\1URCHAN.

23

shales with the Raasay Ironstone, Inferior Oolite limestones and sandstones, and shales belonging to the Great Estuarine Series. The beds dip obliquely seawards, so that higher horizons take on south-westwards along the shore. Their angle of dip is about 30 degrees. Inland, towards the Ring-dyke Complex of Centre 2, the beds are similarly inclined, except for reversals of dip adjacent to strike-faults. The area forms the southern portion of a great dome developed around Centre 2. This structural feature appears to represent the earliest manifestation of activity at Centre 2, for it originated at a date prior to the intrusion of the Outer Cone-sheets, whose inclinations are unaffected by the doming. The Jurassic beds are traversed by numerous minor intrusions, and the sequence is also interrupted by faults. They are considerably indurated and for this reason fossils are somewhat difficult to obtain, except from weathered surfaces. The lower part of the Inferior Oolite, however, yielded a great number of ammonites to Mr. Manson, but this was achieved only after careful and prolonged search. Lower Lias (Pabba Beds).-The section begins at the Jetty in the Pabba Beds, probably close to the top of the Broadford Limestone. In addition to the prevalent sandy micaceous shales, there are occasional beds of sandstone. Elsewhere in the Inner Hebrides the series has yielded rich suites of fossils, but at Kilchoan only a few forms have been found, though some 400 feet of strata are exposed. The most common fossils are Gryphaa obliquata ]. Sow., and G. cymbium Lam. Belemnites and lamellibranchs including Pecten acuticostatus Lam. are also represented. A single ammonite has been collected, which Buckman has suggested to be referable to Gagatoceras. If so. the beds concerned would belong to the Oxynotus zone. A strike-fault heading north-west extends along the shore near high-water mark for nearly half a mile. It is first seen 300 yds. south-west of the Jetty, where it brings a massive fine-grained quartz-dolerite sill against Pabba Shales. In an extension inland (in part shown on Plate 7) it serves to separate the Pabba Shales on the coast from seawardly-dipping Upper Lias shales exposed just above deposits of the lowest raised beach three-quarters of a mile south-west of the Jetty. Middle Lias (Scalpa Sandstone).-White fine-grained sandstone is exposed in disconnected outcrops amongst the Tertiary intrusions, chiefly near low-water mark. The beds contain no fossils, but their stratigraphical position between the Pabba Shales and the overlying Upper Lias shales leaves no doubt that they represent the Scalpa Sandstone of the Skye district. Upper Lias.-West of a small headland formed by an eastwardly dipping quartz-dolerite sheet, dark purple fissile shales I

J.

E. RICHEY,

occur in lenticular outcrops amongst a plexus of cone-sheets. The shales have yielded numerous ammonites, including five species of Dactylioceras, together with Harpoceras aff. falciferum (J. Sow.). and they thus belong to the Falcifer zone. In two places the Raasay Ironstone is partially exposed. This bed is an oolitic limy ironstone, four feet thick as seen at one outcrop near low-water mark just west of a little projection on the shore formed by a composite cone-sheet or sill. A number of ammonites derived from lower beds are contained in the upper and lower portions of the ironstone. They include species of Alocolytoceras, Grammoceras, Dactylioceras and Phylloceras (or perhaps Paroniceras). The ammonites in the ironstone of the Island of Raasay are also derived, the latest forms present indicating the presumed date of deposition of the ironstone as Subcarinatum or a little later. The ammonites Alocolytoceras and Grammoceras found at Kilchoan belong to the much higher Striaiulum zone, and the non-sequence at the base of the ironstone is thus shown to be represented elsewhere by deposits totalling 400-500 ft. in thickness. No estimate of the thickness of the shales on the Kilchoan shore can be made owing to the disconnected nature of the exposures. About three-quarters of a mile beyond Sron Bheag, however, a continuous outcrop shows 20 ft. of beds. These are highly indurated and the ironstone, which is 4 ft. thick, is partly altered to magnetite. Inferior Oolite.-In Ardnamurchan this series is present in situ only near Kilchoan, where it was recognized in the first instance by Judd. The beds concerned are well seen along the shore south-westwards of the outcrops of the Upper Lias to Sron Bheag, and beyond this point they are exposed beneath outliers of the overlying Tertiary basalt lavas (Fig. 7). To north-east of Sron Bheag the strata are more or less indurated, and farther west their alteration by the gabbro ring-dykes of Centre 2 becomes marked. Here limestone and calcareous sandstone form high sea-cliffs whose greyish white colour, due to their contact-alteration, makes them conspicuous from the seaward side. The series is about 120 ft. thick. The lower half, in which many ammonites are found, consists of limestones and calcareous shales and sandstone, with shaly flaggy beds above and below. The upper portion is a massive un fossiliferous yellow or reddish sandstone. In the lower beds the Discites zone of the Bajocian and six zones of the underlying Aalenian have been recognized. Of the latter, Scissum, Murchisona, Bradfordensis and Concava have yielded many characteristic ammonites. The Moorei Beds of the still lower Yeovilian, Buckman considered to be indicated by a few poorly-preserved fossils, including ammonites which

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

he

tentatively referred

to the

genera

Dumortieria

25

and

Caudloceras.

In the following table the zones present are indicated, together with the localities from which fossils have been obtained, and the sediments concerned. STAGES AND ZONES.

LOCALITIES, WITH REFERENCE TO MAOL BUIDHE SUMMIT

SEDIMENTS.

(see Fig. 7)'

Bajocian Stage: Discites zone

Aalenian Stage :

Concaua zone Bradfordensis zone •VIurchisonee ? A ncolioceras Scissuni zone Yeovilian Stage: ? Moorei zone

358 yds. E. 25 S. 390 yds. E. 25 S. { 300 yds. E. 25 S.

flags limestone limestone, 1 ft. 9in.

f

limestone, 1 ft. 9in. limestone, 6 ft.

300 yds, E.25 S. l4I6 yds. E.2I S., E. side of a gully 300 yds. E.25 S. 450 yds, E. IS S. 416 yds. E.21 S., E. side of a gully 500 yds. E.6 S. 500 yds. E.6 S.

limestone, zft. limestone, 2 ft. 6 in. limestone, 8 ft. sandy flags with calcareous daggers sandy flags with calcareous daggers

The Scissum zone has afforded beautiful specimens of Lioceras, including 9 species. The Murchisonce zone is also rich in fossils, which include the zonal form Ludioigia murchisonce (J. de C. Sow.) and several lamellibranchs, The Bradfordensis zone is well represented, but perhaps the richest ammonite fauna has been obtained from the Ludwigella cornu horizon of the Concaoa zone, with numerous species of Ludwigella. The Discites zone according to Buckman includes three distinct horizons, Platygraphoceras, Reynesella and Docidoceras, Great Estuarine Series.-In the sea-cliffs just west of Sron Bheag there is an outcrop of steeply-dipping black fissile shales enveloped by Tertiary intrusions. About 8 ft. of beds are seen. They contain Estheria murchisonce Jones, which is a typical fossil of the Great Estuarine Series of Skye. Tertiary Minor Intrusiolls.-Alongthe shore, west of the Jetty, basic and acid dykes and basic cone-sheets of the Outer Set are well displayed. Dykes, indeed, are mare numerous here than in any other district in Ardnamurchan. In a distance of a mile and a half south-westwards from the Jetty, 36 dykes have been mapped with a total aggregate thickness of 270 ft., an average of 7! ft. for each dyke. In direction the dykes vary mainly between north-west and north-north-west. Many of them

26

J.

E. RICHEY,

probably belong to the Mull Swarm rather than to Ardnamurchan. In the north of Mull, however, the number of dykes and their aggregate thickness are much less than the figures given above. It would therefore appear that some are intrusions from the Ardnamurchan focus. To this focus the acid varieties mav certainly be ascribed, for acid dykes are rare or absent in the case of the Mull Swarm except in the vicinity of the plutonic masses of Central Mull. Of the basic types olivine-dolerite and tholeiite are of chief importance. Both types are frequent in the Mull Swarm, and both are also found amongst the somewhat scattered dykes of the more easterly portion of Ardnamurchan. In the case of basic varieties, therefore, it is impossible on the Ki1choan shore to distinguish the local from the Mull dykes. With few exceptions, the dykes cut the Outer Cone-sheets, and it is probable that they mainly belong to a somewhat late date in the igneous history of Ardnamurchan. They are sometimes intruded along crush-lines which were developed at a late stage in the igneous history of the area. Yet comparatively few dykes cut the plutonic masses, close to the outer margin of which many appear to terminate. Certain of them are contact altered and therefore are concluded to be earlier than the plutonic rocks. Others are quite unaltered. Without doubt dykes were intruded at various periods. Some direct evidence oUhis has been already cited (pp. 5,18). Other examples are as follows. One contemporaneous with a cone-sheet is exposed on the shore two-thirds of a mile south-west of the Jetty. A felspar-phyric dolerite, running north-west, is here seen to be directly continuous with a cone-sheet of similar rock-type. Dykes cut by cone-sheets are also known. For example, a north-west bostonite, 7 ft. wide, traverses the Pabba Shales 300 yds. south-west of the Jetty and is cut by a cone-sheet. A pitchstone dyke, 5 ft. in width, cuts the cone-sheet referred to above at a point two-thirds of a mile south-west of the Jetty (Fig. 7). It provides a beautiful example of a banded spherulitic glass, and has glassy margins free from spherulites. It has been traced inland for more than a mile to where it cuts the hypersthene-gabbro ring-dyke east-south-east of Beinn na Seilg. The following day (the 26th) it was seen there near the twin lochs, the exposure affording a magnificent illustration of overfolded flow-banding. Another remarkable intrusion, 36 ft. wide, is composed of olivine-dolerite strung with pink aplite veins and contains numerous pink-coloured amygdales (Fig. 7). The latter have been described by Dr. H. H. Thomas as consisting of a lining of perthite and centre of prehnite separated by a layer of small colourless garnets. The dyke itself is not metamorphosed, and the amygdale mineral-assemblage cannot therefore be ascribed in this case to re-heating.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

27

The cone-sheets extend almost parallel to the coast-line, and are inclined inland towards Centre 2 at angles of about 35-40 degrees. They are mostly only a few feet in width, and on the whole they are not so abundant as in other parts of the belt. Locally, however, as for example in the Upper Lias shales, the extent of outcrop of the cone-sheets and related sills much exceeds that of the country-rocks which they traverse. Non-porphyritic quartz-dolerite is the main rock-type encountered for some distance south-west of the Jetty, but all nearing Sron Bheag felspar-phyric dolerite sheets become plentiful. Farther west along the coast, and also inland, towards the inner side of the belt, the porphyritic variety predominates. The porphyritic sheets are of interest structurally. They usually bear non-porphyritic margins about 6 ins. or so in width. There is a sharp dividing line between the margins and the felspar-phyric centres, but no signs of chilling of one rock-type against the other are apparent. A similar composite association is known to occur both in dykes and cone-sheets in other Tertiary districts, as in the cone-sheets of Carlingford Mountain and ill certain north-west dykes of Slieve Gullion, in Ireland. A composite sheet or sill with basic margins and acid centre outcrops along the coast adjacent to the exposure of the Upper Lias shales. The central portion is charged with basic xenoliths modified by the acid magma, and the intrusion resembles the composite sill near Mingary Pier (p. 4). Massive basic intrusions extending at right-angles to and traversed by the cone-sheets are noteworthy. One of them forms a slight promontory north-east of the Upper Lias outcrops, and is inclined to the north-east. It may perhaps be an isolated cone-sheet of the massive suite of Centre I, members of which are seen in force some miles to the east extending south-westwardstowards the coast west of Ben Hiant. A second massive basic dyke or sheet forms the conspicuous headland of Sron Bheag, and is flanked on either side by granophyre. The whole complex mass has been termed a composite intrusion, since the granophyre is conspicuously hybridized by interaction with the basic rock. The structural arrangement, however, is different from that characteristic of typical composite minor intrusions, in which the acid rock forms the central member. Basic xenoliths too are inconspicuous in the acid rock except near its contacts with the basic interior portion. The interest of thistriple intrusion lies mainly in the facilities it affords for the study of hybridization, which is especially well exemplified along the western side of the basic member. The latter was originally a quartz-dolerite of Talaidh type, though subsequently it was more or less altered by the later acid magma. The acid rock is a pale-coloured, soda-granophyre or bostonite, but it becomes darker in hue and basified near to the dolerite. This is largely

28

J.

E. RICHEY,

due to the incorporation and recrystallization of augite. Foxred biotite also is plentiful. An interesting feature of the more markedly altered granophyre is the development of abundant phenocrysts of plagioclase felspar. These porphyritic hybrids are best examined just above a scarp or cliff forming the southwest side of the headland. Along the lower part of the scarp itself, close to the level of high tide, another type of contact is to be seen. The marginal acid rock is not so highly basified, .and contains angular basic xenoliths close to the contact. Felspar phenocrysts, apparently similar to those in the adjoining acid rock, occur as xenocrysts in a manner reminiscent of the phenocrysts described from the gabbro-xenoliths of Tregastel in Britanny. I A few feet above the contact a flat plane of movement, noted by Mr. Campbell Smith, can be traced seawards along the scarp for some 20 yds. It is marked by a layer of breccia consisting of fragments of the dolerite and of a finer-grained rock derived from basic cone-sheets that traverse the dolerite. Where last seen, towards the nose of the promontory, a cone-sheet crosses the plane of movement without being affected by it. The plane may therefore be due to stresses connected with the cone-sheet period, to which it is to be dated. At the close of the day's excursion Professor Holmes, acting on behalf of the President, expressed the cordial thanks of members to Dr. Pringle for his much appreciated demonstration of the Mesozoic formations. Tuesday, July 26th. Beinn na Seilg: Ring-dykes and Inner Cone-sheets of Centre2. The route followed was by the road to the Free Church, and thence across country to encircle Beinn na Seilg. The river that flows past the Free Church (the Amhainn Druim an Uillt Mhor) was crossed at a point nearly opposite the church, and the lower eastern slope of Beinn na Seilg was climbed, the party closely following the outer margin of the hypersthene-gabbro ring-dyke. At the top of the slope, when in sight of the twin lochs (Lochain Ghleann Locha) that lie below the base of the main peak of Beinn na Seilg, the party turned north up a low rise (Stachan Dubha) due east of the main peak. Here granulitic strips enclosed in the gabbro are well displayed. A magnificent view was obtained of the gabbro country to north and east, and also of the distant Small Isles-Eigg, Rum, Muck-behind which rose the jagged outlines of Skye. A few members then climbed to the summit of Beinn na Seilg, but the main party skirted around its southern side and then crossed a strip of peat I H. H. Thomas and W. Campbell Smith, " Xenoliths of Igneous Origin in the TregastelPloumanac'h Granite, Cates d u Nord, France," Quart. [ourn, Geol. Soc., vol. lxxxviii., 1932, l'p. 274-295·

-\

K

j

1-

RTNG -DYKF:S lett ered a to j (Ce nt r e 2 )

&A t o N ( Ce nt r e 3 l ~GraTl.op}v ·'·e:- ( e ~&FeJsilc ( J )

')

+++:~+++] Q llarlz -'!'orLY.onile

(N )

+++ ++ 1 &- TonaJil..e, ( M)

_._._ Faull.• . / ~

Sc ate o f

L~

M llc g

1 2 3 1-

----.J.--=--_ - - -

~--- ___J

' ''SHEET SI " " S H E E T 52

FIG. S.-INDEX MAP OF RING-DYKES OF CENTRES 2 AND

3.

ARDNAMURCHAN.

(Reproduced, by permission, from ArdttamurchaH Geol. Surv. Mem., 1930,)

P r r.'1 'aJe " , di/~ ." nl " ~lfe.Yo z o i(' Sr.dil"cn1.~

~ s. w

Be rn n nan or- e

Be i nn na hImeilte AO DA NN CENT RE 2

S S E4

E:-7<-N.NW

N. E,-x--W.

Mea ll

nan M EA LL Con M EA DHO /N : : CENTR E

A CH NAHA CE NTRE

3

1

Ben Hrarrt 172 9 OIl TER CON E'SH E ETS ( OF CENTR E 2 J) -'

'

'of

.\

a.

e

gf

'//+"+11;"+-

: E l K J M N MJ" J ' L .J II

c

R IN (; - DYKES OF CENTR E 2 WITH INNER CON E· SHE ETS

~ -- - - - - - - - '- - - -R /N G - D Y K £ S

OF

Horizon t.al

o

2

S c ale -1'

~

======,=-~~_~=L-

I

Vcrt.ic a .l Sc ale =twicc 'h or-izont.a.l o ,

1000 !

2000

3000

j----o--J

Rin o -d y1< e s

I

!+P+1 fiasall,

t::t:±tt1 L

(I I

·0 ....·

~ Jlk....o za ic

t±±::::::jSpdi",,'liI....

111\l /I~g"'m/t" ffTiT' •s ~ .') (·lt ls l

~ CQ!tc- s l te c (S ISQ;]Eal'O" ~)lqiol' '1.> 'lfl(rll....nons

~V~nt­

~a/lgwIllPJ'alf!S

I

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lJIOl i /" r

encr-ucEucrite:

I

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H.ypP,..ytlteflC _

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2

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FIG. 9.--SECTION ACROSS TERTIARY INTRUSIVE COMPLEX OF ARDNAMURCHAN.

(Reproduced, by permission, from Ardnamurchan Geol. Sun,'. Mem., 1930.)

ft.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

31

and alluvium to a rock scarp to the west. On the scarp a contact of the inner margin of the gabbro with a ring-dyke of quartzdolerite was examined. The steep junction was followed westwards across the valley west of Beinn na Seilg to the eastern slope of Beinn nan Ord. Northwards along this slope successive ring-dykes were then crossed, the quartz-dolerite of Sgurr nam Meann, the quartz-gabbro of Loch Caorach, and the eucrite of Beinn nan Ord. Older quartz-gabbro (of Garbh-dhail) cut by Inner Cone-sheets, which bounds the eucrite on its inner side, was reached at lunch time. The valley was then recrossed, the party keeping close to the contact of the older quartz-gabbro and the eucrite. The older quartz-gabbro was next traversed, and a halt was made on the ridge formed by the old gabbro of Lochan an Aodainn. At a point on the south side of the ridge, east of the Garbh-dhail hollow, the old gabbro was seen to be intruded by the older quartz-gabbro. From this viewS:-7''€- N, W.

S.E.4

Beinn

na SeiIg.

>0.

FIG. lO.-SECTION ACROSS SOUTH-WEST PART OF ARDNAMURCHAN.

r , Schists;

Trias; 3, Lower Lias; 4, Inferior Oolite; 5. Tertiary ban' lavas. For explanation of lettering, see Fig. 8.

2.

(Reproduced, by p mnission, [rem: Ardnamurchuw Geol. Surv. Mem., 1930.)

point the curving course of the old gabbro was observed, and thereafter older rocks along its inner side were examined, including highly-altered basalt lavas and agglomerates. A short way south of Lochan an, Aodainn (or Lily Loch) the party turned east, mainly across peat, to reach the Achosnich road, along which they returned to Kilchoan, Outer Margin of Hypersthene-gabbro Ring-dyke.-On the lower eastern slope of Beinn na Seilg, the outer margin of the hypersthene-gabbro is very steep, for it extends almost directly uphill. Farther west, beyond the twin lochs that lie in a hollow near a junction with basalt lavas forming a steep bluff, the margin crosses a series of hills above the southern coast to the summit of Beinn nan Codhan, and extends southwards for short distances obliquely down the intervening valleys. This steeply outward

32

J.

E. RICHEY,

inclination of the margin is still better displayed west of Beinn nan Codhan, where it obliquely descends cliffs 400 feet high to reach the coast near An Acairseid. A wide fine-grained outer margin is characteristic of the intrusion. As already mentioned (p. 20) much of the marginal portion is an olivine-free quartz-dolerite in composition, but like the main mass it contains rhombic pyroxene.· On the eastern slope of Beinn na Seilg a few cone-sheets of porphyritic basalt or dolerite traverse the marginal rock, and may be best grouped with the partially developed set of cone-sheets referred to Centre 3 (p. 2). Wall-rocks consisting of contact-altered basalt lavas and Pabba Shales are abundantly cut by older conesheets (the Outer Set of Centre 2). The sheets are likewise altered, and as elsewhere (p. 20) it is more especially in their chilled marginal portions that this fact is most clearly disclosed. Granulitic Strips in Hypersthene-gabbro.-Xenoliths of finegrained basic granulite are locally abundant. They sometimes occur as irregular-shaped masses, but often they take the form of strips many yards in length and a foot or more wide. Similar bands of basic xenolithic rock, more or less recrystallized, are known from the Cuillin Gabbro of Skye and the Ben Buie Gabbro of Mull; in microscopic structure all of these resemble beerbachite. I Xenolithic strips are well seen on the low eminence of Stachan Dubha. Here and elsewhere their elongate shape suggests that they are minor intrusions cutting the gabbro. But their margins are unchilled, and it can frequently be seen that they have been invaded by the adjoining gabbro, while under the microscope their recrystallized condition is evident. Many consist of granulitic gabbro, often containing hypersthene, and these are presumably cognate to the intrusion itself. On Stachan Dubha the strips are almost horizontal and are parallel to flow banding in the gabbro. Elsewhere, however, the strips and the banding when present, are vertical or steeply inclined. It is thought that the strips have been mainly derived from an earlyconsolidated portion of the intrusion and that they were scaled off the sides of the intrusive mass and carried upwards by a fresh influx of magma. To some extent also the elongate form is to be ascribed to the streaking-out of the xenolithic rock while the latter was in a plastic state. Junction of Hypersthene-gabbro and Quartz-dolerite Ringdyke of Sgiur nam Meann.-Much of the inner edge of the hypersthene-gabbro is bounded by the quartz-dolerite ringdyke. The contrast between the two rocks is so marked that their mutual junction is one of the easiest plutonic contacts to follow in the field. The quartz-dolerite is a fine-grained lightI For a useful summary see A. G. MacGregor, "Scottish Pyroxene-Granulite Hornfelses and Odenwald Beerbachites," Geol, Mag. (1931), pp. S06-S2I.

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

33

weathering rock with well-developed acid mesostasis, and is frequently intruded by abundant net-veins of granophyre (Fig. 12). Both basic and acid rocks usually contain a rhombic pyroxene. The net-veining is practically confined to the outcrop of the quartz-dolerite, straying but little into the adjoining hypersthene-gabbro. The latter is for the most part a dark coarsely-crystallized rock, sometimes banded, consisting of basic plagioclase, augite, hypersthene, olivine and iron-ore. Age relations are clearly demonstrable at an exposed contact which was examined on a scarp bordering the western side of a strip of peat and alluvium west of the main peak of Beinn na Seilg. A steeply inclined cone-sheet belonging to the Inner Set of Centre 2 cuts the scarp at the contact, but the latter is well seen at the base of the scarp. The margin of the quartzdolerite, steeply inclined outwards from Centre 2, cuts across the inwardly-inclined steep banding of the gabbro, so that individual bands abut against the contact almost at a right angle. Xenoliths of the gabbro are contained in the marginal quartz-dolerite. Farther north along the scarp, a few yards within the quartz-dolerite, an immense xenolith of the gabbro was observed by Miss Dingwall. From this point the junction was traced without difficulty westwards and downhill to where it crosses the steep-sided valley between Beinn na Seilg and Beinn nan Ord. The very steep nature of the junction is here a scenic feature. Ring-dykes within the Quartz-dolerite of Sgilrr nam Meann.Two ring-dykes that accompar.y the quartz-dolerite along its inner side are exposed on the eastern slope of Beinn nan Ord. The combined width of outcrop of all three intrusions is here less than a quarter of a mile (see Fig. 8). The quartz-gabbro of Loch Caorach next to the quartz-dolerite is a dark rock streaked and spotted with a pale acid mesostasis. Its dark colour is largely due to its turbid plagioclase felspar, which resembles the clouded felspars of contact-altered rocks (see below, p. 35). The clouding in this case appears, however, to be an original character, for the quartz-gabbro is later than the two ring-dykes on either flank. A contact with the quartzdolerite has been observed on Beinn nan Ord, at which this rock is recrystallized. No junction with the innermost of the three intrusions (the eucrite of Beinn nan Ord) has been observed here, but about a mile and a half to the north-west, south of Grigadale Loch, the quartz-gabbro becomes finer in texture at an intrusive contact with eucrite. On this somewhat slender evidence it has been assigned to a later date than the eucrite. 1he eucrite throughout its extent is a moderately coarsegrained, black and white rock with abundant olivine. In the hand-specimen it is easily distinguished from the quartz-gabbro. PROC. GEOL. Assoc .• VOL. XLIV., PART

I,

1933·

3

34

J.

E. RfCHEY,

A bolder type of scenery is characteristic of it, and its icerounded reddish-weathered surfaces are in contrast with the much-weathered dark-coloured quartz-gabbro. The difficulty experienced in locating contacts between the two rocks along the Beinn nan Ord ridge is due to the fact that the eucrite is comminuted by explosion and that this minute brecciation has also affected the adjoining part of the quartz-gabbro. Within the eucrite, an older quartz-gabbro (Garbh-dhail) continues from Beinn na Seilg along the ridge of Beinn nan Ord, and extends inwards to the old gabbro of Lochan an Aodainn. It is traversed by numbers of cone-sheets of the Inner Set of - ---- -- -._-

--

-

FIG. II.-VIEW OF THE WESTERN SIDE OF BEINN NA SEILG..

[For explanation of lettering, see Fig. 8.] (Reproduced, by permission, front Ardnamurchan. Geot, SUyt'. J1e111., I930.)

Centre 2, whereas not a single sheet cuts the eucrite. On the lower western slope of Beinn na Seilg the contrast between the two masses in this respect is striking. The quartz-gabbro becomes finer in texture outwards towards the eucrite, but this portion of it is contact-altered near to the latter. The eucrite would appear to have been intruded near to the original outer margin of the quartz-gabbro. The latter rock is also highly sheared next to the eucrite, which is not affected in this way. On Beinn nan Ord, indeed, dark irregular veins of flinty crush rock are to be found in the sheared quartz-gabbro.

--,

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

35

The steep nature of the ring-dykes above described is evident on the eastern side of Beinn nan Ord and also on Beinn na Seilg opposite. A view of the latter slopes is depicted in Fig. II. lt will be seen that on this hillside junctions abruptly cross successive ridges and the intervening hollows, which are eroded along N.-S. lines of crush. An interesting feature of the eucrite is also shown in the view. The northern summit is formed by an inwardly-projecting radial offshoot from the ring-dyke, which is intrusive into the old quartz-gabbro. South of Grigadale Loch a second radial projection has been mapped. lt may be remarked that similar offshoots are found in the ring-dyke of the Ossipee Mountains, New Hampshire. ' The Inner Cone-sheets cut the quartz-gabbro on the lower western slopes of Beinn na Seilg. They are invariably steeply inclined at angles of 65-70 degrees, and are composed of felsparphyric dolerite or basalt. In width they usually measure about 5 feet, but individuals as much as 60 feet wide have been mapped. Eastwards they are cut off by the Beinn nan Ord eucrite and by a later quartz-gabbro (of Faskadale) belonging to Centre 3. Near to the latter mass the sheets are baked and streaked with acid veins. Old Gabbro of Lochan an Aodainn.-This mass is probably one of the oldest intrusions in the Earlier Ring-dyke Complex. Its early date is suggested by its general metamorphosed condition. It everywhere shows the effect of re-heating in the clouding of its felspars by myriads of dust-like particles. The cloudiness observed under the microscope is to be correlated with the remarkable black or bluish-black hue of the gabbro, especially characteristic of weathered surfaces. In composition the rock is an olivine-gabbro. It is often spotted with acid patches, and includes considerable amounts of fine-grained, apparently xenolithic material. The outcrops of the old gabbro form a half-circle around Centre 2, though for the most part the mass is in contact with later intrusions of quartz-gabbro. An original arcuate shape, however, can be demonstrated along part of its inner margin, where the gabbro is in intrusive contact with basalt lavas and agglomerates. These earlier rocks are highly baked. In the lavas amygdales appear as mere ghosts on fractured surfaces, but they stand up fairly prominently on the weathered crusts. The agglomerates are composed largely of basalt, but they also contain fragments of rhyolite, quartz-dolerite, etc. The gabbro lies inside the main belt of the Inner Cone-sheets, and is cut by only one or two thin sheets.

I Louise Kingsley, "Cauldron-Subsidence of the Ossipee Mountains,' Amer. [oum, Science, vol. xxii, (I93I), pp. I39-I68.

J.

E. RICHEY,

Wednesday, July 27th. Grigadale to the Lighthouse: Ring-dykes of Centre 2.

The day was wet, with intermittent heavy rain, and, while waiting for a fine interval, a brief account of the ring-dykes was given by the Director at Professor Holmes' suggestion. The question of the age relationships of the prevailing gabbroic masses was chiefly dealt with, and the means whereby the timescale had been ascertained. It was emphasized that in some cases relative ages could be established beyond doubt, while in others sufficient evidence had perhaps not yet been obtained. Evidence may be gained from either of two masses in contact at or near their mutual junction. For example, a structure in one mass, such as flow-banding or traversing veins, may be cut across by the margin of another gabbro. The latter is then obviously the younger of the two. Again, one mass may contain xenoliths derived from an older gabbro, or an older gabbro may be contact-altered in proximity to another unaltered intrusion, or a later mass may present a marginal facies against another rock, such as a chilled edge or a border of slightly different composition to the main mass. Again, the age of one intrusion relatively to another may be disclosed by its preceding history: a mass shattered by explosion or sheared by crustal movement may be regarded as older than an adjoining mass that is quite sound. Most convincing of all, when it can be demonstrated, thin veins proceeding from the margin of one intrusion may penetrate an adjoining mass. At about I I a.m. the party left the hotel to motor to Grigadale. Members then followed the road to the Lighthouse, where they lunched. The weather improved and the shore section west of Sgurr nam Meann was visited. The rocks examined included the quartz-dolerite ring-dyke abundantly net-veined by granophyre, an acid sheet intruded in the hypersthene-gabbro. south-east of Eilean Carrach and a strip of porphyritic granulite enclosed in the hypersthene-gabbro on Eilean Carrach. Alongside the road to Grigadale, where motors awaited the party, material was collected from the dark quartz-gabbro of Loch Caorach and from the shattered and recrystallized eucrite of Beinn nan Ord. Details of the rocks of this area are given below. Hypersthene-gabbro Ring-dyke.-The rugged western coast of Al dnamurchan, well known to passing ships, is formed of the hypersthene-gabbro. At the Point of Ardnamurchan, on the shore west of the Lighthouse, there is exposed the outer marginal facies of the gabbro, approximating to quartz-dolerite in composition and texture. The curving coast-line farther south also closely approximates to the original outer edge of the mass. Two-thirds of a mile east of the Lighthouse, close to.

se~IMARY

OF GEOLOGY OF ARDNAMeRCHAN.

37

where the gabbro is intruded by the quartz-dolerite of Sgurr nam Meann, it becomes a fine-grained fluxion gabbro. This rock may represent the original inner marginal facies of the ring-dyke, and the distance given may be little short of the original width of the intrusion at this point. Bands of fluxion gabbro and of normal gabbro alternate with one another, suggesting the intrusion along the inner margin of two magmas, one partly crystallized, that became interbanded by flow. The central coarser portion of the gabbro east of the Lighthouse locally contains a good deal of acid mesostasis, and such rocks approach the quartz-gabbros in composition. Masses of basic granulite are often included. Farther south, on a promontory bounding a bay south of the Point itself, xenolithic dyke-like strips are especially well displayed. There are also several good examples two-thirds of a mile E. 35 N. of the Lighthouse, on Eilean Carrach, a rocky promontory isolated as an island at high tide. On the seaward side, near high-water mark, a strip of porphyritic basic granulite is of special interest, since it bears evidence of having been subjected to plastic flow. It contains abundant phenocrysts of felspar which sometimes show their original angular shape, but more often are slightly elongated or drawn out into folise parallel to the length of the strip. Quartz-dolerite of Sgurr nam Meann.-Abundant net-veins of granophyre are characteristic of exposures along the bay between the Lighthouse and the Eilean Carrach promontories. Good instances may be seen on a scarp bordered by sand on the foreshore 1,100 yds. E. 14 N. of the Lighthouse (Fig. 12). West of the beach-sands along rocks near tow-water mark, an intricate relationship between the quartz-dolerite and the hypersthene-gabbro may be examined. The quartz-dolerite, strung with acid veins, encloses abundant gabbro blocks of all sizes. Still more amazing is the condition of the gabbro itself adjacent to the dolerite. This is best seen farther to the northnorth-east, on flat ground above high-water mark west-northwest of Sgurr nam Meann. There is a semblance of gradation from one rock into the other, through an apparent transition zone some 30 yds. wide. Microscopic examination of the rocks of this zone shows that they represent the hypersthenegabbro completely shattered and subsequently contact-altered. Xenoliths of the shattered gabbro are enclosed in the adjoining quartz-dolerite. In fact, prior to the emplacement of the dolerite, the wall of the ring-fissure up which the magma rose was subjected to intense brecciation. This is ascribed to explosive gases that preceded the intruding magma and were emitted from it. Certain xenoliths of basic· granulite contained in the marginal quartz-dolerite are of special interest, since they contain thin bands of spinel and magnetite. Dr. H. H. Thomas has suggested

J.

E . RICHEY.

as one possibility that th e spinel may have resulted .Irom the re-fusion of a xenolith compo sed of olivine and anorthite. The quartz-dolerite is cut by a few cone-sheets of nonporphyritic quartz-dolerite , notably on the summit of Sgurr nam Meann , where the sheet s and the net-veined dolerite are both highly contact-altered. The alteration is t o be ascribed to adjoining later ring-dyke s, the quartz-gabbro of Loch Caroach and the eucrite of Beinn nan Ord. Granophyre Sheets east of Eilean Carrach.-A westwardlydipping granoph yre sheet, up t o 14 ft. thick, cut s across the

F IG . 1 2 .- 0 UARTZ-D oLER ITE N E T -V E I NED B Y G R A~ OP H Y R E . SCUR R r; A M '-

M EAN N

RI ~G-D YKE .

(Rl'pro L-Ua d J by perm i.. . .sion ; fr om Ardna mnrcho n, Cecl. S u r..,.

Al ~ UI . . I<)JO. )

broad headland formed of hypersthen e-gabbro north-west of Sgurr nam Meann. Near the coast , east of a stretch of sand bordering Eilean Carrach, it splits into two, and these two sheets are well exposed crossing gabbro cliffs near high-water mark. The southerly intrusion was exa mined during our visit. The gabbro above the sheet is riddled with acid veins to a height of ten feet or mor e. Continuity can be established between th e veins and the underlying granophyre. The veins are more slender and the brecciation less pronounced ,t han in the case of the net-veins in the quartz-dolerite ring-dyke.

IJci u JI

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SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

39

The granophyre sheets contain numerous basic xenoliths in all stages of acidification and resorption. Less altered and more angular fragments resemble the hypersthene-gabbro, from which the majority would appear to have been derived Quartz-gabbro of Loch Caorach.-The quartz-gabbro exposed alongside the road about a quarter of a mile west of Grigadale is typical of the ring-dyke throughout its extent (see p. 33). About 100 yds. or less to the north of the road. a contact with the quartz-dolerite to the west has been found. Close to the contact the quartz-gabbro becomes finer in texture and is strung by veins of gabbro-aplite and of augite-rich gabbropegmatite. The veins run parallel to the margin of the intrusion. At the contact the fine-grained gabbro becomes porphyritic, and resembles the adjoining felspar-phyric dolerite. The latter, however, can be distinguished by its intensely baked appearance. Eucrite of Beinn nan Ord.-The eucrite exposed near the Grigadale road is typical of the ring-dyke farther south, both in its original composition and present condition. It is a eucrite rich in olivine, which has been subjected to intense brecciation followed by re-heating. Northwards of Grigadale the eucrite changes to a less olivinerich type, with large plate-like crystals of augite. No contacts with the quartz-gabbro to the west have been located in this neighbourhood, but to the east a junction with another late quartz-gabbro is well seen. About 800 yds. N.N.E. of Grigadale, on the north-western side of Beinn Bhuidhe and 100 yds. N.N.W. of the northern summit of this hill, augitic eucrite, baked in appearance, is in vertical contact with quartz-dolerite, which grades uphill into quartz-gabbro. The intrusion concerned is the later quartz-gabbro of Beinn Bhuidhe. It may be mentioned that near the southern summit of Beinn Bhuidhe the later quartz-gabbro cuts an earlier mass traversed by the Inner Cone-sheets and forms the rugged southern slope of the hill, to north of the road from Grigadale to Achosnich. Below the road, low ground largely covered by peat coincides with the outcrop of the Grigadale Granophyre. This irregularly-shaped mass is also cut by the Inner Cone-sheets, which are well seen to the west adjoining the Allt Grigadale. To the east the granophyre intrudes the old gabbro of Lochan an Aodainn, a contact being exposed 200 yds. southwards of the road on the lower slope of a rocky hill formed of the old gabbro. On the way back from Achosnich to Kilchoan the contrast is noteworthy between the broken country, partly rock-ridge, partly grass and peat, which constitutes the Earlier Ring-dyke Complex (Plate 3), and high ground to the left formed of the great eucrite of Centre 3. Ice-rounded rock-knolls cover the

- 1-

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13.-l\;IAP OF SOUTHERN PORTION OF INTERIOR (CENTRE 3).

COMPLEX OF RING-DYKES

(R.Fodute
1930,)

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

41

steep curving hillsides that mark the outcrop of this massive ring-dyke. Its intrusive outer margin,of which details are given in the Ardnamurchan Memoir (pp. 297-304), may be examined at a few points: at the north end of Achosnich adjacent to the track-road leading to Portuairk; at a point 550 yds. east of Aodann, where the margin of the great eucrite is inclined outwards from Centre 3 at about 70 degrees; and a mile to the south-east, where the eucrite veins the older rocks in contact, 350 yds. south-east of Lochan Crannaig, round the southern end of which the road skirts. Thursday, July 28th. Achnaha District: Ring-dykes of Centre 3.

The road from Kilchoan to Achnaha was followed, and members examined adjacent exposures of two massive ringdykes of Centre 3, viz., the quartz-gabbro of Faskadale, the outermost and oldest mass, and the great eucrite. The latter rises in steep mountain-sides, largely bare of vegetation and with ice-rounded crags. After passing through a rocky defile eroded across the massive ridge formed by the eucrite, the road enters upon a wide stretch of lower ground that surrounds the village of Achnaha. At this point the inner side of the eucrite ridge is well seen, extending as a magnificent circular escarpment around the central area, broken at intervals by hollows eroded along lines of crush. This amphitheatre of mountains is without doubt the most striking scenic feature of Ardnamurchan (see Plate 6). Erosion-modelling is also well seen in the area enclosed by it, where curving ridges and hollows mark the course of various ring-dykes composing the Interior Complex. This complex was crossed from where the road first touches it, at a wide peat-covered flat (Fig. I3), and in a traverse, which ended at Achnaha, various ring-dykes were examined. These included the narrow quartz-dolerite net-veined by granophyre, the fluxion biotite-gabbro of Sithean Mor that forms a curving ridge, the inner eucrite, quartz-biotite-gabbro, very coarse in texture, which partly surrounds a ring-ridge formed of the fluxion biotite-gabbro of Glendrian, and the more central tonalite, itself centred by a boss-like mass of quartz-monzonite. Intrusive junctions, so difficult to locate in the Interior Complex, were seen between an apophysis of the Sithean Mor gabbro and the (earlier) great eucrite, between the Glendrian ringdyke and the quartz-biotite-gabbro, and between the tonalite and quartz-monzonite. From Achnaha the new road north-westwards to Sanna was followed. Where the road crosses the great eucrite ridge, here much lower than around its southern side, an excavation in bouldery gravels interbedded with sand was noted. The

J.

E. RICHEY,

deposit belongs to one of the higher raised beaches. At the top of the rise, north-east of the road, numerous veins of gabbropegmatite such as are typical of the eucrites of Centre 3 were examined where they cross the rock-walls of a slack in the great eucrite. From the road-end at Sanna, low ground covered with blown sand, largely composed of broken shell, was crossed to Sanna Bay. On a rocky promontory on the northern side of the bay the hypersthene-gabbro near a contact with the great eucrite was visited. A contact between hypersthene-gabbro and a darker rock much richer in pyroxene was noted by Miss D. Reynolds and Professor Holmes. It was observed that the joints in the darker rock, which are most probably cooling joints, are not parallel to the contact. Hence the hypersthene-gabbro which cuts them must be younger. Unfortunately the tracing of this new contact had to be curtailed for lack of time. r Near this new junction the party also found light-coloured " schlieren" which appear to represent metamorphosed impure limestone. Mr. Campbell Smith informs me that thin sections of one of these shows them to contain abundant pale green diopside, numerous small grains of green spinel, and much colourless garnet with several other minerals not yet certainly identified. Return to Ki1choan was made by motors that awaited the party at the road-end at Sanna. The following descriptions are mainly in illustration of the rocks along the route detailed above, but these are supplemented by general statements in regard to the various masses concerned. Quartz-gabbro of Faskadale.-This ring-dyke, the outermost member of the complex of Centre 3, is exposed around half the circumference of the complex. To the north-east it is last seen on the shores of Faskadale Bay (p. 49). To the south-west it disappears on the eastern side of Beinn na Seilg under a roof formed of the Earlier Ring-dyke Complex. What is perhaps the same intrusion is seen again, to the north-west, in the mass termed the later quartz-gabbro of Beinn Bhuidhe, which emerges from beneath an inclined roof formed of the older quartz-gabbro of Beinn Bhuidhe (p. 39). It is to be noted that contact-alteration is a marked feature of certain rocks composing the Earlier Complex between these two points, which cannot be ascribed to any plutonic intrusion exposed. The quartz-gabbro, as exposed along the Achnaha road in a quarry near the middle of its outcrop, is remarkable for its elongate crystals of augite, which exhibit the cross-striation of I The two rocks have since been cut and examined microscopically by Miss Reynolds and Professor Holmes. The well-jointed darker rock is a schillerized pyroxenite with interstitial plagioclase and pools of quartz, and it closely resembles a melanocratic variety of the old gabbro (A.dnamurehan Memoir, p. 238). The lighter rock in contact is a typical hypersthene-gabbro (ap. cit., p. 223).

SUMMARY OF GEOLOGY OF ARD);AMURCHAN.

43

sahlite, An acid mesostasis is well developed, and acid veins are of common occurrence. Fine-grained xenolithic material is prevalent. It appears to be chiefly granulitized quartz-dolerite, consanguinous to the gabbro enclosing it. Towards its outer margin next to the hypersthene-gabbro, the quartz-gabbro becomes finer in grain. The actual contact of the two rocks has not been located, and is perhaps not exposed. It is.however.easy to distinguish in the different rock-exposures between brown-weathering fine-textured quartz-gabbro and the dark grey surfaces of the older and probably altered hypersthenegabbro. Farther east, on the western side of the Glas Bheinn ridge, the hypersthene-gabbro is finely brecciated close to the quartz-gabbro, and this may also be its condition, and explain its fine grain, in the exposures near the Achnaha road. The rocks of the latter locality have not, however, been examined microscopically. From the Achnaha road the continuation of the quartzgabbro up the eastern slopes of Beinn na Seilg may be observed, as it forms smoother ground than the contact-altered rocks of the Earlier Complex, beneath which it finally disappears. To the east, on the steep western face of Meall an Tarmachain, the intrusion may also be viewed to its upward termination beneath a grey rocky capping of baked agglomerate and basalt lavas. The rocks of the capping are traversed by close-set joints at right angles to the gabbro-contact. Similar jointing has been observed elsewhere in older rocks bordering a plutonic intrusion (e.g., the quartz-dolerite of Sgurr nam Meann summit). Great Eucrite.-Where Meall an Tarmachain comes into view along the Achnaha road there is an unexposed gap of a few yards, beyond which the great eucrite outcrops. The marginal rock is a quartz-gabbro, fortunately unlike the quartzgabbro nearby. The augite occurs in ophitic relationship to the felspar, and not in elongate prisms. Moreover, the rock has a eucritic aspect, although olivine is inconspicuous and crystals of iron-ore are plentiful. This marginal facies is developed around the periphery of the great eucrite, and, adjoining the Achnaha road, it grades into normal eucrite in a distance of 30 yards. In the view of the western side of Meall an Tarmachain it may be observed that the ice-rounded rock-knolls, marking the course of the eucrite up the hillside,' extend across the capping above the quartz-gabbro. The structural relation suggests that the eucrite is the later of the two intrusions, and this is borne out by a study of contacts. One such contact, already cited (p. 4I), occurs near the Achosnich road south-east of Lochan Crannaig, where the marginal eucrite sends veins into an older rock mapped with the quartz-gabbro. Other suggestive evidence is supplied by the fact that sparse· cone-sheets referred to Centre 3 and traversing the quartz-gabbro and capping on Meall

44

J. E. RICHEY,

an Tarmachain are not found to cut the great eucrite and are contact-altered near to it. The typical eucrite is, as already mentioned, rich in olivine, but varieties in which augite is the predominant ferromagnesian mineral are frequently encountered. Both kinds are exposed alongside the Achnaha road. Here, too, a pale felspathic variety may be seen, which forms areas or somewhat irregular bands, a foot or more in breadth, surrounded by the normal darker eucrite. Another feature is the occurrence of areas of gabbropegmatite composed mainly of large crystals of augite and basic plagioclase felspar. Similar pegmatite also forms veins, which are locally abundant. The broader veins can be traced for many yards and at any particular locality they are parallel to one another and also to the course of the ring-dyke itself. They are found almost everywhere around the ring outcrop of the intrusion and evidently mark contraction cracks that developed in the cooling mass and became filled with residual magma rich in volatiles, such as is also represented by the pegmatite patches. The latter are sometimes seen to be continuous with the veins. The great eucrite forms a complete ring outcrop, a mile in annular width, around Centre 3 (Fig. 8). Along its inner side it is in contact with later intrusions belonging to the Interior Complex, and its original inner limit is not known. Possibly it may have been originally an immense boss, or stock, and its central portion may have been replaced by the ring-dykes now found within it. But the development of the longitudinal contraction cracks filled with pegmatite and a cross-jointing, well seen from the interior traversing the circle of crags along their inner face, seem to suggest that the intrusion is a ring-dyke. Quartz-dolerite Ring-dyke, veined by granophyre.-This extensive but narrow intrusion was the first ring-dyke to be recognized in Ardnamurchan, and it well illustrates the concentric habit of these intrusions around Centre 3. As will be seen in Fig. 8, it is discontinuous to the north, like many other ring-dykes belonging to this centre. As in other cases, it follows a roughly oval course, the long axis of which is directed eastnorth-east. The rock is a fine-grained biotite-bearing quartz-dolerite. Granophyre abundantly net-veins the dolerite and likewise contains biotite. The veins occur almost everywhere, but they are curiously restricted to the outcrop of the dolerite itself and do not wander into adjoining older intrusions. A contact with the great eucrite, exposed on the south side of the ringdyke, was examined by members beyond the peat west of the Achnaha road (Fig. 13). The best exposure of the junction, however, occurs near the eastern end of the ring-dyke at the base of Meall Meadhoin. There, as in the case of so many other

SUM~IARY

OF GEOLOGY OF

ARD~A)HJRCHA~.

45

ring-dykes, the contact is definitely inclined outwards from the CEntre at an angle of 70 degrees. Fluxion Biotite-gabbro of Sithean Mor.-The age relations of this intrusion to the great eucrite are determined by an apophysis which extends southwards out into the eucrite (Fig. 13). The outer side of the apophysis is marked by a curving scarpfeature, behind which rises the actual ridge of Sithean Mor. The apophysis consists of two parts. Marginally there is a finetextured fluxion gabbro (or dolerite), within which a coarser quartz-gabbro occurs, except along the narrower western portion of the apophysis. These two different rocks are in sharp contact with one another, the central member being the later intrusion of the two, having baked the marginal rock. A contact is to be seen on the south-eastern side of the apophysis on the outer side of the central portion. To the north-west, where the fine-grained rock only is encountered. a chilled margin is exposed on its outer side against eucrite. To the south-west, near the middle of the course of the apophysis, and east of a stone wall, xenoliths of eucrite in all stages" of disintegration and resorption are included in the marginal rock. The eucrite bounding the apophysis is similar to the inclusions and forms part of the great eucrite. The apophysis, at any rate its marginal portion, resembles in composition the Sithean Mor ring-dyke and is therefore regarded as an offshoot from it ; however, it is to be noted that exposures fail just where the two should join. The main mass of the Sithean Mor intrusion is composed of a dark medium-grained gabbro with fluxioned felspars and numerous small crystals of biotite. Midway between the apophysis and the Achnaha road to the east, on the southern half of the ridge, this rock is interbanded with white-weathering coarser-textured gabbro containing biotite and quartz. A small mass of the latter rock here extends alongside the fluxion biotite-gabbro. The two magmas concerned would seem to have entered the same ring-cavity and thus to have become interbanded with one another. I The bands are vertical or highly inclined, as are also the planes of fluxion in the ring-dyke generally. Inner Eucrite.-Lower ground north of Sithean Mor is largely occupied by very coarse-textured gabbroid rocks. In the northern part of this area quartz-biotite-gabbro outcrops. In the southern half, biotite is not found; the rocks contain olivine, and typical eucrite of medium grain, sometimes with the characteristic pegmatite veins, also occurs. This area appears to belong to the inner eucrite, which to west and east is a normal or else fluxioned eucrite and is easily distinguished from the I For further details see the Ardnamurchan Memoir, p. 330.

J.

E. RICHEY,

quartz-biotite-gabbro on its inner side. The planes of fluxion, where developed, are practically vertical and are parallel to the sides of the arcuate intrusion. To the east, pegmatite veins are frequent, and are usually highly inclined outwards from Centre 3. Though the exact relationship of the mass to its neighbours is not proved by a sufficient number of contacts, it seems probable that it is a distinct ring-dyke. To the northwest of Achnaha, alongside the road leading to Plocaig, a sharp contact has been located between the fluxioned marginal portion of the olivine-rich inner eucrite and biotite-eucrite that here separates the inner from the great eucrite. Quartz-biotite-gabbro.-When all the other rocks in the Interior Complex had been mapped out from one another, there remained areas of very coarse textured gabbro, which may represent parts of a single intrusion of earlier date than the intervening ring-dykes (see Fig. 8). The rock is remarkable for its coarse texture. The main constituent minerals, plagioclase felspar, augite, biotite, and iron-ore, form crystals as large as those characteristic of gabbro-pegmatite. Quartz is often seen as large blebs, and is especially conspicuous in an elongate mass enclosed in the tonalite. Epidote is frequently to be seen. Fluxion Biotite-gabbro of Glendrian.-The ring ridge formed of this intrusion is a conspicuous feature of the Interior Complex. It is interrupted only at one point, to the south-west, where the tonalite breaks through the gabbro. Dark in colour, medium grained, with fiuxioned felspars and abundant octahedral crystals of magnetite, the intrusion is easy to distinguish from the adjoining light-weathering, coarse-textured, quartz-biotitegabbro. A contact, highly inclined inwards, was examined near the top of the ridge due north of the road-bridge over the Allt Uamha Muice. No indication of the relative ages of the two gabbros is forthcoming here, but farther east large enclosures in the fluxion biotite-gabbro resemble the adjacent quartz-biotitegabbro. Also, the occurrence of quartz-biotite-gabbro within the ring of fluxion gabbro (to the east), as well as around much of its outer side, suggests that the fluxion gabbro is a later intrusion separating portions of a once-continuous mass of quartz-biotite-gabbro. East of a hollow eroded along a north-west basic dyke, through which runs an old track to Glendrian, members collected samples of the fluxion gabbro. From this locality material analysed by the Geological Survey was obtained. Tonalite.-This mass occupies a low-lying area within the Glendrian ring-dyke; it interrupts the latter to the south-west and so extends to the road at a point 500 yds. south-east of Achnaha. The Glendrian intrusion is cut by acid veins near to the tonalite, which is undoubtedly later in age. The

SC:\IMARY OF GEOLOGY OF ARDNAMURCH.·\S.

47

'tonalite is also later than the quartz-biotite-gabbro, an elongate mass of which is enclosed in it, as mentioned above. The tonalite is finer-grained and of more acid composition along its outer margin ; internally it is quite coarsely crystallized locally. Biotite is fairly abundant. Plagioclase felspar, grey in colour, predominates over orthoclase, which constitutes (with quartz) residual areas of very fine texture which are distributed evenly through the rock. Near the centre of the mass, about 130 yds. east of the central boss of quartzmonzonite, material was collected bv members. This resembles 'the analysed rock, which was obtained from a rock-knoll nearer 'the outer margin of the tonalite soo yds. north-north-east of the quartz-monzonite. A basic north-west dyke cutting the tonalite and exposed on the low ground 300 yds. north-north-east of the quartzmonzonite was also collected. The rock is a dolerite approaching crinanite in composition, containing a good deal of analcite, together with small flakes of biotite. Dykes of quartz-felsite also traverse the tonalite. An example may be seen in a stream 430 yds. north-east of Achnaha. Quartz-monzonite.-This tiny mass, only IOO~I40 yds. across, forms a knoll in the centre of the low ground occupied by the tonalite. From the latter rock the quartz-monzonite differs in its finer grain, lighter colour and more acid composition. A compact orthoclase-quartz matrix is plentiful, from which the darker crystals of plagioclase may be distinguished. Biotite is the dominant ferromagnesian constituent, and augite and hornblende are sparse, in contrast to their abundance in the tonalite. A contact with the tonalite was examined on the north-west side of the knoll, about 60 yds. from the summit. The quartzmonzonite is slightly chilled for an inch or so from the tonalite. A cairn of stones was erected at the spot as a guide to future visitors. Friday, July 29th. Faskadale Bay and Meall nan Con: Vent-Agglomerates and Major Intrusions of Centre 1, cut by Outer Cone-sheets of Centre 2 ; Ring-dykes of Centre 3. Members journeyed by motor to Faskadale: the road north of Camphouse crosses low ground occupied by the Northern Vents, and is featured to the east by prominent scarps and dip slopes which mark the outcrops of the massive cone-sheets of Centre I. The party proceeded around Faskadale Bay, examining the quartz-gabbro of Faskadale (Centre 3) and its contact with conesheets, agglomerates of the Northern Vents, and the dyke-like

J.

E. RICHEY,

major intrusions of Centre I (quartz-gabbro and granophyre west of Faskadale). The steep eastern side of Meall Bhuidhe Mor to the west was then climbed, close to the contact of the two intrusions just mentioned, here partly separated by a screen of agglomerate (Fig. 14). From the summit of the hill; the party turned south across the quartz-gabbro and agglomerates, both cut by the Outer Cone-sheets, to a ridge formed of these rocks, metamorphosed by the great eucrite. The ridge was reached at a point north of Lochan Dubh. The extensive slopes on the outer side of the great eucrite were next climbed, the party keeping to the west of the summit of Meall an Fhir-eoln, till the top of the ridge was reached and the Interior Complex came into view. After lunch, members proceeded south-eastwards around the great eucrite and obliquely recrossed its outcrop, keeping a level course and reaching the screen of Meall nan Con at about three-eighths of a mile north-west of the summit. They then ascended along the N.

Meall

s

Buidhe Mar 312,fT.

7~

+

SCAL.E

~8

560

1000, FEET

FIG. 14.-SECTION WEST OF FASKADALE BAY.

I. basalt lavas; 2 and 3. vent-agglomerate; 4, gabbro, and 5. granophyre, Centre I ; 6, dyke; 7, cone-sheets. Centre 2 ; E. great eucrite, Centre 3.

(Reproduced, by permission, [rem Ardnamurchan Geol. Sun'. Mem, 1930.)

escarpment that marks the western edge of the screen to a watershed at three little lochs which divides Meall nan Con from Meall Meadhoin to the west. Opposite the central one of the three lochs the escarpment includes also a portion of the eucrite (Fig. 15). Contact of the eucrite with the granulitized basic rocks of the screen was examined where the junction crosses the escarpment in a cleft just. north of the loch. Thence the screen was crossed, the party keeping south of the summit of Meall nan Can to follow the course of the AUt Rath a' Bheulan. In this stream-section ring-dykes of Centre 3 outside the screen are exposed-the outer eucrite, and the fluxion, gabbro and quartz-gabbro of Faskadale. East of the ring-dykes, the much earlier composite intrusion of Beinn an, Leathaid outcrops. Specimens were collected from its lower basic portion, which is traversed by thin cone-sheets. Just as heavy rain set in, the party reached the Camphouse road, and returned to Kilchoan by motor.

49

SUl\liYIARY OF GEOLOGY OF ARDNAMURCHAK.

Quartz-gabbro of Faskadale (Centre 3).-On the eastern side of Faskadale Bay an extensive exposure of the quartzgabbro is separated by a patch of beach-shingle from highly altered quartz-dolerite to the north. The quartz-dolerite forms a rocky coast-line farther east, along which many chilled margins can be detected. These show that the quartz-dolerite represents a complex of cone-sheets. In a cliff bordering the beach at Faskadale Bay the quartz-dolerite is traversed by veins of gabbro, and there is no doubt that the quartz-gabbro is later than the cone-sheets. Inland from Faskadale similar highly-altered rocks bordering the quartz-gabbro ring-dyke form a conspicuous mountain-ridge culminating in Beinn an Leathaid and bounded to the west by an extensive escarpment, at the base of which the more easily-eroded quartz-gabbro outcrops. Agglomerates of Northern Vents.-Beach-shingle at the head of Faskadale Bay separates outcrops of the quartz-gabbro from smooth-weathering grey crags formed of contact-altered agglomerates. Farther along the western side of the bay, on a wide beach-platform, the alteration decreases rapidly in intensity, and the included blocks stand up above the more easily-eroded matrix, in which only their lower portions remain embedded. Many completely detached boulders are also strewn along the shore. The blocks are often a foot or more in diameter. They consist of many rock-types, chiefly porphyritic felsite, spherulitic rhyolite, grey trachyte, quartz-dolerite, and amygdaloidal and felspar-phyric basalt lavas. The occurrence of the quartz-dolerite, which resembles the type prevalent among the cone-sheets, is of interest, for no such intrusions older than the agglomerates are known in situ, The agglomerates are hemmed in by major intrusions of later date, but they doubtless belong to the Northern Vents of Centre 1. Like the main outcrops farther east, masses of basalt lavas are associated with the agglomerates. One such elongate mass, partly shattered and probably down-faulted into its present position, has been mapped on the hillside to the west of Faskadale Bay (Fig. 14). Major Intrusions of Centre 1.-Two dyke-like major intrusions of quartz-gabbro and granophyrealready referred to (p. 48) extend westwards from Faskadale Bay and are aligned as though they belonged to Centre 1. An early date is directly proved by the fact that both are traversed by numerous conesheets belonging to the Outer Set of Centre z. They are in part separated by narrow screen-like outcrops of agglomerate (Fig. 14). Where seen on the western shore of Faskadale Bay, the quartz-gabbro is represented by a narrow outcrop that includes only a felspar-phyric marginal facies. The rock is here greatly sheared, the shear lines running parallel to the length of the PROC. GEOL.

Assoc.,

VOL.

XLIV.,

PART I, 1933.

4

50

J.

E. RICHEY,

intrusion. Inland, near the summit of Meall Bhuidhe Mor, the marginal facies was observed again. It was here seen in contact with baked agglomerate, and southwards grades rapidly into the normal quartz-gabbro, which contains at this place a good deal of olivine. The granophyre is well exposed on the coast to north of the sheared felspar-phyric rock. It encloses many xenoliths of schist, which are largely resorbed. Great Eucrite.-The highly-altered rocks bordering the great eucrite to the north of Lochan Dubh consist of grey:. weathering agglomerates traversed by brown-weathering conesheets, whose chilled edges are largely obliterated by recrystallization. They are cut by acid veins. Some of these, traversing the agglomerate, are dark in colour and appear to be contactaltered. Others are pale, and were presumably derived from the adjoining margin of the eucrite, which here, as elsewhere, is composed of quartz-gabbro (p. 43). This marginal zone is exposed just south of the baked rocks and grades southwards into the normal eucrite. The ice-rounded crags that rise southwards of Lochan Dubh are characteristic of the great eucrite ring-dyke. In the traverse made to the top of the ridge west of Meall an Fhlr-eoln a number of pegmatitic veins were seen, and at the summit a characteristic view of the Interior Complex was obtained (Plate 6). Meall nan Con Screen.-The recrystallized rocks of the screen north of the summit of Meall nan Con are believed to have originally been basalt lavas. South of the summit, rocks resembling tuffs occur, and farther south again there are undoubted agglomerates. The steep or vertical nature of the screen is apparent on the hillside north of the summit, and is also demonstrable at a contact examined west of the summit (Fig. IS and see p. 48). Acid veins traverse the granulites, and are especially profuse close to the eucrite intrusions to west and east. Ring-dykes east of the Meall nan Con Screen.-East of the screen the first rock encountered in the AlIt Rath a' Bheulan is fluxion gabbro, which is probably an isolated portion of the fluxion gabbro of Faskadale exposed farther east, beyond the outer eucrite. The latter intrusion would, in fact, seem to be later than the fluxion gabbro. A contact between it and the fluxion gabbro downstream was located by Miss Reynolds and Professor Holmes, but did not afford evidence of the relative ages of the two rocks. As in other fluxion gabbros, biotite is locally conspicuous in the Faskadale intrusion. The mass is traversed by many fairly thick veins of an interesting rocktype approximating to tonalite in composition. No contact is seen in this section between the fluxion gabbro and quartz-

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

5I

gabbro of Faskadale. The latter is somewhat basic in character as compared with exposures near the Achnaha road (p. 42), and locally contains a good deal of olivine. During the evening Professor Holmes voiced the gratitude of members to Mr. Richey for leading the excursion and for the arrangements made on their behalf.

Saturday, July 30th. Staffa and Iona, In the early part of the morning a heavy storm of rain necessitated cancellation of the original plan for crossing the Sound of Mull by motor-boat to connect with the Iona steamer at Tobermory. Later, the mists began to lift during the passage

¥~

FIG.

IS.-VIEW OF EUCRITE CRAGS ON WEST SIDE OF MEALL NAN CON.

Junction of eucrite and" screen" indicated by broken line. (Reproduced, by permission, from Ardnamurchan Geol. Suru. Mem. "930.)

to Tobermory by the S.S. Locheam, and, by courtesy of the captain, the party numbering 22 members were allowed to tranship directly to the Iona steamer off the north of Mull. On the way to Staffa, good views were obtained along the north-west coast of Mull and also the distant Treshnish Isles of the extensive pre-Glacial raised-beach platform at about !IS ft. O.D.• first described by Dr. W. B. Wright.

52

J.

E. mCHEY,

At Staffa, members landed by motor-boat (the sea being sufficiently calm) and visited the famous Fingal's Cave (Plate 4). On the way along the Causeway formed of the well-known columnar basalt, the apparent junction of the lower tier of upright hexagonal columns and middle tier of smaller irregularlyrunning columns was examined. It was observed that there was slight interdigitation of the two sets of columns and that no exact plane of separation existed. At Iona, the hill at the head of the main street was climbed in order to see the basal Torridonian conglomerates and underlying Lewisian Gneiss. All the rocks appeared to be greatly sheared. After visiting the Cathedral, where the Stone of Destiny was examined, the party returned to the landing stage and collected from the dark grey Torridonian flagstones well exposed along the coast. The pink Caledonian granite of the low-lying Ross of Mul] opposite lona, seen in sunlight, made a striking contrast to the mist-shrouded mountains behind. As the steamer passed eastwards along the south side of Mull, the granite area was succeeded by dark craggy ground composed of Moine Schists in faulted contact with terraced lava-hills well seen towards Carsaig. At Carsaig and beyond, coastal cliffs showed light-weathering Jurassic sediments capped by the Tertiary basalt lavas; but on crossing the entrance of Loch Buie to the Laggan peninsula terraced slopes gave place to formless lava-country adjacent to the Tertiary plutonic complex of Central Mull. Numerous dykes ofthe Mull Swarm could be detected in the extensive cliffs along the Laggan coast. Ben Buie, composed of gabbro, and other Mull peaks came into view for the first time from beneath the rising mists as the steamer crossed the Firth of Lome to enter the Sound of Kerrera. There, a well-known example of the broad platform characteristic of the post-Glacial 25-ft. raised beach was seen in the Dutchman's Hat, so like the Dutchman's Cap of the Treshnish Isles, except for the much lower level of the wide, flat "brim."

Sunday, July 31st. Oban and Dunollie. In the afternoon, rocks typical of the Oban district were examined along the coast northwards of Oban to beyond Ganavan Sands. Colonel MacDougall of MacDougall kindly acted as guide to the exposures described below. Later, members were entertained at Dunollie by Colonel and Mrs. MacDougall, and after tea had the opportunity of examining various objects of historic interest, including the famous Brooch of Lome. On the north side of Oban Bay, on the beach below the esplanade and a short distance south-east of the War Memorial,

PLATE 4.

}JIWC. GEOL. Assoc. , VOL. X LIV. (1933).

A .-

T H E G HE AT F A CE , WITH BOAT CAVE ON LEFT, A N)) FI N G AL' S CA VE E XTIm~lE H IGHT .

B . -F IN GAL ' S C A V E , STA F FA .

ON

S H O W ING D OU llL E-TIE H COLU MNA H J OI NTI N G IN BASALT L AVA.

(J'lw los rep roduced by kin d pe nn issic-n of sl eesrs . M ' Lsaac &

l.\[ll cJ ' !lCI SO" ,

Otan.)

[T o [uce p. 52

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

53

an ice-smoothed surface of Easdale Slates, bearing glacial strise directed W. 30 N., was first examined. A fault evidently intervenes between this exposure of the basement Dalradian Schists and the succeeding Lower Old Red Sandstone conglomerates, since the latter are seen below the War Memorial to be dipping towards the schists at an angle of 20 degrees. Farther round the coast, just south of the end of a small bay opposite Dunollie Castle, an exposure shows fiat-lying Old Red Sandstone breccias resting unconformably upon the highly inclined and contorted Easdale black slates and limestones. On the south side of the outcrop there is an excellent example of a crush-breccia, developed along a plane of movement, with fragments sheared out into the form of lenses in the centre of the crushed zone. Variation in the composition of the conglomerates was observed as the party proceeded northwards from Oban Bay. At the War Memorial pebbles of basalt lava predominate. Farther north, Highland quartzites become plentiful. West of Dunollie Castle pebbles of granite are fairly frequent. Farther north, in the high coastal cliffs beyond Ganavan Sands, granite boulders up to a few feet in diameter are common in a mixed assemblage that includes the ubiquitous pebbles of basalt lava and many of quartzite and porphyritic biotite-andesite. On the shore west of Dunollie Castle, basalt lavas are faulted down against the conglomerates. They provide good examples of the fissured and cavernous tops with infillings of bedded sediment characteristic of lavas of Lower Old Red Sandstone age. Within the grounds of Dunollie, opposite to the War Memorial, a sea-stack, the Dog Stone, stands on the 2S-ft. raisedbeach platform, here backed by high cliffs of conglomerate. The base of the stack shows the effect of undercutting by the post-Glacial sea. The legendary interpretation of this localized erosion was related by Colonel MacDougall, how Finn M'Cuil, the Irish giant, once hitched his hound, Bran, to this massive pillar, the base of which became worn by the chain in the great dog's efforts to escape. A cave in the raised-beach cliff below the Castle was also visited. Human bones were found here, but this particular cave has not yielded evidence of occupation by Azilian man, who seems to have preferred more sheltered sites nearer the head of Oban Bay. In the wall of an outbuilding at Dunollie the surface of one of the stones shows circular depressions resembling cup markings. r Some uncertainty as to this interpretation was expressed by I Colonel MacDougall informs me that he has noted similar markings on a grey weathered rock in tbe Easdale Slates of Kerrera on the south shore of a small bay over half a mile S.W.

of Little Horse Shoe Bay, opposite the island of Sgeirean Dubha,

J.

54

E. RICHEY,

j

1'\

KERRERA

LAVA S ON EROD ED

,,

S lJR F A CC: OF 4-

,, '"

4'

2 0 .:.:;'

P.N ' U;S I

I

O L D REO S A N DST ONE

B as a lt: L a Y II, I'

~'-:-:ll/mr;lt)m/'rtlt~~

~ I) . Jj/'(~t..<..·ULS

A ll t[rl.·/w oflulh·d

FIG, (Reproduced, bJ' permission, from

H

16,-]\IAP OF KERRERA.

The Pre-Tertiarv Geology of Mvtt, Loch Aline and Oban:" Mem; Geol.. Sun'. Scotland, 1925,)

SUMMARY OF GEOLOGY OF ARDNAMURCltAN.

55

members acquainted with the cup markings of other regions. As an alternative it was suggested that the hollows might represent the casts left by concretions that had fallen out. The rock itself is a dark calcareous grit. Monday, August 1st. Island of Kerrera.

Kerrera was reached from Oban by motor bus to the Ferry, two miles distant, which crosses the Sound of Kerrera. The party had the advantage of Colonel MacDougall's guidance throughout the day. From the Ferry House, a road skirting the eastern coast of the island was followed for two miles southwestwards to Gylen Castle. At Little Horse Shoe Bay, a mile southwards of the Ferry House, a hollow marking the course of an extensive north-east fault was observed. This fault and a parallel fracture farther to the north-west are responsible for a ridge of Lower Old Red Sandstone lavas, down-faulted on either side against the less durable Easdale Slates and Lower Old Red Sandstone conglomerates (Fig. I6). On the south-west side of the bay a ridge marks the outcrop of a sheet of grey porphyrite inclined to north-west at 45 degrees and intruded into the Easdale Slates. A north-west Tertiary dyke crosses the porphyrite and also the lava-cliff behind. Other basic dykes of the Mull Swarm, consisting of olivine-dolerite and tholeiite, were examined at various places around the coast. Gylen Castle is situated on a rock-spur formed of conglomerate just west of the trough-faulted lava-axis of the island (Plate 5, A). Its varied historical and architectural interests were explained to members by Colonel MacDougall. The southern coast of Kerrera was then traversed. It constitutes the western side of a hollow of Lower Old Red Sandstone times in which 300 feet of sediments, surmounted by basalt lavas, were deposited. Unconformable junctions of successively higher beds with the Easdale Slates were examined as the party proceeded westwards. On a promontory west of Gylen Castle a thick series of conglomerates (I of Fig. I6) rests upon folded black slates and limestones (Plate 5, B), and is succeeded by a thick series of dark red sandstones (2 of Fig. I6). Westwards, the conglomerates thin out and the sandstones, containing bands of conglomerate, come to rest directly upon the slates. At the south-west point of the island the sandstones in turn appear to thin out so that a higher conglomerate (3 of Fig. I6) is there found above the unconformity. At a junction of the sandstones (2) and underlying conglomerates in the bay east of Ardmore Farm, about 20 feet of grey flaggy shales have yielded to the Geological Survey collectors' characteristic Lower Old Red Sandstone fossils (Ceplialaspis

56

SUMMARY OF GEOLOGY OF ARDNAMURCHAN.

lornensis Traq. and Kantpecaris obanensis Peach). During a brief search fossil fragments were obtained by Miss Dingwall from a bed of purple compact shale near the base of the group. Lavas are probably interbedded in the upper group of conglomerates (3). One of these, composed of biotite-andesite, was examined at the south-west point of Kerrera. Overlying sandstones (4) were seen to the north along the western coast. These are succeeded by the upper suite of lavas that outcrop so extensively in the Lome volcanic plateau. A lower suite, from which the lava-pebbles of the conglomerates of the ObanKerrera hollow were derived, outcrops from beneath the intervening sediments near Ardentallen, north of Loch Feochan, z! miles south of Kerrera Ferry. An eroded hollow in the sandstones (4) is filled with representatives of the upper Oban lavas half a mile north of the south-west point of Kerrera. The party halted near the base of sea-cliffs formed of columnar jointed lava, and observed in sectional view the southern side of the old hollow with the beds of sandstone evidently cut off against the lava-infilling. Dr. Raw found that slag was absent below an apparent junction of two columnar lavas in the cliff, but time did not permit of a complete re-examination of the section. A sill with well-chilled margins cutting the lavas was seen a short way farther north. The cliffs were then dimbed, and the steep ridges and valleys of Kerrera crossed to the Ferry House, where tea was served, ACKNOWLEDGMENTS. To all those who contributed to the success of the meeting the thanks of the Association are accorded. To Mr. Fletcher, factor for the Ardnamurchan estate, we are much indebted for permission to traverse Ben Hiant and other deer-forest areas, and to Mr. Cameron and Mr. P. MacLellan, Kilchoan, for general arrangements in connection with the Ardnamurchan visit. The Association gratefully expresses its thanks to Colonel and Mrs. MacDougall of MacDougall for contributing so much to the interest and enjoyment of the week-end spent at Oban, The information contained in this report is drawn in part from memoirs written by my colleagues on the Geological Survey. For this the writer desires to make full acknowledgment, more especially to Prof. E. B. Bailey and Dr. H. H. Thomas. To Prof. A. Holmes and Prof. H. H. Read for correcting this report in MS., and to Miss D. Reynolds for the photograph reproduced in Plate I, B. the writer offers his cordial thanks. Permission to publish illustrations from Geological Survey Memoirs has been granted by the Controller, H.M. Stationery Office. We are also indebted to the Council of the Geological Society of Glasgow for blocks of Figs. I and 8.

PROC. GEOL.

Assoc.,

VOL.

XLIV. (1933).

PLATE

5.

[Piloto by H.M . Geol. Su rvey. U NCONF O RMIT Y OF CONGLOMERATE OF L OW E R OLD R E D SANDSTONE OK S LAT ES AND L IM ESTONES. SO UT H EN D OF K E RR E RA.

EASD ALE

A.-At Gy len Cast le, with 25-ft . rai sed -beach platform, cut in cong romerat es,

[I' iloto by H.M . Geol. Survey.

B.-AT

HEADLAND WEST OF CAs·rLE .

[B. from ' Pre-Tert iary Geology o f Mull,' etc. , A/em . Geol, Surv., 1925. Plate I. J I.] [To I Me

p. 56.

PLATE 6.

PROC. GEOL. Assoc., VOL. XUV. (1933) ,11call M eadhoin.

M cull

~

a ll

Turma chain , ~

__

- - _..

Creal]

all

B cin n nil S eilg, ~

A i rgi d,

.v

Bcinn

1/U

h 'Lm cilt e. ~

i11call S ann a .

M eall Clacb

t

a ll

Da ra ich:

~

.._----_._----------------------------------------------------------------------~

/: (

c,

c:

l ~ \.. - :.- - (.- _ ._- - - -

--'

... .

<,

.--;.~~-

------

3, ARDNAMURCHAN, FROM NORTH.EAST, WITH MEALL AN TARMACHAIN AND BElNN NA SEILG IN DISTANCE. Low inner ring surrounding central knob of Quartz-monzonite is the fluxion Biotite-gabbro of Glendrian. The distance from Mcall Meadhoin across the Interior Complex to Meall Sanna is three miles.

PANORAMA OF GREAT EUCRITE AND INTERIOR COMPLEX OF H.ING-DYKES OF CENTRE

Outer ring of hills and dark foreground mark the outcrop of the great eucrite.

(Reprolluced, by permission, from Ardnamurchan: Geol, Surv. Mem,

1930.)

[To face p. 56.

PROC. GEOL.

Assoc.,

VOL.

PLATE

XLIV. (1933).

7.

'·F.N T ROC.S

(~~=~r:l~::;~,,_) A

~,.;:::::: f' ltUII1MU iff"" _'.

Om Hid,.' .

LAYA S

-:-n -

RtU>1lt

MESOZO IC SliDI M IiS 1'S

III

SIGNS

. 'S'

-g' cD

g'

-G

"""r o-,, II1U!u9

'.

,,

'0'.

MAP OF

E

I

~

.,

COMPLEX OF ARDNAMURCHAN. Ardnatnurctuu:. Geoi, Suru, Mem«, '930.)

Y _. GNEOUS T' RTIAR of onuunent, from W~"til. udditiow

(Uepl'otluced

[To face p. 56.