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Summer Field Meeting in the Isle of Man
Summer Field Meeting in the Isle of Man 15-21 July 1965 Report by the Director: DR. A. SIMPSON Received 26 October 1965
PARTY OF fourteen members assembled at the Headquarters for the meeting, Peveril Hotel, Douglas, Isle of Man, during Thursday, 15 July. A pamphlet outlining the itinerary and summarising the geology of the Island was issued to the members by the Director. The main object of the Field Meeting was to examine the stratigraphy and tectonics of the Manx Slate Series which outcrop over three-quarters of the Island and to study the contained pre-tectonic and syn-tectonic basic and acid igneous intrusions. These aspects of Manx geology have been studied in recent works by the Director. The magnificent, continuous coastal sections provide an unrivalled display of Caledonian polyphase tectonics. Fj, F2 and Fa folding on all scales with accompanying cleavages and lineations are clearly displayed and the tectonic chronology can easily be demonstrated. The two Lower Carboniferous outliers around Castletown and Peel were also visited (Larnplugh, 1903; Lewis, 1930). Other incidental geological features included metalliferous lodes, normal and transcurrent faults, Tertiary dykes, glacial and post-glacial geology, etc. This was the first Field Meeting held in the Isle of Man by the Association. The weather remained perfect throughout the five days spent in the field.
THE
Friday, 16 July In the morning the lithology and tectonics of the Lonan Flags! were examined at selected localities on the east coast of the Island between Port'' Groudle (1)3 and Port Grenaugh (5). Along this stretch of coastline the geological structure is relatively simple and provides a convenient introduction to the polyphase tectonics of the Isle of Man. While the Fl structures are strongly developed, the later F2 and Fa deformation is only mild and sporadic. I The distribution of the various formations comprising the Manx Slate Series and the axial traces of the large-scale folds (F,. Fa, Fa) are shown in Fig. I. Further information concerning the stratigraphy and tectonics of the Manx Slate Series is given in Simpson (l963a). 2 'Port' is the Manx word for 'bay'. 3 The locality numbers are shown in Fig. I.
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After breakfast the party travelled by coach to Port Groudle (1), three miles north-east of Douglas. Leaving the coach by the side of the main road, the descent to the small bay below was made by a steep winding path. In the coastal cliffs the Lonan Flags, on the south-eastern limb of the Fr Douglas syncline, were seen to be vertical or steeply inclined to the north-west at 65-85°. The first axial-plane cleavage (81) preserved a steep, consistent dip to the south-east. Looking across the bay from a vantage point on its south side, acute FI folding, with gentle Caledonoid plunges, could be seen on the northern shoulder. In the same section occasional, more open, F2 flexures were also observed. The party followed a path for 250 yds. southward from Port Groudle along the top of the low cliffs, and examined the lithology of the Lonan Flags in the headland. The relationship of 81 to the steep bedding and the occurrence of small-scale current-bedding confirmed that the sequence youngs to the north-west. In this section a steeply inclined dyke, some 50 ft. thick and described by Lamplugh (1903, 152 & 302) as hornblende mica-trap, strikes north-west and intersects the cleaved Lonan Flags at right-angles. The marginal few feet of the intrusion are vesicular with irregular quartz blebs and xenoliths of the Lonan Flags. In places, the dark massive interior of the dyke contains veins and patches of pink acidic material. The dyke is clearly post-Fi in age. The severe 81 developed in the country rock does not penetrate the intrusion. Undeformed, wedge-shaped quartz veins penetrate into the flags from the dyke. The contact alteration at the margins of the intrusion is slight, the flags being merely hardened and rendered paler in colour. Returning to the coach, the party continued to Onchan Bay (2) on the outskirts of Douglas. On the northern side of this small bay, the hinge of the Fl Douglas syncline intersects the coast. From a dip of 45-60° to the southeast in the interior of the bay (on the north-western limb of the fold), the Lonan Flags curve (with some subsidiary undulation) round the synclinal hinge and become vertical on the south-eastern limb. In the hinge 81 is intensely developed and the bedding stripe plunges gently south-west. Abundant current-bedding in the flags demonstrated the upward-facing character of the fold. A small, sinistral transcurrent fault of Caledonian age runs towards the north-north-west into Onchan Bay. The actual fault-plane is concealed but its position is marked by a pebbly gully in the wave-cut platform. A few related fractures with horizontal displacements of a few feet were observed on the west side of the bay and a couple of small Fa flexures, plunging to the south-east down the dip of the Lonan Flags, were also noted. As the coach continued round Douglas promenade, the Director pointed out that the Onchan fault shifts the hinge of the Douglas syncline a short way out to sea, and across Douglas Bay (3) the Lonan Flags dip to the
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south-east on the north-western synclinal limb. The concealed hinge ultimately re-enters the coastline at the harbour on the south side of Douglas. The Director also showed that the coastal fringe of Douglas is built on a post-Glacial raised beach, the surface of which lies 10-12 ft. above high-water mark. The steep bluffs of the old sea-eliff behind are clearly visible though now largely built upon. The coach continued across the harbour to the top of Douglas Head (4) where the view north-eastwards across Douglas Bay was enjoyed by the party. The high cliffs of Lonan Flags are here, once again, on the southeastern limb of the Douglas syncline and the bedding is inclined steeply to the north-west. S1 retains its consistent, steep dip to the opposite southeastern quarter. F1 folding was observed in roadside exposures. Since the Marine Drive, which follows the precipitous coastline to the south-west from Douglas Head, is closed to heavy traffic, the coach continued southwards by the inland road (via Quine's Hill) and rejoined the coast again at Port Grenaugh (5). Leaving the coach, the party crossed the bay and followed a path for about 400 yds. to the south-west along the top of the cliffs. The country rock of Lonan Flags is somewhat thinnerbedded in this area than farther north. Innumerable pre-Fj greenstone bodies were observed in the coastal cliffs. These originally constituted a plexus of basic dykes and sills but have been much altered by the F1 chlorite-grade metamorphism (Simpson, 1964a) and disrupted or lenticularised by the F1 deformation. Where the descent was made to the coastal section, F1 cross-folding was clearly displayed (Simpson, 1965b). The folds plunge steeply to the south-eastern quarter more or less down the dip of the severe accompanying axial-planar S1. In one instance a greenstone sill was seen to be flexed by an F1 cross-fold and crudely cleaved by S1. Sporadic signs of the later F2 deformation were observed in the form of a weak, gently inclined second axial-plane cleavage (S2) and occasional small open F2 flexures superimposed on the F1 structures. In the same section a vertical 2-ft. olivine-dolerite dyke of Tertiary age, striking NW.-SE., cuts discordantly across all the pre-existing Caledonian structures. Although much deformed by F1 cross-folding, the Lonan Flags along this stretch of the east coast are primarily steeply inverted to the south-eastern quadrant on the north-western limb of the major F1 Dhoon anticline. Returning to the coach, the party proceeded to Port St. Mary in the south-western tip of the Island (Fig. 1). On the way the Director pointed out such features as the high ridge of resistant Barrule Slates forming the skyline to the north and the drift-filled depression between Port St. Mary and Port Erin which separates the Cregneish promontory from the rest of the Island. After lunch at Port St. Mary the coach drove up Mull Hill to the village
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of Cregneish where a large roadside quarry (6) was examined.' The Cregneish promontory is floored by the Maughold Banded Group which contains, in this localised part of the Island, two thick psammitic units termed the Mull Hill Quartzite and the Chasms Quartzite. The Cregneish quarry displays the top of the Mull Hill Quartzite with its intercalations of dark pelite. The bedding dips south-south-east at 350 and is rumpled by small F2 folds plunging to the east-north-east with an accompanying strong S2. Clearly superimposed on the F2 structures is a severe third axial-plane cleavage (Sa), dipping steeply east-north-east and occasional Fa folds which plunge down the dip of the bedding to the south-south-east. In one comer of the quarry Fi boudins with a Caledonoid trend occur in the quartzite and a thin greenstone sill was also examined. The party then walked southward along the path to the Chasms (7), where the high cliffs of white Chasms Quartzite were seen. The bedding retains its relatively gentle, right-way-up inclination to the south-south-east on the northern limb of the F 1 Spanish Head syncline. Part of the hinge of this major flexure was observed on the east side of Bay Stacka (7) though the inverted southern limb lies out to sea. Following a path westward towards Spanish Head (8), the Bay Stacka fault was noted in the cliff (Fig. 1). This normal dislocation, presumably Armorican in age, truncates the Cregneish promontory and, downthrowing to the east, terminates the outcrops of the two thick quartzite units. On the east side of Spanish Head the hinge of the F2 Mull Hill antiform flexes the Fl Spanish Head syncline through a right-angle so that farther west both its limbs and its axial plane dip to the north-north-west. The southern limb of the Fl syncline was examined in the spectacular cliffs of Spanish Head (8), dipping gently northward. The synclinal hinge is clearly exposed on the west side of the headland; the very strong Sl dips to the north-western quarter and shows sigmoidal flexing of F2 age. The party then continued along the cliff top to the Sound (9), over the northern synclinal limb. The latter, which also embraces the Calf of Man, is at first vertical and then assumes a steep north-western inversion. Just before the party rejoined the coach at the Sound (9), a north-south fault, containing fault-breccia and vein-stuff, was examined in a recess west of the small headland of Burroo Ned (176664). Throughout the Cregneish promontory the successive superimposition of folds, cleavages and lineations belonging to the Fi, F2 and Fa phases of Caledonian deformation was repeatedly observed. There was no difficulty in verifying the tectonic chronology. Saturday, 17 July
After breakfast the party returned to the south-western tip of the Island • Permission to examine this quarry, and other quarries at Dhoon (24) and Crosby (27), was kindly given by the Surveyor-General, Isle of Man Highway and Transport Board.
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and alighted from the coach at Rhenwyllan, three-quarters of a mile north-east of Port St. Mary. The broad wave-cut platform on the west side of Bay ny Carrickey, between Rhenwyllan and Gansey Point was traversed at low tide (10). This section consists of the Maughold Banded Group and the first half of the traverse lies within the eastern prolongation of the Mull Hill Quartzite. The overall inclination of the strata is to the south-east at a moderate angle on the right-way-up short limb of the Fi Cregneish foldpair. Small-scale folds, cleavages and linear structures belonging to the Flo F2 and Fa movement-phases are clearly displayed and the tectonic sequence was demonstrated by the Director. In addition, boudins plunging to the south-eastern quarter, more or less down the dip of the sequence, were attributed to the Fa phase by the Director. Such boudinage was observed in quartzite beds and also in a greenstone sill. The existence of small-scale Fa slides was noted. At the southern end of the traverse the nature of certain structures in thinly banded rock, whether sedimentary or tectonic, was discussed by members of the party. The party then proceeded by coach to Port Erin. From a vantage point on the southern headland of the bay, the Director outlined the large-scale structural features of this part of the Island and the relationship of the Port Erin section to the Cregneish promontory farther south. F2 quartz-veining (Simpson, 1963b), associated with minor F2 contortion, was examined in a neighbouring exposure. The coach then drove to the interior of the bay (II) and the excellent section of Maughold Banded Group was examined in the cliffs. Besides several normal and transcurrent faults, and the presence of pre-Fj greenstone intrusions, many structural features of Fi and F2 age were demonstrated. The latter included the refolding of acute Fl folds by more open F2 flexures and the presence of minor Fl slides. The sequence possesses an overall steep dip to the south-east on the northern limb of the Fr Port Erin anticline. The down-dip style of subsidiary Fl folding and the presence of small-scale current-bedding at one point confirmed the inversion of this fold-limb. The anticlinal hinge runs westward through the southern side of Port Erin Bay but is concealed by sand on the coast. After lunch at Port Erin the party followed the path through Bradda Glen along the northern headland. The position of the large-scale F2 Manx synformal hinge on the northern side of Port Erin Bay was pointed out by the Director, and the significance of this important flexure was outlined. On Bradda Head (12) the impressive north-south, and presumably Armorican, Bradda lode was examined. The party then climbed up to Bradda Tower and followed the clifftop until the junction of the Maughold Banded Group with the overlying Barrule Slates was reached (13). Much Fl folding was visible in the high cliffs, outlined by thicker psammitic bands PROC. GEOL. ASS., VOL. 77, PART 2, 1966
IS
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in the Maughold Banded Group. The Director pointed out that the sharp, almost knife-edge, junction of the latter formation with the uniform dark blue Barrule Slates contrasted with the transitional passage displayed elsewhere in the Island (e.g. in Port e Vullen, see below). Localised F1 sliding had effectively cut out the passage zone in this section. Returning to the coach at the entrance to Bradda Glen, the party drove northward to the crest of the Island formed by the prominent southeastern ridge of Barrule Slates. From the roadside (14) at Cronk Fedjag (240750) the Director demonstrated the surface expression of the fundamental geological structure in the Island, the F 1 Isle of Man syncline. The limbs of this fold are outlined by two NE.-SW. ridges of Barrule Slates. The party was standing on the south-eastern slaty belt while the reappearance of the same formation on the north-western synclinal limb formed the other, more subdued, ridge one mile away. The intervening belt of low ground was floored by the stratigraphically younger Injebreck Banded Group occupying the core of the syncline. The coach then proceeded downhill to the west coast at the prominent reef called the Niarbyl (15). The lithology of the Niarbyl Flags (=Lonan Flags) and the Maughold Banded Group were examined on the northwestern limb of the Isle of Man syncline. The junction between these two formations in this coastal section is marked by the most pronounced F1 slide in the Island, the Niarbyl slide (Fig. 1). The intense deformation associated with this dislocation was studied. Since the zone of tectonic lamination and boudinage is deformed by small-scale F2 and Fa structures, the slide is clearly F1 in age. The usual sequence of F1-Fa polyphase deformation was observed in the contiguous flags, banded group and greenstones. The return to Douglas was made via Peel and the central valley. En route the Director indicated the distribution of the various formations in the Manx Slate Series. Unfortunately, there was insufficient time at the end of the day to visit the Ballanayre Slump-breccia in the coastal cliffs three miles north-east of Peel. Sunday, 18 July In the morning the party proceeded by coach up the east coast to the small bay of Port e Vullen (16) in the north-eastern corner of the Island. Severe small-scale F1 folding, almost isoclinal in places, was examined in the dark blue Barrule Slates which were fissile along the accompanying Sl. Superimposed on these structures were F2 folds and the strain-slip S2. The section also contained a variety of other interesting features such as "deformed greenstone intrusions, F2 quartz-veining and small faults occupied by breccia and vein-stuff. Moving across the bay the incoming of
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beds of hard psammite provided a passage to the underlying Maughold Banded Group which forms the headland to the east (Fig. I). At this point the traverse was terminated. In these more competent beds larger acute Fr fold-pairs, displaying an up-dip style, were observed. The predominant dip of the bedding along this part of the Island's coastline is northward and the sequence is right-way-up on the south-eastem limb of the Fl Isle of Man syncline. The headland on the east side of Port e Vullen lies within the belt of F2 porphyroblasts (Simpson, 1964a), and the Maughold Banded Group is spotted by small pale cordierite blebs. Looking northward, the red extra-insular drift (Lamplugh, 1903) forming the cliffs between Ramsey and the Point of Ayre could be seen in the distance. The party then drove to the village of Maughold, one and a half miles farther east. Leaving the coach, a path was followed towards the lighthouse on the high cliffs of Maughold Head (Fig. I). The coastal section (17) was traversed between this point and Port Mooar, three-quarters of a mile to the south-west. The party examined the hinge of the large-scale F2 Manx synform which intersects the coast in the third recess south of Maughold Head. In this hinge the sedimentary pile is flexed through a right-angle. From the rightway-up, north-westerly disposition displayed farther north (e.g. at Port e Vullen) on the upper limb of the synform, the Maughold Banded Group passes through the vertical and becomes inverted to the south-east on the lower synformal limb. This overall inversion characterises the whole east coast of the Island. The accompanying rotation of the Fl fold-system from an up-dip style to a down-dip style was observed and the comparable rotation of SI was noted. In the hinge of the F2 flexure, mesoscopic F2 folding is particularly impressive and thick pods of F2 vein-quartz are frequently emplaced along the axial planes (Simpson, I963b). Throughout this coastal section the Fr structures plunge consistently to the south-west while the superimposed F2 system plunges to the east-north-east. The flatlying second axial-plane cleavage (S2)is everywhere very strongly developed and, forming broad pavements, is frequently the dominant plane of fissility. A short way south of the major F2 hinge, the inverted Maughold Banded Group gives way to the older, but structurally overlying, Lonan Flags. The passage beds between these two formations were examined. There was no shortage of current-bedding to show that the flags young to the north. On the north side of Port Mooar, the party crossed the hinge of the major FI Port Mooar anticline over which the rumpled Lonan Flags become essentially flat-lying. The coastal section between Maughold Head and Port Mooar lies within the belt of F2 porphyroblasts (Simpson, 1964a). Consequently, the strata are everywhere spotted by white lenses of leucoxene after ilmenite. The effect of this F2 metamorphism on the numerous greenstone intrusions was
224
A. SIMPSON
also noted. Several thin olivine-dolerite dykes of Tertiary age were seen running NW.-SE., cutting through the Caledonian deformation. After lunch in Port Mooar (Fig. 1), the party rejoined the coach and proceeded to the farm of Ballafayle, three-quarters of a mile farther southwest. The fields were then crossed to the headland of Gob ny Garvain (18) on the south side of Port Mooar. The right-way-up Lonan Flags, dipping gently south-east on the short limb of the FI Ballajora fold-pair (consisting of the Port Mooar anticline and the complementary Gob ny Garvain syncline), were examined. The superimposition of the F2 deformation on the FI fold-system was again observed. A steeply inclined sheet of porphyritic microgranite, 11 ft. thick and emplaced along Sj, was seen cutting across the headland in an east-northeasterly direction. The intrusion displayed shearing parallel to its margins and S2 penetrated into it from the country rock. The syn-Fs age of such acid sheets (Simpson, 1964b) was commented on by the Director. At one point the microgranite is offset II ft. in a sinistral sense by a vertical transcurrent fault striking at 9°. The coastal cliffs were followed southward for about half a mile until the hinge of the major FI Gob ny Garvain syncline was reached (Fig. 1). In this hinge the Lonan Flags were seen to be compressed into striking concertina-style folding with orthorhombic symmetry. Farther south the upper limb of the structure was observed to be steeply inverted to the south-east and crenulated by mesoscopic FI down-dip fold-pairs. In this limb a second thick syn-Fg microgranite sheet, emplaced along SI, was examined (Simpson, 1964b). The intrusion was rendered fissile by severe F2 shearing parallel to its margins. The party then returned to the coach which awaited by the roadside half a mile inland. The coach then drove through Ramsey (Fig. 1) and up the mountain road to the Bungalow (19) below Snaefell. En route the Director indicated the stratigraphic constitution of the surrounding terrain and pointed out the high ridge of Barrule Slates carrying such prominent summits as North Barrule (1860 ft.) and Snaefell (2034 ft.). The coach continued down the road leading north-westward to Sulby Glen. At the sharp corner overlooking the glen (374891) the party left the coach and followed a footpath southward to the Lhergyrhenny stream where the typical striped lithology of the Injebreck Banded Group was examined (20). Farther downstream, at the junction with the Sulby River, the Sulby Slump-breccia was studied in water-smoothed pavements (21). The origin of this formation by penecontemporaneous slumping was confirmed; its deformation by F2 and F3 folds, with the associated cleavages, was noted. Where the party rejoined the coach, convenient roadside crops (22) of the uniform, dark blue Slieau Managh Slates were hammered. The coach then continued to the foot of Sulby Glen where another roadside outcrop
SUMMER FIELD MEETING IN THE ISLE OF MAN
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(23) of Sulby Flags, high in the Manx sequence, was briefly inspected. At the entrance to Sulby Glen the coach turned left and proceeded through Ballaugh and Kirk Michael. This stretch of road follows the southern fringe of the low-lying northern plain which has a roughly triangular shape and stretches northward to the Point of Ayre (Fig. I). The Director pointed out that this plain is primarily composed of glacial drift, some 200-300 ft. thick and largely of extra-insular origin, which was deposited on the pre-Glacial sea-floor during the Pleistocene period (Lamplugh, 1903).The hills forming the slaty massif to the south terminate sharply in steep, northward-facing bluffs which overlook the drift-plain below. The striking change of slope clearly marks the position of the preGlacial coastline. The coach then returned to Douglas via St. John's and the central valley. Monday, 19 July The day's itinerary was concerned with the various Caledonian acid igneous intrusions emplaced in the Manx Slate Series. In the morning the party drove northward from Douglas to the Dhoon granite. The intrusion was examined in a large quarry (24) situated a quarter-mile east of the Douglas-Ramsey road. The Director demonstrated the deformation displayed by the granite which indicated a syn-tectonic emplacement near the beginning of the F2 episode (Simpson, 1964b). The coach then carried the party, via Foxdale, to the south-western coast of the Island. Alighting at 227741 the party crossed the southern spur of Cronk ny Arrey Laa to the steep slope of Barrule Slates overlooking the sea (25). In a southward traverse a cluster of deformed Syn-F2 acid sheets, emplaced along the steep SI of the uniform pelite, was examined (Simpson, 1964b). The white or pinkish aspect of these discontinuous bodies contrasted sharply with the enclosing dark blue slate. Within the incompetent country rock, F2 shearing parallel to the intrusions' margins is especially severe. In some instances the pre-existing igneous fabric has been largely destroyed with the production of fissile acid phyllonite. The deformed acid sheets were also seen to be flexed by F2 minor folds, cut by S2 and injected by F2 quartz-veining. In one sheared intrusion small F3 folds were also noted. Returning by coach to Foxdale, the party had lunch and then examined the Foxdale granite in a working quarry" opened on the north-eastern fringe of the exposed intrusion (26). Specimens were collected, especially of a vein containing beryl which had recently been uncovered. The Director outlined the evidence for the syn-Fs emplacement of the intrusion and its sub-surface continuation to Archallagan, one and a half miles farther north-east (Simpson, 1965a). • Permission to examine this quarry was kindly given by the Directors, Manx Flux and Mica Co. Ltd., Foxdale.
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Continuing by coach, via Eairy and Garth, the party next alighted at Crosby in the central valley. Just south of the railway station the Crosby microgranite sheet is well exposed in a large quarry (27). The intrusion, about 30 ft. thick, is emplaced concordantly within inverted Lonan Flags dipping at 65° to the east-south-east. In the country rock of the foot- and hanging-walls the three cleavages Sl, S2 and Sa are successively superimposed on one another. The Director pointed out that the deformation which characterises the swarm of syn-Fg acid sheets elsewhere in the Island was absent in this intrusion. Moreover, while the latter bodies produced virtually no contact alteration at their margins, the Crosby sheet was flanked by an obvious development of thermal biotite in the contiguous flags. The intrusion was in the course of examination and would be described in a later paper. The available evidence, however, indicated that this microgranite was emplaced at a somewhat later date in the Caledonian tectonic chronology than the other acid sheets in the Island, namely during the F2-Fa static interval. The porphyroblastic biotite in the flanking hornfels overgrew S2 but appeared to be deformed by Sa. The last stop of the day was made at the small Oatlands composite intrusion (28) in the south-east of the Island. This oval body possesses an outer ring of gabbro and a central core of alkali-granite (Taylor & Gamba, 1933). The gabbro is nowadays very poorly exposed. The party, however, examined the central granite in a large, flooded quarry. Steeply inclined shearing was observed here and there, similar in many respects to the F2 deformation displayed by the larger Dhoon granite. This point, however, required further investigation. On the north-western side of the intrusion the baked Lonan Flags at the contact were studied. The party then returned to Douglas. Tuesday, 20 July During this last day in the field, attention was concentrated on the two outlying patches of Lower Carboniferous around Castletown in the southeast of the Island (Lamplugh, 1903, 187; Lewis, 1930) and at Peel on the west coast (Lamplugh, 1903, 263). In the morning the party drove south-westward to Port St. Mary. On the east side of Perwick Bay (29) the boundary fault separating the Lower Carboniferous from the Manx Slate Series was observed. The lithology of the limestone was studied. The coach then continued to the north-east, round Bay ny Carrickey, as far as Balladoole (30). Leaving the coach the party followed the coastal section. The stratigraphy ofthe Visean sequence, consisting primarily of limestone, was investigated and fossils were collected. The disturbance along the Balladoole fault, and the associated dolomitisation of the contiguous limestone, were examined. Numerous
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olivine-dolerite dykes of Tertiary age, trending NW.-SE., were seen cutting the Lower Carboniferous. A short way south of Poyllvaaish the junction of the Scarlett Volcanic Series, the highest exposed unit in the Carboniferous, with the underlying limestone sequence (Posidonomya Beds) was well exposed (31). Continuing round the coastal section towards Scarlett Point (32), the tuff and agglomerate of the volcanic pile were examined. Occasional sheets of vesicular basalt were also noted. The coach was rejoined at Scarlett Farm and the party proceeded into Castletown for lunch. After lunch the party drove to Derbyhaven (33), a mile farther east. Leaving the coach, a footpath was followed southward along the west side of the peninsula of Langness. Around the southern tip of Langness (34) the striking unconformity between the flat-lying basal conglomerate of the Lower Carboniferous and the steeply inclined, much deformed Manx Slate Series was seen. The party then returned to the coach and drove across the Island to Peel, where the red Peel Sandstone was examined in a crag at the eastern end of the promenade (35). In the evening Professor Scott Simpson proposed a vote of thanks for Dr. A. Simpson who had directed the meeting throughout the five days in the field. Dr. Simpson thanked the Professor for his kind remarks. He also moved a vote of thanks for Mr. B. Ainsley, Secretary for Field Meetings, who had arranged the accommodation for the party and dealt with the correspondence. On Wednesday, 21 July, some of the party members returned to the mainland by sea or by air. The remainder stayed on the Island for a few days' vacation.
REFERENCES
LAMPLUGH, G. W. 1903. The Geology of the Isle of Man. Mem. geol. Surv. U.K. LEWIS, H. P. 1930. The Avonian Succession in the South of the Isle of Man. Q. Jl geol. Soc. Lond., 86, 234-88. SIMPSON, A. 1963a. The Stratigraphy and Tectonics of the Manx Slate Series, Isle of Man. Q. Jl geol. Soc. Lond., 119, 367-400. - - - . 1963b. F2 Quartz Veining in the Manx Slate Series. Nature, 199, 900-1. - - - . 1964a. The Metamorphism of the Manx Slate Series, Isle of Man. Geol. Mag., 101, 20-36. - - - . 1964b. Deformed Acid Intrusions in the Manx Slate Series, Isle of Man. Geological Journal, 4, 189-206. - - - . 1965a. The Syntectonic Foxdale-Archallagan Granite and its Metamorphic Aureole, Isle of Man. Geological Journal, 4, 415-34. - - - . 1965b. Fr Cross-folding in the Manx Slate Series, Isle of Man. Geol. Mag., 102,440-4. TAYLOR, J. H. & E. A. GAMBA. 1933. The Oatland (LO.M.) Igneous Complex. Proc, Geol. Ass., 44, 355-76.
A. Simpson Geology Department Birkbeck College Malet Street, W.C.I
PARTY OF fourteen members assembled at the Headquarters for the meeting, Peveril Hotel, Douglas, Isle of Man, during Thursday, 15 July. A pamphlet outlining the itinerary and summarising the geology of the Island was issued to the members by the Director. The main object of the Field Meeting was to examine the stratigraphy and tectonics of the Manx Slate Series which outcrop over three-quarters of the Island and to study the contained pre-tectonic and syn-tectonic basic and acid igneous intrusions. These aspects of Manx geology have been studied in recent works by the Director. The magnificent, continuous coastal sections provide an unrivalled display of Caledonian polyphase tectonics. Fj, F2 and Fa folding on all scales with accompanying cleavages and lineations are clearly displayed and the tectonic chronology can easily be demonstrated. The two Lower Carboniferous outliers around Castletown and Peel were also visited (Larnplugh, 1903; Lewis, 1930). Other incidental geological features included metalliferous lodes, normal and transcurrent faults, Tertiary dykes, glacial and post-glacial geology, etc. This was the first Field Meeting held in the Isle of Man by the Association. The weather remained perfect throughout the five days spent in the field.
THE
Friday, 16 July In the morning the lithology and tectonics of the Lonan Flags! were examined at selected localities on the east coast of the Island between Port'' Groudle (1)3 and Port Grenaugh (5). Along this stretch of coastline the geological structure is relatively simple and provides a convenient introduction to the polyphase tectonics of the Isle of Man. While the Fl structures are strongly developed, the later F2 and Fa deformation is only mild and sporadic. I The distribution of the various formations comprising the Manx Slate Series and the axial traces of the large-scale folds (F,. Fa, Fa) are shown in Fig. I. Further information concerning the stratigraphy and tectonics of the Manx Slate Series is given in Simpson (l963a). 2 'Port' is the Manx word for 'bay'. 3 The locality numbers are shown in Fig. I.
217
218
A. SIMPSON
After breakfast the party travelled by coach to Port Groudle (1), three miles north-east of Douglas. Leaving the coach by the side of the main road, the descent to the small bay below was made by a steep winding path. In the coastal cliffs the Lonan Flags, on the south-eastern limb of the Fr Douglas syncline, were seen to be vertical or steeply inclined to the north-west at 65-85°. The first axial-plane cleavage (81) preserved a steep, consistent dip to the south-east. Looking across the bay from a vantage point on its south side, acute FI folding, with gentle Caledonoid plunges, could be seen on the northern shoulder. In the same section occasional, more open, F2 flexures were also observed. The party followed a path for 250 yds. southward from Port Groudle along the top of the low cliffs, and examined the lithology of the Lonan Flags in the headland. The relationship of 81 to the steep bedding and the occurrence of small-scale current-bedding confirmed that the sequence youngs to the north-west. In this section a steeply inclined dyke, some 50 ft. thick and described by Lamplugh (1903, 152 & 302) as hornblende mica-trap, strikes north-west and intersects the cleaved Lonan Flags at right-angles. The marginal few feet of the intrusion are vesicular with irregular quartz blebs and xenoliths of the Lonan Flags. In places, the dark massive interior of the dyke contains veins and patches of pink acidic material. The dyke is clearly post-Fi in age. The severe 81 developed in the country rock does not penetrate the intrusion. Undeformed, wedge-shaped quartz veins penetrate into the flags from the dyke. The contact alteration at the margins of the intrusion is slight, the flags being merely hardened and rendered paler in colour. Returning to the coach, the party continued to Onchan Bay (2) on the outskirts of Douglas. On the northern side of this small bay, the hinge of the Fl Douglas syncline intersects the coast. From a dip of 45-60° to the southeast in the interior of the bay (on the north-western limb of the fold), the Lonan Flags curve (with some subsidiary undulation) round the synclinal hinge and become vertical on the south-eastern limb. In the hinge 81 is intensely developed and the bedding stripe plunges gently south-west. Abundant current-bedding in the flags demonstrated the upward-facing character of the fold. A small, sinistral transcurrent fault of Caledonian age runs towards the north-north-west into Onchan Bay. The actual fault-plane is concealed but its position is marked by a pebbly gully in the wave-cut platform. A few related fractures with horizontal displacements of a few feet were observed on the west side of the bay and a couple of small Fa flexures, plunging to the south-east down the dip of the Lonan Flags, were also noted. As the coach continued round Douglas promenade, the Director pointed out that the Onchan fault shifts the hinge of the Douglas syncline a short way out to sea, and across Douglas Bay (3) the Lonan Flags dip to the
SUMMER FIELD MEETING IN THE ISLE OF MAN
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south-east on the north-western synclinal limb. The concealed hinge ultimately re-enters the coastline at the harbour on the south side of Douglas. The Director also showed that the coastal fringe of Douglas is built on a post-Glacial raised beach, the surface of which lies 10-12 ft. above high-water mark. The steep bluffs of the old sea-eliff behind are clearly visible though now largely built upon. The coach continued across the harbour to the top of Douglas Head (4) where the view north-eastwards across Douglas Bay was enjoyed by the party. The high cliffs of Lonan Flags are here, once again, on the southeastern limb of the Douglas syncline and the bedding is inclined steeply to the north-west. S1 retains its consistent, steep dip to the opposite southeastern quarter. F1 folding was observed in roadside exposures. Since the Marine Drive, which follows the precipitous coastline to the south-west from Douglas Head, is closed to heavy traffic, the coach continued southwards by the inland road (via Quine's Hill) and rejoined the coast again at Port Grenaugh (5). Leaving the coach, the party crossed the bay and followed a path for about 400 yds. to the south-west along the top of the cliffs. The country rock of Lonan Flags is somewhat thinnerbedded in this area than farther north. Innumerable pre-Fj greenstone bodies were observed in the coastal cliffs. These originally constituted a plexus of basic dykes and sills but have been much altered by the F1 chlorite-grade metamorphism (Simpson, 1964a) and disrupted or lenticularised by the F1 deformation. Where the descent was made to the coastal section, F1 cross-folding was clearly displayed (Simpson, 1965b). The folds plunge steeply to the south-eastern quarter more or less down the dip of the severe accompanying axial-planar S1. In one instance a greenstone sill was seen to be flexed by an F1 cross-fold and crudely cleaved by S1. Sporadic signs of the later F2 deformation were observed in the form of a weak, gently inclined second axial-plane cleavage (S2) and occasional small open F2 flexures superimposed on the F1 structures. In the same section a vertical 2-ft. olivine-dolerite dyke of Tertiary age, striking NW.-SE., cuts discordantly across all the pre-existing Caledonian structures. Although much deformed by F1 cross-folding, the Lonan Flags along this stretch of the east coast are primarily steeply inverted to the south-eastern quadrant on the north-western limb of the major F1 Dhoon anticline. Returning to the coach, the party proceeded to Port St. Mary in the south-western tip of the Island (Fig. 1). On the way the Director pointed out such features as the high ridge of resistant Barrule Slates forming the skyline to the north and the drift-filled depression between Port St. Mary and Port Erin which separates the Cregneish promontory from the rest of the Island. After lunch at Port St. Mary the coach drove up Mull Hill to the village
220
A. SIMPSON
of Cregneish where a large roadside quarry (6) was examined.' The Cregneish promontory is floored by the Maughold Banded Group which contains, in this localised part of the Island, two thick psammitic units termed the Mull Hill Quartzite and the Chasms Quartzite. The Cregneish quarry displays the top of the Mull Hill Quartzite with its intercalations of dark pelite. The bedding dips south-south-east at 350 and is rumpled by small F2 folds plunging to the east-north-east with an accompanying strong S2. Clearly superimposed on the F2 structures is a severe third axial-plane cleavage (Sa), dipping steeply east-north-east and occasional Fa folds which plunge down the dip of the bedding to the south-south-east. In one comer of the quarry Fi boudins with a Caledonoid trend occur in the quartzite and a thin greenstone sill was also examined. The party then walked southward along the path to the Chasms (7), where the high cliffs of white Chasms Quartzite were seen. The bedding retains its relatively gentle, right-way-up inclination to the south-south-east on the northern limb of the F 1 Spanish Head syncline. Part of the hinge of this major flexure was observed on the east side of Bay Stacka (7) though the inverted southern limb lies out to sea. Following a path westward towards Spanish Head (8), the Bay Stacka fault was noted in the cliff (Fig. 1). This normal dislocation, presumably Armorican in age, truncates the Cregneish promontory and, downthrowing to the east, terminates the outcrops of the two thick quartzite units. On the east side of Spanish Head the hinge of the F2 Mull Hill antiform flexes the Fl Spanish Head syncline through a right-angle so that farther west both its limbs and its axial plane dip to the north-north-west. The southern limb of the Fl syncline was examined in the spectacular cliffs of Spanish Head (8), dipping gently northward. The synclinal hinge is clearly exposed on the west side of the headland; the very strong Sl dips to the north-western quarter and shows sigmoidal flexing of F2 age. The party then continued along the cliff top to the Sound (9), over the northern synclinal limb. The latter, which also embraces the Calf of Man, is at first vertical and then assumes a steep north-western inversion. Just before the party rejoined the coach at the Sound (9), a north-south fault, containing fault-breccia and vein-stuff, was examined in a recess west of the small headland of Burroo Ned (176664). Throughout the Cregneish promontory the successive superimposition of folds, cleavages and lineations belonging to the Fi, F2 and Fa phases of Caledonian deformation was repeatedly observed. There was no difficulty in verifying the tectonic chronology. Saturday, 17 July
After breakfast the party returned to the south-western tip of the Island • Permission to examine this quarry, and other quarries at Dhoon (24) and Crosby (27), was kindly given by the Surveyor-General, Isle of Man Highway and Transport Board.
SUMMER FIELD MEETING IN THE ISLE OF MAN
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and alighted from the coach at Rhenwyllan, three-quarters of a mile north-east of Port St. Mary. The broad wave-cut platform on the west side of Bay ny Carrickey, between Rhenwyllan and Gansey Point was traversed at low tide (10). This section consists of the Maughold Banded Group and the first half of the traverse lies within the eastern prolongation of the Mull Hill Quartzite. The overall inclination of the strata is to the south-east at a moderate angle on the right-way-up short limb of the Fi Cregneish foldpair. Small-scale folds, cleavages and linear structures belonging to the Flo F2 and Fa movement-phases are clearly displayed and the tectonic sequence was demonstrated by the Director. In addition, boudins plunging to the south-eastern quarter, more or less down the dip of the sequence, were attributed to the Fa phase by the Director. Such boudinage was observed in quartzite beds and also in a greenstone sill. The existence of small-scale Fa slides was noted. At the southern end of the traverse the nature of certain structures in thinly banded rock, whether sedimentary or tectonic, was discussed by members of the party. The party then proceeded by coach to Port Erin. From a vantage point on the southern headland of the bay, the Director outlined the large-scale structural features of this part of the Island and the relationship of the Port Erin section to the Cregneish promontory farther south. F2 quartz-veining (Simpson, 1963b), associated with minor F2 contortion, was examined in a neighbouring exposure. The coach then drove to the interior of the bay (II) and the excellent section of Maughold Banded Group was examined in the cliffs. Besides several normal and transcurrent faults, and the presence of pre-Fj greenstone intrusions, many structural features of Fi and F2 age were demonstrated. The latter included the refolding of acute Fl folds by more open F2 flexures and the presence of minor Fl slides. The sequence possesses an overall steep dip to the south-east on the northern limb of the Fr Port Erin anticline. The down-dip style of subsidiary Fl folding and the presence of small-scale current-bedding at one point confirmed the inversion of this fold-limb. The anticlinal hinge runs westward through the southern side of Port Erin Bay but is concealed by sand on the coast. After lunch at Port Erin the party followed the path through Bradda Glen along the northern headland. The position of the large-scale F2 Manx synformal hinge on the northern side of Port Erin Bay was pointed out by the Director, and the significance of this important flexure was outlined. On Bradda Head (12) the impressive north-south, and presumably Armorican, Bradda lode was examined. The party then climbed up to Bradda Tower and followed the clifftop until the junction of the Maughold Banded Group with the overlying Barrule Slates was reached (13). Much Fl folding was visible in the high cliffs, outlined by thicker psammitic bands PROC. GEOL. ASS., VOL. 77, PART 2, 1966
IS
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A. SIMPSON
in the Maughold Banded Group. The Director pointed out that the sharp, almost knife-edge, junction of the latter formation with the uniform dark blue Barrule Slates contrasted with the transitional passage displayed elsewhere in the Island (e.g. in Port e Vullen, see below). Localised F1 sliding had effectively cut out the passage zone in this section. Returning to the coach at the entrance to Bradda Glen, the party drove northward to the crest of the Island formed by the prominent southeastern ridge of Barrule Slates. From the roadside (14) at Cronk Fedjag (240750) the Director demonstrated the surface expression of the fundamental geological structure in the Island, the F 1 Isle of Man syncline. The limbs of this fold are outlined by two NE.-SW. ridges of Barrule Slates. The party was standing on the south-eastern slaty belt while the reappearance of the same formation on the north-western synclinal limb formed the other, more subdued, ridge one mile away. The intervening belt of low ground was floored by the stratigraphically younger Injebreck Banded Group occupying the core of the syncline. The coach then proceeded downhill to the west coast at the prominent reef called the Niarbyl (15). The lithology of the Niarbyl Flags (=Lonan Flags) and the Maughold Banded Group were examined on the northwestern limb of the Isle of Man syncline. The junction between these two formations in this coastal section is marked by the most pronounced F1 slide in the Island, the Niarbyl slide (Fig. 1). The intense deformation associated with this dislocation was studied. Since the zone of tectonic lamination and boudinage is deformed by small-scale F2 and Fa structures, the slide is clearly F1 in age. The usual sequence of F1-Fa polyphase deformation was observed in the contiguous flags, banded group and greenstones. The return to Douglas was made via Peel and the central valley. En route the Director indicated the distribution of the various formations in the Manx Slate Series. Unfortunately, there was insufficient time at the end of the day to visit the Ballanayre Slump-breccia in the coastal cliffs three miles north-east of Peel. Sunday, 18 July In the morning the party proceeded by coach up the east coast to the small bay of Port e Vullen (16) in the north-eastern corner of the Island. Severe small-scale F1 folding, almost isoclinal in places, was examined in the dark blue Barrule Slates which were fissile along the accompanying Sl. Superimposed on these structures were F2 folds and the strain-slip S2. The section also contained a variety of other interesting features such as "deformed greenstone intrusions, F2 quartz-veining and small faults occupied by breccia and vein-stuff. Moving across the bay the incoming of
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beds of hard psammite provided a passage to the underlying Maughold Banded Group which forms the headland to the east (Fig. I). At this point the traverse was terminated. In these more competent beds larger acute Fr fold-pairs, displaying an up-dip style, were observed. The predominant dip of the bedding along this part of the Island's coastline is northward and the sequence is right-way-up on the south-eastem limb of the Fl Isle of Man syncline. The headland on the east side of Port e Vullen lies within the belt of F2 porphyroblasts (Simpson, 1964a), and the Maughold Banded Group is spotted by small pale cordierite blebs. Looking northward, the red extra-insular drift (Lamplugh, 1903) forming the cliffs between Ramsey and the Point of Ayre could be seen in the distance. The party then drove to the village of Maughold, one and a half miles farther east. Leaving the coach, a path was followed towards the lighthouse on the high cliffs of Maughold Head (Fig. I). The coastal section (17) was traversed between this point and Port Mooar, three-quarters of a mile to the south-west. The party examined the hinge of the large-scale F2 Manx synform which intersects the coast in the third recess south of Maughold Head. In this hinge the sedimentary pile is flexed through a right-angle. From the rightway-up, north-westerly disposition displayed farther north (e.g. at Port e Vullen) on the upper limb of the synform, the Maughold Banded Group passes through the vertical and becomes inverted to the south-east on the lower synformal limb. This overall inversion characterises the whole east coast of the Island. The accompanying rotation of the Fl fold-system from an up-dip style to a down-dip style was observed and the comparable rotation of SI was noted. In the hinge of the F2 flexure, mesoscopic F2 folding is particularly impressive and thick pods of F2 vein-quartz are frequently emplaced along the axial planes (Simpson, I963b). Throughout this coastal section the Fr structures plunge consistently to the south-west while the superimposed F2 system plunges to the east-north-east. The flatlying second axial-plane cleavage (S2)is everywhere very strongly developed and, forming broad pavements, is frequently the dominant plane of fissility. A short way south of the major F2 hinge, the inverted Maughold Banded Group gives way to the older, but structurally overlying, Lonan Flags. The passage beds between these two formations were examined. There was no shortage of current-bedding to show that the flags young to the north. On the north side of Port Mooar, the party crossed the hinge of the major FI Port Mooar anticline over which the rumpled Lonan Flags become essentially flat-lying. The coastal section between Maughold Head and Port Mooar lies within the belt of F2 porphyroblasts (Simpson, 1964a). Consequently, the strata are everywhere spotted by white lenses of leucoxene after ilmenite. The effect of this F2 metamorphism on the numerous greenstone intrusions was
224
A. SIMPSON
also noted. Several thin olivine-dolerite dykes of Tertiary age were seen running NW.-SE., cutting through the Caledonian deformation. After lunch in Port Mooar (Fig. 1), the party rejoined the coach and proceeded to the farm of Ballafayle, three-quarters of a mile farther southwest. The fields were then crossed to the headland of Gob ny Garvain (18) on the south side of Port Mooar. The right-way-up Lonan Flags, dipping gently south-east on the short limb of the FI Ballajora fold-pair (consisting of the Port Mooar anticline and the complementary Gob ny Garvain syncline), were examined. The superimposition of the F2 deformation on the FI fold-system was again observed. A steeply inclined sheet of porphyritic microgranite, 11 ft. thick and emplaced along Sj, was seen cutting across the headland in an east-northeasterly direction. The intrusion displayed shearing parallel to its margins and S2 penetrated into it from the country rock. The syn-Fs age of such acid sheets (Simpson, 1964b) was commented on by the Director. At one point the microgranite is offset II ft. in a sinistral sense by a vertical transcurrent fault striking at 9°. The coastal cliffs were followed southward for about half a mile until the hinge of the major FI Gob ny Garvain syncline was reached (Fig. 1). In this hinge the Lonan Flags were seen to be compressed into striking concertina-style folding with orthorhombic symmetry. Farther south the upper limb of the structure was observed to be steeply inverted to the south-east and crenulated by mesoscopic FI down-dip fold-pairs. In this limb a second thick syn-Fg microgranite sheet, emplaced along SI, was examined (Simpson, 1964b). The intrusion was rendered fissile by severe F2 shearing parallel to its margins. The party then returned to the coach which awaited by the roadside half a mile inland. The coach then drove through Ramsey (Fig. 1) and up the mountain road to the Bungalow (19) below Snaefell. En route the Director indicated the stratigraphic constitution of the surrounding terrain and pointed out the high ridge of Barrule Slates carrying such prominent summits as North Barrule (1860 ft.) and Snaefell (2034 ft.). The coach continued down the road leading north-westward to Sulby Glen. At the sharp corner overlooking the glen (374891) the party left the coach and followed a footpath southward to the Lhergyrhenny stream where the typical striped lithology of the Injebreck Banded Group was examined (20). Farther downstream, at the junction with the Sulby River, the Sulby Slump-breccia was studied in water-smoothed pavements (21). The origin of this formation by penecontemporaneous slumping was confirmed; its deformation by F2 and F3 folds, with the associated cleavages, was noted. Where the party rejoined the coach, convenient roadside crops (22) of the uniform, dark blue Slieau Managh Slates were hammered. The coach then continued to the foot of Sulby Glen where another roadside outcrop
SUMMER FIELD MEETING IN THE ISLE OF MAN
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(23) of Sulby Flags, high in the Manx sequence, was briefly inspected. At the entrance to Sulby Glen the coach turned left and proceeded through Ballaugh and Kirk Michael. This stretch of road follows the southern fringe of the low-lying northern plain which has a roughly triangular shape and stretches northward to the Point of Ayre (Fig. I). The Director pointed out that this plain is primarily composed of glacial drift, some 200-300 ft. thick and largely of extra-insular origin, which was deposited on the pre-Glacial sea-floor during the Pleistocene period (Lamplugh, 1903).The hills forming the slaty massif to the south terminate sharply in steep, northward-facing bluffs which overlook the drift-plain below. The striking change of slope clearly marks the position of the preGlacial coastline. The coach then returned to Douglas via St. John's and the central valley. Monday, 19 July The day's itinerary was concerned with the various Caledonian acid igneous intrusions emplaced in the Manx Slate Series. In the morning the party drove northward from Douglas to the Dhoon granite. The intrusion was examined in a large quarry (24) situated a quarter-mile east of the Douglas-Ramsey road. The Director demonstrated the deformation displayed by the granite which indicated a syn-tectonic emplacement near the beginning of the F2 episode (Simpson, 1964b). The coach then carried the party, via Foxdale, to the south-western coast of the Island. Alighting at 227741 the party crossed the southern spur of Cronk ny Arrey Laa to the steep slope of Barrule Slates overlooking the sea (25). In a southward traverse a cluster of deformed Syn-F2 acid sheets, emplaced along the steep SI of the uniform pelite, was examined (Simpson, 1964b). The white or pinkish aspect of these discontinuous bodies contrasted sharply with the enclosing dark blue slate. Within the incompetent country rock, F2 shearing parallel to the intrusions' margins is especially severe. In some instances the pre-existing igneous fabric has been largely destroyed with the production of fissile acid phyllonite. The deformed acid sheets were also seen to be flexed by F2 minor folds, cut by S2 and injected by F2 quartz-veining. In one sheared intrusion small F3 folds were also noted. Returning by coach to Foxdale, the party had lunch and then examined the Foxdale granite in a working quarry" opened on the north-eastern fringe of the exposed intrusion (26). Specimens were collected, especially of a vein containing beryl which had recently been uncovered. The Director outlined the evidence for the syn-Fs emplacement of the intrusion and its sub-surface continuation to Archallagan, one and a half miles farther north-east (Simpson, 1965a). • Permission to examine this quarry was kindly given by the Directors, Manx Flux and Mica Co. Ltd., Foxdale.
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Continuing by coach, via Eairy and Garth, the party next alighted at Crosby in the central valley. Just south of the railway station the Crosby microgranite sheet is well exposed in a large quarry (27). The intrusion, about 30 ft. thick, is emplaced concordantly within inverted Lonan Flags dipping at 65° to the east-south-east. In the country rock of the foot- and hanging-walls the three cleavages Sl, S2 and Sa are successively superimposed on one another. The Director pointed out that the deformation which characterises the swarm of syn-Fg acid sheets elsewhere in the Island was absent in this intrusion. Moreover, while the latter bodies produced virtually no contact alteration at their margins, the Crosby sheet was flanked by an obvious development of thermal biotite in the contiguous flags. The intrusion was in the course of examination and would be described in a later paper. The available evidence, however, indicated that this microgranite was emplaced at a somewhat later date in the Caledonian tectonic chronology than the other acid sheets in the Island, namely during the F2-Fa static interval. The porphyroblastic biotite in the flanking hornfels overgrew S2 but appeared to be deformed by Sa. The last stop of the day was made at the small Oatlands composite intrusion (28) in the south-east of the Island. This oval body possesses an outer ring of gabbro and a central core of alkali-granite (Taylor & Gamba, 1933). The gabbro is nowadays very poorly exposed. The party, however, examined the central granite in a large, flooded quarry. Steeply inclined shearing was observed here and there, similar in many respects to the F2 deformation displayed by the larger Dhoon granite. This point, however, required further investigation. On the north-western side of the intrusion the baked Lonan Flags at the contact were studied. The party then returned to Douglas. Tuesday, 20 July During this last day in the field, attention was concentrated on the two outlying patches of Lower Carboniferous around Castletown in the southeast of the Island (Lamplugh, 1903, 187; Lewis, 1930) and at Peel on the west coast (Lamplugh, 1903, 263). In the morning the party drove south-westward to Port St. Mary. On the east side of Perwick Bay (29) the boundary fault separating the Lower Carboniferous from the Manx Slate Series was observed. The lithology of the limestone was studied. The coach then continued to the north-east, round Bay ny Carrickey, as far as Balladoole (30). Leaving the coach the party followed the coastal section. The stratigraphy ofthe Visean sequence, consisting primarily of limestone, was investigated and fossils were collected. The disturbance along the Balladoole fault, and the associated dolomitisation of the contiguous limestone, were examined. Numerous
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olivine-dolerite dykes of Tertiary age, trending NW.-SE., were seen cutting the Lower Carboniferous. A short way south of Poyllvaaish the junction of the Scarlett Volcanic Series, the highest exposed unit in the Carboniferous, with the underlying limestone sequence (Posidonomya Beds) was well exposed (31). Continuing round the coastal section towards Scarlett Point (32), the tuff and agglomerate of the volcanic pile were examined. Occasional sheets of vesicular basalt were also noted. The coach was rejoined at Scarlett Farm and the party proceeded into Castletown for lunch. After lunch the party drove to Derbyhaven (33), a mile farther east. Leaving the coach, a footpath was followed southward along the west side of the peninsula of Langness. Around the southern tip of Langness (34) the striking unconformity between the flat-lying basal conglomerate of the Lower Carboniferous and the steeply inclined, much deformed Manx Slate Series was seen. The party then returned to the coach and drove across the Island to Peel, where the red Peel Sandstone was examined in a crag at the eastern end of the promenade (35). In the evening Professor Scott Simpson proposed a vote of thanks for Dr. A. Simpson who had directed the meeting throughout the five days in the field. Dr. Simpson thanked the Professor for his kind remarks. He also moved a vote of thanks for Mr. B. Ainsley, Secretary for Field Meetings, who had arranged the accommodation for the party and dealt with the correspondence. On Wednesday, 21 July, some of the party members returned to the mainland by sea or by air. The remainder stayed on the Island for a few days' vacation.
REFERENCES
LAMPLUGH, G. W. 1903. The Geology of the Isle of Man. Mem. geol. Surv. U.K. LEWIS, H. P. 1930. The Avonian Succession in the South of the Isle of Man. Q. Jl geol. Soc. Lond., 86, 234-88. SIMPSON, A. 1963a. The Stratigraphy and Tectonics of the Manx Slate Series, Isle of Man. Q. Jl geol. Soc. Lond., 119, 367-400. - - - . 1963b. F2 Quartz Veining in the Manx Slate Series. Nature, 199, 900-1. - - - . 1964a. The Metamorphism of the Manx Slate Series, Isle of Man. Geol. Mag., 101, 20-36. - - - . 1964b. Deformed Acid Intrusions in the Manx Slate Series, Isle of Man. Geological Journal, 4, 189-206. - - - . 1965a. The Syntectonic Foxdale-Archallagan Granite and its Metamorphic Aureole, Isle of Man. Geological Journal, 4, 415-34. - - - . 1965b. Fr Cross-folding in the Manx Slate Series, Isle of Man. Geol. Mag., 102,440-4. TAYLOR, J. H. & E. A. GAMBA. 1933. The Oatland (LO.M.) Igneous Complex. Proc, Geol. Ass., 44, 355-76.
A. Simpson Geology Department Birkbeck College Malet Street, W.C.I