The triassic sandstones of Yorkshire and Durham

12 5

THE TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM: Their Petrography and their Relation to the Trias of other Parts of the British Isles. By FRANK

S~llTHSON,

Ph.D., F.G.S.

[Read January and, 1931.] [Received AP"il 30th, 1930.]

I. INTRODUCTION. H E present paper is a record of an investigation into the mineral constitution of the Triassic sandstones of Yorkshire and Durham. The investigation has largely taken the form of a determinative and quantitative examination of the heavy minerals, but the lighter minerals also have been studied in thin section and in samples of the crushed rocks. The area from which samples have been collected extends from Doncaster to the River Tees. For comparison, samples of the Millstone Grit and Coal Measures Sandstones and samples of the superficial deposits in the Triassic belt have also been examined. The region is one of extremely low relief and is very largely covered with Glacial and post-Glacial deposits of considerable thickness, and in only a few places do the Triassic rocks rise out of the drift-covered plain. The Trias of Yorkshire consists chiefly of soft sandstones in its lower part and of marls and clays in the upper part. Pebble beds are characteristic of the Triassic rocks of the Midlands, but on being traced northwards these deposits thin out and disappear in the south of Yorkshire. In the present paper attention will be directed chiefly to the sandstones. These are commonly red, sometimes yellow and occasionally mottled. The rock varies considerably in grade and hardness. In places it is hard enough to be used as a very inferior building stone, whilst in others it is little more than an unconsolidated sand. In some of the quarries, e.g, near Warlaby, the texture is fine and the soft sandstone breaks readily along bedding planes which are silvery with white mica. In other exposures, e.g., at Doncaster and Hambleton Haugh, the rock is coarse and has no well-defined direction of cleavage. Current-bedding is visible in many of the exposures, and on a small scale is seen in some of the thin sections. At Bilbrough, near York, small lenticles of buff coloured clay occur amongst the current-bedded sandstone, and in some samples there are what appear to be fragments of sun-dried clay. The Geological Survey divide the sandstone, placing part of it with the overlying marls and clays in the Keuper and the rest in the Bunter, which, in Yorkshire, is not further sub-divided.

T

126

F RANK SM IT H SO~ ,

The whole of the outcropping Tri as from th e neighbourhood of Bo rou ghbridge northwards is placed in the Keuper , and south of that place the Tri as belt is divided into two parts, Bunter t o the West an d Keuper (almost entirely concealed by dr ift) to the E ast. * Dr. R. L. Sherlock (1926) has recently shown th at there are strong reasons for believing that th e lithological classification of the Permo-Triassic rocks does not pro vide a true chronological classification, and that the Keuper of one region may correspond chronologically with th e Bunter, or even with th e Magnesian Limest one, of an other. The pr esent writer. whilst den oting samples as " Bunte r " and " Keuper " in agreement with the Geologica l Survey map s, fully recognises th e possibility of Dr. Sherlock's contention.

II. EXAMINATION OF THIN SECTIONS. The examination of the sandst one in t hin section has not been exhaust ive, being based on the st udy of only fourteen slides made from representative samples. The sa ndstones consist chiefly of qu artz grains t ogether with grains of certain ot her minerals, am ongst which orthoclase is the m ost ab undant. Th e ro ck is best describ ed as a poorly con solidated arkose. The qu artz gra ins vary considerab ly in size and shape, lar ge grains and small, rounded and an gular being found together in the same sect ion . In th e Bunter sand stones of Hambleton H au gh the av erage size of t he grains is about 0 .2 mm. or 0.25 mm. as seen in t hin sectio n , and the m axim um measurements are ab out 0.5 mm. The Keuper sandst ones are finer grained and in a num ber of secti ons none of th e grains ex ceed 0. 2 mm . When examined betwee n crossed n icols stra in shadows are frequently observed, whilst in some grains there is a st rong banding pr odu ced by strain. Th e gra ins of or thoclase ar e generally more an gular, their margins being determined by cleavage. The grains are usually clear and untwinned , although Carlsbad twinning has been observed . Microcline, although mu ch less abundant than orthoclase, is represented by a few gra ins in nearl y every slide. Plagioclase sho wing polysyntheti c twinning is sca rce , but certain cloud y grains of decomposed felspar may also be plagioclase. • T he geologic al map of th e British Isles on t he sca le of 25 mil es t o the in ch (I912) sh ows th e Bunt er ex t ending no farther n or th than B oro ughb ridge. T he one-in ch ma p , Shee t 62 (Drif t Edn. 1917) shows th e p robab le boundary of th e Bu n t er Sa ndst on e (Jr- g} and the Ke uper Sands to ne (is ) funning ne ar the vill ages of Grea t Ou sebur n , Gra ft on an d Minskip and convergin g t owards t he P ermia n -B un te r boundary as if t o mee t it ab ou t tw o and a ha lf m iles wes t of B OIOUg b bridg e. T he quarter- in ch ma ps, Sheets 4. a n d 7 (190 7). divide th e Trias in to fr -S (B un ter a nd Keuper, Sandsto nes toget her ) a nd 16 (Keupe r )larls). It may be me n t ione d that Mr. L tnsd all Ri ch ardson states (19 29) .. Nor t h of Ri pon t he line of demarcation be t ween 'B un ter an d Keuper has not be en determi ned ," whilst P ro fesso r P . f. K endall (19 17) has e xp ressed t he vi ew tha t th e B unt er exte-nds as fa r a s th e R iver T ee s. a

TRIASSIC SA:\DSTO:\ES OF YORKSHIRE AXD DURHAM.

127

The following heavy minerals have been observed in thin sections: muscovite, biotite, chlorite, iron-ores, tourmaline, apatite and zircon. As these minerals are best examined as grains in the heavy residues they will be described later. The aggregate grains may be divided into (a) those consisting of only one mineral, and (b) those consisting of more than one. The following are the more important types : (a) (i) Quartz. The grain is composed of elongated granules which show strong strain shadows and have crenulate margins. Probably from schist. (ii) Quartz. The grain consists of a fine mosaic of more or less equidimensional granules. (iii) Quartz. The grains consist of only a few individual parts with different optical orientations and irregular or interlocking margins. (iv) White mica. The grain consists of mica flakes roughly parallel to one another. Probably from schist. (b) (i) Quartz + white mica. The grain consists of granules of quartz with parallel flakes of white mica as in mica schist. (ii) Quartz + chlorite. Similar in structure to the above. (iii) Quartz + biotite. Similar in structure to the above, but less frequent. III.

EXAMINATION OF MINERAL GRAINS. Methods of Working. For the purpose of heavy liquid. separation the amount of material treated was usually between 10 and 20 ounces, but in the case of the Keuper sandstone from Aldborough and of the Bunter sandstone from Bilbrough samples of 4lbs. and 7lbs. respectively were employed. The samples were crushed so as to pass through a 4o-mesh sieve, washed, and then carefully panned and repanned several times until the accumulated residues amounted to a few grams. When the residue had been dried it was transferred to a Spaeth separator containing bromoform. The amount of heavy residue obtained varied considerably, even from samples of apparently similar material from the same quarry. The residue, or part of it, was next placed in a watch glass with a little benzol and examined under the microscope. Only after the residue had been thoroughly searched in this manner was a permanent mount made. Material left over from the permanent mount was employed in a number of ways:(a) For further working over and picking out grains of interest. (b) For treatment with a permanent, and an electromagnet, dividing the material usually into the following fractions:

FR .-\NK S:.\IITHSO); ,

(i) Magnetite fra ct ion ; (ii) Ilmenite fraction ; (iii) Tourmaline fraction ; (iv) Non-magnetic fra ction . Th e last fraction was furth er divided by again using th e Spaet h separato r, now filled with methylene iodide, an d so obtaining (iv .a) Apatite fra ction, lighter than th e separating liquid and (iv .b) Zir con fra cti on , den ser t ha n th e liquid. I t is important th at it was never found necessary to t reat samples with acid , and hen ce it was possible t o obtain residues containing an abund ance of apatite. It was the possibility of ob taining apatite-ri ch conce nt rates and of study ing in detail th e variet ies of this somewhat elus ive detrital m ineral t ha t dr ew th e writer 's at t ent ion t o t he York shire Trias. Previous Work.

The petrography of th e Triassic rocks of the British I sles has probably received m ore attention than that of an y ot her formation and yet no systema tic account of th e petrography of th e Yorkshire Trias has bee n published. Samples from Yorkshire have been exam ined previously by Professor P . G. H. Boswell, who has made referen ce to the occurrence of Monazite (1927) . Professor B oswell has kindly placed slides a t the writer 's disposal ; and, as four of th ese represent Bunter sam ples from South Yorkshire localities at which there now appear t o b e no exposures , it may be mentioned that the suite s shown agree closely with t he Bunter suites described in t he pr esent paper. List of Minerals. The following m ineral s have been found in the Tri assic sands tone s of York shire. Fluorite, Garnet , Magnetite, C UBI C P yrite, Spinel. T ETRAGOl\AL Anatase, Rutile, Zircon . H EXAGO N AL-RH ol\IBOHE V R AL Apatite , Calcit e, Dolomite, Haematite, Ilmenite, Quartz, Tourmaline. O RTH ORHOMBIC •. Barytes, Brookite, Orthorhombic pyro xene, St aurolite. M ON OCLINIC Biotite, Chlorit e, Muscovite, Monazite, Orthoclase, H omblende*. T RI CLINIC •• Microcline, Plagioclase. AM ORPHOUS OR DOC BTF UL Leucoxene, Lim onit e, Pinite. * P roiessor

B {) ~ \n: LI : S

obser vation .

TRIASSIC

J. 2.

3. 4.

5.

SA~DSTONES

OF YORKSHIRE AND DURHAM.

129

FIG. 10. KEY TO LOCALITIES. COATHA:\I STOB. Yellow sandstone exposure in disused whinstone working on the line of the Cleveland dyke, near Stockton-on-Tees. CROFT. Natural exposures .of red sandstone in the river bed both east and west of the road bridge crossing the Tees. HURWORTH. Natural exposure of red sandstone in the middle of the River Tees opposite the village. XEASHAM. A succession of natural exposures of red sandstone in the bed of the River Tees on the Durham side of the river and about a quarter of a mile below the village. OVER DJKSDALE. A natural exposure of argillaceous red sandstone in the bed of the Tees. Position shown by a dip arrow on oneinch Geo!. Surv , map, Sheet 33.

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A. Northern Portion. B. Southern Portion. FIG. 10 -KEY MAP TO LOCALITIES ON THE YORKSHIRE TRIAS. Dotted Area = Trias. Scale 12 miles = I inch. 6. 7. S. g. 10.

LEVEN BRIDGE. Natural exposure of red sandstone in the left bank of the River Leven below the bridge. HILL NORTH OF \VARLABY. Pit in soft red sandstone worked as moulding sand. GATENBY. Small pit in soft yellow sandstone, as marked on six-inch Geo!. Surv. map. CATTOK-ON-SWALE. Exposures of red sandstone in ditch and small pit marked on six-inch Geo!. Surv. map. :NEAR EIRKLIKGTOK. Red sandstone exposed in a cutting in a field about one mile south by east of Eirklington.

130

II. 12.

13. 14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

FRANK SMITHSON,

HUTTO::-l MOOR. Quarries in red and yellow sandstone. (A) East of Leeming Lane and (B) West of Leeming Lane. NEAR RIPON. Quarry in red and mottled sandstone to the left of the Thirsk road. NEAR COPT HEWICK. Small pit in soft red sandstone on left of road leading from Dishforth to Ripon. NEAR ALDBOROUGH. Quarry in red sandstone at crossing of the Aidborough-Minskip and Boroughbridge-Green Hammerton roads. GREAT OUSEBURN. Exposure of soft yellow sandstone in the village near Inn. BILBROUGH. Quarry in soft red sandstone visible from the YorkTadcaster road. HAMBLETON HAUGH. Cutting in the soft red sandstone near the summit. BRAYTON BARF. Small pits in soft red sandstone near road north of the hill. WHITLEY BRIDGE.* NEAR HE::-lSALL. Pits in soft red sandstone. (A) 'Vest of village. (B) South-west. (C) South-east. SNAITH* HECK.* ARMTHORPE.* DONCASTER. Exposures of soft red sandstone near the L. & N.E.R. main line. (A) East of the line near the Station. (B) West of the line at Balby Bridge.

Descriptions of Mineral Grains. Anatase has been found in nearly all the samples examined, but is never abundant. Nearly all the grains consist of euhedral rectangular plates, usually yellowish or whitish, but occasionally bluish in colour. Apatite (Plate 5 1-12) is present in almost all the samples examined and where it has not been found there are good reasons for believing that it has been removed after deposition. The frequency of the mineral varies considerably even amongst samples taken from the same quarry. It appears to be rather more abundant in the Keuper than in the Bunter. In the latter zircon is numerically more abundant than apatite in all the samples examined quantitatively, but in the Keuper apatite frequently equals or exceeds zircon. The largest apatite grain observed was found in the Bunter sandstone of Hambleton Haugh and measured 0.25 X 0.185 mm., whilst grains measuring from 0.15 to 0.2 mm. are not uncommon. Except in a few cases, where the grains have been partially dissolved or have been broken, they are beautifully rounded. Some still retain their prismatic habit, but many are almost circular in outline. Partially dissolved grains show ragged terminations (Plate 5 NO.3) or present irregular outlines. Inclusions of zircon are occasionally seen, and bubble holes occur in some of the grains. • Source-s of Professor

BOSWELL'S

samples.

TABLE SHOWING THE OCCURRENCES OF HEAVY MINERALS IN THE TRIASSIC SANDSTONES OF YORKSHIRE. For positions of localities see Fig. 10 and Key. t Altered to limonite. ~

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There are three interesting peculiarities to be observed in certai.n of the grains-(a) some of the grains are lifted by the electromagnet; (b) some exhibit pleochroism, sometimes the whole of the grain being pleochroic, sometimes only part of it ; (c) the grains from one locality show secondary growth. Characteristics (a) and (b) arc occasionally, but by no means always, observed in the same grain. The writer found that certain apatite grains in the Leinster Granite were lifted by the electromagnet when the poles were very close together, and gave reasons for believing that in these grains part of the CaO was replaced by FeO (1928). Dr. A. Brammall (1928) has observed that most of the apatite from the Dartmoor Granite behaves in a similar manner. Some of the grains from the electromagnetic separations from the Yorkshire Trias exhibit a series of fine dark lines parallel to the c-axis, and these on examination with a high magnification are seen to consist of very small opaque inclusions. When such grains are immersed in benzol and examined under a microscope whilst a steel needle is brought near to them they are seen to turn and move readily in response to the needle. The inclusions are evidently magnetic, and there is little doubt that they consist of magnetite. Other apatite grains in the electromagnetic separations are not marked with these fine lines and are not attracted by a steel needle. Coloured grains appear to be marc frequent amongst the apatite grains of the electro-magnetic separation than amongst those of the non-magnetic residue. The colour is sometimes distributed uniformly throughout the grain, especially in the case of the yellow ones, but sometimes it is patchy or confined to a more or less central core. The cores present an almost endless variety of types: some have well-defined boundaries, whilst others are ill-defined at their margins ; some are well defined on the margins parallel to the prism planes and fade away at the ends; some are thin and rod-like, and elongated parallel to the c. crystallographic axis, whilst others are stout and only a small part of the grain is uncoloured. In some cases there is a core within a core, and the outlines of the cores represent the crystal faces of the growth stages. In some samples very pale green apatite has been observed. Coloured grains form only a small proportion of the total number of apatite grains. In a count of II92 apatite grains from various localities only 18 were recorded as coloured or cored. The nature of the pleochroism of the apatite is indicated in the following table, whilst data for apatite grains from certain granites are included for comparison.

T RIASS IC

SA~ DSTO~ES

OF YORK SHIRE

F A S T RA Y

Hu rworth Do n caster B ilbrough, etc. Hurworth Doncaster Ca t t on

I

1

Gr ey

Blue-black

f Yellow Yellow -b rown

l Orange-b r own Black ish

Ross of 1\1uIl Gr a nit e

B la ck Alm ost bl a ck

Leinste r Gra nite

D yke in .N e wry G ramte (Cas tleweIla n )

{

Gre y Ind igo

D URH AM.

133

S L OW RA Y

(Ordin a ry), (E xtra ord inary ). i .e ., vib ra tion s i.e . . v ibrati ons perpend ic u la r to pa ra lle l to G . a x is. c. a xi s . , Vari ous sh a d es Bl a cki sh of sepia

O CCU R RE );C E S .

M ost lo cal it ies, es p ecia lly Ald bo r ough H a mbleton Bi lb ro ugh Do ncaster Ribble E stuar y H a mbleton

c\ ~ D

D I STR IBUTI O K

OF COLOU R ,

U su all y i n the for m of a co re .

P a t chy or un iform or sura ro u ndin g darker co re . P urplish brown Co re with d efini te, b u t rounded ou t line V er y pa le yellow U ni fo r m or y "llow p atchy, n ev er as a co re . Uniform or P al e y ell ow p atchy ; one co n tains a bl ack core . D ark sepia A s a core ,

Ve ry pale b r own

Bla ck Brown o r purlish brown Pale p urplish brown R eddi sh b rown

A s a core, A s a core . As a core with ill -defined bo un dar ies . Ill-d efin ed co res; s t reaks o f colo ur parallel to b as al p lane

Coloure d or dark-cored apatite grains appear t o be widesp read , if not ab undan t, in British igne ous and sedimen tary roc ks . A few years ago Dr. Fleet and the present writer collected a number of records of occurr ences (Fleet and Smithson 1928), an d drew atten ti on t o th e desirability of recording occur renc es of such grains. Through delay in the publication of Dr. W , Mackie's ex haustive survey of th e heavy min erals of the Scottish grani tes (1928) we were un able to include any Scottish localities. Dr. Mackie has found grains wit h dark cores (blac k, brownish and purplish) in the gran it es of Galloway and a number of ot her localities. In a pers onal comm unica t ion to the writers he state s that he has seen black- cored apatite gra ins and ot hers with cloudy purple or pink cores in Scottish sedimentary rocks. Dr. A. W. Groves and 1\1r. A. E. Mourant (1929) hav e found in igneous rocks of Normandy, Brittany and the Channel Islands, certain apatite grains with dark or coloured cores, which they attrib ute to the presence of minute particles of chlorite, biotite, haematite, manganite, etc . Some of these grains show pleochroism , which the aut hors believe is due t o th e inclusions , t he colour changes being from grey to a bluish or purplish t int , or to a greenish gre y or brown.

134

FRANK SMITHSON,

When dealing with the da rk cored apatite grains from Leinste r (1928) the wri ter had some t housands of grains at his disposal , and afte r chemical test s had failed to give any positi ve clue as t o the nature of the colouring matter, he was left with a strong suspicion that it migh t be ca rbo n. Dr. V. M. Gold schmidt , who kindly ex amined some of the Irish cored apat ite, ex presse d the view that the appearance of pleoch roism might be some kind of dispersion phenomen on , due t o t he presence of minut e include d particles. Accord ing to th is view it is not necessary that the included particles sho uld be pleoch roic, as any d ark particles would be capable of cau sing dispersion. Dark-cored ap atite ha s not been found in sufficient abu ndance in t he Y orkshire Trias to justif y chemical tests. The blackish and greyish cored grains st rongly resemble those from Leinster and R oss of Mull. The ye llow gr ains are of a distinctly different type and there appears t o be no record of such grains from British igneous rocks. Mr. F. J. Frazer (1927) has found simil ar apatite in sediments in Saskatchewan, Canada. The pleochroism in t his t ype seems more likely t o be an inherent property of the mi neral , for even under high magni fication the colouring of most of t hese gra ins is quite uniform . T he ap ati te grains from the yellow sands to ne (Keuper) at Coa tham Stab in Coun ty Durham , unlike those from any ot he r sam ples, show a secondary growth of apa tite. As the sam ples from Coatham Stob were collected a very sh ort dist ance (at most, a few yards) from the " Cleve land Dyk e " it is not unreasonable to connect the t wo ph en omen a , and t o attribute the secondary gr owth t o a mild me t amorphism . In many of t he grains the shape of the original ro unded detrital grain ca n be seen , some wh at cloudy and often crowded with cavit ies. The seconda ry apatite, which is in op tical con tinuity with the origina l grain, is clea r and free from cavit ies, and has ev iden tly restored the grain to as complete a crystal as the spaces available would perm it . Barytes is not common in any of t he Bunter sa mples, but in some of the K euper sa nds tones, especially along t he Tees, it is ver y abundant. Brookite has been found in both t he Bunter an d the Keuper, though it is an extremely rare miner al; but where a zircon concen trate has been p repared, a few grains of b rook it e always appear in the mount . The grains are of the cha racteristic striated tabular form, the larger ones attaining a maximum measurement of rather more than 0.2 mm. Many of the grains are well rounded, and there are strong reasons for suspecting that these are detrital, not authigenic.

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

135

Frequently feeble pleochroism is observed, the colour change being from dark yellow (for light vibrating parallel to the striations, i.e. to the c-axis) to pale yellow. The mineral has been recorded only where it could be verified by an interference figure. Dolomite and Calcite. Some of the Keuper samples contain a rhombohedral carbonate. Where the mineral occurs in the heavy residue from the bromoform separation it has been recorded as dolomite, but as a portion floats in a mixture of bromoform and benzol with a specific gravity of about 2.7 it appears that calcite is also present. Garnet (Plate 5 Nos. IS and 16) has been found in all the Bunter samples and in a number of Keuper samples. In general the grains of garnet are larger and more abundant in the Bunter, where they frequently attain a size of 0.3 mm. or more. In the coarser sandstones, especially at Doncaster, many of the grains are well rounded, whilst others are sub-angular. In the finer grained sandstones the garnet is more angular or more ragged in outline. The colour varies from pink to almost colourless. Dark inclusions similar to those in the staurolite and tourmaline are seen in some of the grains. Haematite as a colouring matter is widespread, but it has been recorded in the table only where actual grains of a reddish iron-ore have been founded in the heavy residue. Ilmenite is present in almost every sample examined, being usually the most abundant mineral in the heavy residues. Both rounded and sub-angular grains occur, the usual sizes being from 0.1 to 0.3 mm. The grains are usually fresh, but coatings of limonite and leucoxene, and outgrowths of anatase are observed. It is noteworthy that the yellow sandstones contain little or no ilmenite. Magnetite is present in most of the samples examined, but in the yellow sandstones it is either very scarce or quite absent. This fact suggests that the development of limonite in these yellow rocks is due in part to the alteration of magnetite. It is difficult from the examination of the mounted residues to form an opinion as to the relative abundance of this mineral, for it is so readly confused with ilmenite. * But when the residues were examined in a watch glass a magnet was oscillated beneath the stage of the microscope and the movement of the magnetite grains allowed their relative abundance to be roughly estimated. In nearly every case ilmenite was seen to be distinctly more abundant than magnetite. Among the grains of magnetite a small proportion are very perfectly rounded, but most are sub-angular. The majority are • Dr. W. F. FLEET (r927) records high percentages of magnetite in the Red Rocks of the Midlands, but, as he mentions, unaltered ilmenite is included with magnetite and only leucoxenic ilmenite is recorded as ilmenite. Mr. T. H. Burton (r9r7) records ilmenite as abundant in the Nottinghamshire Bunter and magnetite as frequent. PROC. GEOL.

Assoc.,

VOL.

XLII.,

PART 2, 1931.

10

FRANK SMITHSON,

nearly equidimensional, measurements ranging from about 0.04 to 0.4 mm., the average being about 0.10 to 0.15 mm. In only a few of the grains is the octahedral form recognisable. Mr. T. H. Burton (1917) has recorded that part of the magnetic separation from the Trias of Nottinghamshire has the appearance of pyrrhotite. One of the most abundant magnetic separations from the Yorkshire Trias was tested chemically but no sulphur was found. Monazite is recorded in a large number of samples, and its occurrence in others is suspected. In the Bunter sandstones and in some of the coarser sandstones of the Keuper there are grains large enough to show the characteristic absorption spectrum of Neodymium when an eye-piece spectroscope is employed. but grains of much less than 0.1 mm. in width show the absorption only faintly, and grains of much smaller dimensions are very difficult to distinguish from yellowish zircon. The largest monazite grain noticed was one measuring 0.19 X 0.14 mm. and occurring in the Bunter Sandstone of Doncaster. It is difficult to estimate the abundance of this mineral, but in some of the Bunter residues it may amount to as much as 0.5 per cent. of the non-opaque minerals. Most of the grains are well rounded. Inclusions of zircon and of an undetermined opaque mineral have been observed in some of the grains. Muscovite, Biotite and Chlorite are probably present in all the samples, but where panning is adopted they tend to be lost and are not recorded unless a special amount of the flaky minerals is prepared. The records in the table must, therefore, be regarded as incomplete. Muscovite and chlorite appear to be much more abundant than biotite. The chlorite is usually in the form of cleavage plates, as though an alteration product of biotite, but some grains present various non-platy forms. Orthoclase. A concentrate of this mineral extracted from the Bunter sandstone of Bilbrough shows both angular and rounded forms, the former being the more abundant. Many of the grains appear to consist of a more or less rounded grain on which secondary orthoclase has been deposited in optical continuity with the original orthoclase. At the correct focus the outline of the original grain can be seen, with clearer secondary felspar surrounding, or, where the growth of the grain has, apparently, been restricted, only partly surrounding it. A few grains may be entirely authigenic, similar to those described by Miss D. Reynolds (1929) from the Trias of Ulster and elsewhere; but in the present case most of the secondary orthoclase has been deposited on grains of detrital orthoclase.

TRI ASSI C SAN DSTONES OF Y ORKSHIRE AND DURHAM.

137

Pinite. Amongst the light minerals from the san dst one at Ri pon , Doncaster and elsewhere there occur certain grains which have the characters of pinite. When seen in air they ar e pale g reenish grey and have a somewhat waxy lustre. Wh en immersed in liquid or mounted in Canada balsam and examined bet ween crossed nicols t he gra ins are seen to consist of cryptocrys talline aggregates of a doubly refracting mineral. Pyrite has been foun d in only one sample, nam ely the red sa nds to ne from th e bed of th e River Tees at Croft, which also yields fl uor i te ; bu t as both t hese minerals are common in th e sand of th e Ri ver Tees even before it reaches th e T rias it is possible tha t their presence m ay be du e t o cont am ination of the samples. In th e yellow sandstones, however, especially at Hutton Moor an d Gatenby there occur sma ll cubes of limonite, which ar e evidently altered pyrite. Man y of the grains exhibit the striated faces characteristic of pyrite. In a few of the samples the se grains are extremely abundant and in one residue from Hutton Moor 95 per cent. of th e residu e consist s of well form ed cube s of lim onite. The pyrite in the first instance may hav e resulted from the alteration of ilmenite and m agnetite, as these minerals are abse nt or scarce in the samp les which contain limonite cubes . Pyroxenes and Hornblende. These are extremely scarce. A grain of an orthorhomb ic pyroxe ne (enst atite ?) has been obtai ned from th e Keu per at Ripon, while gra ins which may be augite have been observed in t he Keup er from Aldbo rough and Catton-on-Swale. H orn blende has been noted by Professor Boswell* from th e Bunter a t Whitley Bridge. Rutile is pre sent in every samp le examined, being always less abundan t than zircon or tourmaline. Th e largest grains a re about 0.3 mm. in length. Th e colour varies from reddish brown t o yellowish, gr ains of the form er colour being more ab undant. Spinel (Ceylonite) (Plate 5 Nos. 17-r 9) has been found in a min ority of th e samples examined. It occurs both in the Bu nter and in the Keu per , bu t not more than three or four grains have been foun d in anyone sample. The gra ins are more or less equidimensiona l-sizes of gra ins mea sured range from 0.138 X 0.08r mm. to 0.227 X 0.r6 5 mm .s-iand the gra ins ar e well ro unded. The colour is usually a rich green, but one bluishgreen grain was observed. In the permanent mount th ere is a little danger of confu sing grains of spinel with round ed plate s of chlorite, but in most cases the occur rence has been verified by examining and " rolling " them over in benzol before making the mount. • Personal communication.

FRANK

SMITHSON,

Staurolite (Plate 5 Nos. 13 and 14) is present at all the Bunter localities from which samples have been examined. At Hambleton Haugh staurolite is almost as abundant as tourmaline, whilst in all the Bunter samples which have been examined quantitatively it is more abundant than the garnet or rutile. In almost all the Keuper samples staurolite was scarce or absent. The largest grain noted measured 0.35 X 0.25 mm. and was obtained from the Bunter sandstone at Hambleton Haugh. In the Keuper the grains are much smaller. In some of the samples a few of the grains are fairly welI rounded, but usually they are angular and ragged. The colour varies from dark yellowish brown (accompanied by strong pleochroism) to pale yellow (accompanied by weak pleochroism). Most of the grains are clear and free from inclusions, but in some there are black rounded inclusions, similar to those in the garnet and tourmaline, whilst grains containing ovoid or irregular cavities are not uncommon. Tourmaline is present in every sample examined, both from the Bunter and the Keuper. It is numerically less abundant than zircon, but it is always more abundant than rutile, staurolite or garnet. The size of the grains varies with the coarsenessof the deposit, the largest noted (0.50 X 0.37 mm.) being found in the Bunter sandstone of Hambleton Haugh. Grains exceeding 0.2 and 0.3 mm. are not uncommon. The grains are usually well rounded, yet a large proportion of them still preserve their prismatic habit. The colour is usually brownish, although a great variety of shades are to be notedbrownish mauve, greenish brown, pale green, blue, etc. Occason ally a bicoloured grain (brown and blue) has been observed. A few of the grains contain inclusions, e.g., zircon, rutile (?) and a black mineral which occurs as roundish or elongated patches, frequently confined to a band running down the middle of a prismatic grain. Zircon occurs in every sample examined. It is always present in considerable abundance and occasionally forms more than half of the non-opaque heavy residue. It is somewhat more abundant in the fine-grained, than in the coarse-grained, deposits; and since the Keuper is, on the whole, finer grained than the Bunter, the latter, on the average, shows lower percentages. The largest grain observed (from the Bunter at Hambleton Haugh) measured 0.385 x 0.II5 mm., but grains exceeding 0.2 mm. in length are quite scarce. In a large proportion of the grains a considerable degree of rounding has occurred, and many appear to have passed through more than one cycle of erosion. The prismatic habit is usually preserved in spite of the obliteration of faces.

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

139

In a small proportion of the grains the faces of the crystals are clearly preserved. Some of these crystals show the simple combination of (no) and (In) faces, and grains belonging to this type are frequently zoned. Other crystals present sharper terminations, due apparently to the presence of the form (3n). A scarcer type presents the forms (no) and (III), but two pairs of bipyramid faces are abnormally developed, and a flattened hexagonal grain results. The ratio of length to breadth reaches 6: I or more in a few exceptional grains. Purplish, yellowish and dusky zircons occur in most of the samples. Quantitative Results. The present writer has shown (1930) that separations made from the same material first with, and then without a preliminary panning give different quantitative results when counting is adopted. It has been concluded that when slides are to be used for quantitative work panning should be omitted, and that the residue should be obtained from a small sample by washing, drying and treating with bromoform, and that the whole of the residue should be mounted. Furthermore, it does not appear to be profitable to make a complete count of all the mineral species present, as more valuable results may be obtained by fixing the attention upon the more stable and easily determined minerals. In a few cases where a total count has been made it has been found that the opaque minerals constitute the greater part of the residue, e.g. the residue from the Keuper of Aldborough contains 91 per cent. of opaque and only 9 per cent. of non-opaque grains. The difficulty of distinguishing between ilmenite and magnetite when attempting to estimate their relative abundance has already been alluded to, and any estimate which might be obtained would be of doubtful value. The removal or alteration of certain minerals by weathering or percolating waters introduces another source of confusion. Suppose, for example, that a certain rock when fresh yields a heavy residue containing 50 per cent. of apatite, the remaining 50 per cent. consisting of various stable minerals. A sample taken from the same bed, but from a part which has been much weathered or much altered by percolating water may yield no apatite whatever, so that the percentages of all the other minerals will be doubled. Where alteration of some of the minerals occurs, the number of heavy mineral grains in the sample may be increased. In the yellow Triassic sandstones in which the iron-bearing minerals have been altered to pyrite and then to limonite, the limonite cubes may be much more numerous than were the iron-ore grains in the original rock. Titanium dioxide from ilmenite and other titaniferous minerals may also add to the number of heavy

FRANK

140

SMITHSON,

mineral grains. Suppose, for example, that the heavy residue of a certain rock consists of 50 per cent. ilmenite and 50 per cent. stable minerals. If each grain of ilmenite on decomposition yields two plates of anatase,* then the percentage of the stable minerals in an altered sample will be less by one third than in an unaltered sample, whilst if the iron from the ilmenite remains as some iron-bearing compound the decrease may be still greater.

In simple cases such as those mentioned it might not be difficult to recognise the peculiarity of the conditions and draw reasonably sound conclusions; but as alterations and decomposition are often only partly completed and as the size and nature of the resulting grains may vary considerably, it will be seen that a series of complete counts may present such a complex assemblage of facts as to be almost valueless. The stable non-opaque detrital minerals of the Yorkshire Trias are tourmaline, staurolite, garnet, rutile, zircon and monazite (excluding scarce minerals, such as spinel, which never form more than a very small fraction of the heavy residue). In the process of counting it is not practicable to pause to distinguish monazite from yellowish zircon, and therefore for quantitative estimations monazite has been included with zircon. By counting the above minerals in a series of fields, regularly spaced about the slide on a definite plan so as to rule out personal selection, and reducing the totals to percentages, a set of frequencies were obtained. The data for Keuper and Bunter samples are given in the following table, in which T, S, G, Rand Z represent the five minerals named above, whilst in the Column A the comparative figure for apatite is given. The data for the Brearton Beds which may also be Triassic (Versey, 1925) is appended. KEUPER. Hurworth Gatenby Kirklington Hutton {Yellow .. Moor Red ..

T.

Aldborough Gt. Ouseburn

29. I 14·4 19·4 33. 6 7·5 26.6 23. 6 4 2. 0 29·4 13·3

..

24·9

{

..

Ripon Copt Hewick

Mean

S.

G.

R.

3·4 0·4 3. 1

5. 8

3. 1

6.0 9·9 11.4 9·2 6·3 7. 0 11.5 3. 2 16·5 6.8

0·9

1.0

S.8

Tr.

3. 2

Z.

A.

64·9 7 2.7 69. 2 53. 8 85.9 63·3 65. 0 51.6 54. 1 71.0

64·9 55·9 68.9 26.9 19. 2 69·5 29·9 71.0 44. 0 22·9

65. 2

47·3

• No such extreme case has been observed in the Trias, hut in some of the Lower Estuarine Sandstones of Cleveland the write- has obtained" floods" of anatase which might make quantitative data quite valueless for correlation.

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

T.

S.

BUNTER.

{

Bilbrough Hambleton H. Hensall Doncaster

f

Mean

l

G.

R.

Z.

141 A.

27·7 32.8 28·3 22·3 28.8 27. 6

15·5 19·4 26.6 13·9 9. 8 5. 1

5. 8 7·5 3·5 0·4 3. 8 1.6

9. 0 7·5 6·4 7·3 5. 2 6·7

41.9 32.8 35·5 56. I 5 2.3 59. 1

32.2 25·4 9.8 3·5 19.6 18·5

27·9

15. 0

3. 8

7. 0

4 6.3

18.2

10·5 44·5 24·0 25·5

Tr. 1.5 3. 0 1.0

1.2 1.2 4. 0

7. 0 3·3 15. 0 18.6

81.4 49. 6 56.0 5 0. 0

Nil Nil Nil Nil

26.

1.4

2.1

II

.0

59·3

BREARTON. Yellow Red

{ {

Mean

I

2.0

Nil

These few quantitative examinations are sufficient to confirm the impression that staurolite and, to a less extent, gamet, are more abundant in the Bunter than in the Keuper of Yorkshire. The frequencies of the other minerals afford no definite means of differentiation. When these data are studied graphically it is found that there is "sympathy" and "antipathy" between the minerals, e.g, the garnet percentage tends to be highest where the zircon percentage is lowest. It is, of course, to be expected that when one important constituent increases, the percentages of the other minerals will decrease. But this will not account completely for the relationship, for it is noticed that the rutile percentage does not fall, but frequently rises with increase of zircon. Indeed, the minerals may be divided into two groups (a) tourmaline, staurolite, garnet and (b) rutile and zircon, so that members of each group are more or less sympathetic to one another and members of different groups are antipathetic. * The relationship is probably due to a sorting action, whether by wind or by water, before deposition, and may be compared with the action of panning in the laboratory. The Brearton Beds. At Brearton near Knaresborough there is an outcrop of red and yellow sands or soft sandstones: Dr. H. C. Versey (1925) has described their heavy mineral suite and has suggested that they represent a Triassic outlier, although the Geological Survey map them as Permian. On the suggestion of Dr. Versey the writer has made a count of the residues from four Brearton samples, and the results are included in the table for purposes of comparison. They agree more closely with the Keuper average than with the Bunter, but in view of the * The antipathy between zircon and garnet has also been observed in other formations, e.g., by Dr. A. Gilligan in the Millstone Grit of Yorkshire (1919).

FRANK

SMITHSON,

difficulties of correlating by means of heavy mineral suites it would be rash to offer this as a proof that the Brearton beds are of Keuper age. The absence of apatite from the Brearton residues distinguishes them from the other samples, but this is not an important difference, for it may be due to the removal of this mineral after deposition. IV. COMPARISON WITH THE ROCKS BELOW THE TRIAS. Before considering the significance of the heavy mineral suite of the Yorkshire Trias and the possible source of the grains, it will be advisable to make comparisons firstly with Carboniferous and Permian sediments of the North of England, and, secondly with the Triassic deposits of other parts of the British Isles. For the knowledge of the heavy mineral suites of the Carboniferous and Permian rocks we are indebted to the work of Prof. A. Gilligan (I9I8, I9I9), Dr. H. C. Versey (I925), Messrs. H. P. Lewis and W. J. Rees (I926) and others, from whose papers the data for the Palseozoic in the following table is derived. It will be seen that the only mineral definitely recorded in the Trias which has not been found in the lower formations is spinel, which is scarce even in the Trias. The minerals which have been recorded in the lower formations but which the writer has not found in the Trias are epidote, bvrotusite, topaz and xenotime, and these minerals do not appear to be abundant in the Palseozoic, When we consider some of the minerals which are recorded both in the Palseozoic rocks and in the Trias, some interesting facts are brought to light. Apatite, which is not recorded in the Millstone Grit, makes its appearance in the Coal Measures and in the Permian sands, and becomes abundant in the Trias. The same is true of staurolite, except that this mineral is scarce in the Keuper. V. COMPARISON WITH THE TRIAS OF OTHER DISTRICTS. The occurrences of heavy minerals in the Trias of various parts of the British Isles, as recorded by various workers, are shown in the Table on p. I45. N.E. Scotland. Dr. W. Mackie (I923) has examined samples of the Triassic sandstones from the neighbourhood of Lossiemouth. Gamet is plentiful; apatite is present but is nowhere abundant; whilst staurolite was not found in the Trias although it is abundant in the Middle Old Red Sandstone of that district. Irish Sea Area (including S.W. Scotland). Published data concerning the heavy mineral content of the Triassic rocks of a number of districts surrounding the Irish Sea are available. (See Bibliography. *) * Professor Boswell has kindly supplied information from his own unpublished records concerning the Trias of Ulster, Arran, etc., which has allowed the writer to make the tablemore complete.

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FRANK

SMITHSON,

T he heavy mineral suites of these deposits have m an y fea t ur es in com mon ; staurolite is either absent or pre sen t in only small amo unts, whilst garnet , alt hough rec ord ed in all the districts, is n ot an abundant mineral. Apatite does n ot appear to have been found in any abundance , probably beca use preliminary treatment with acids has been adopted ; a nd a pat ite grains exhibiting colour, pleochroism or cores have not been recorded in any of the papers published. The writer has examined a sm all sample of Bunter sa ndst one from the R ibble Estuary and observed amongst the apatite grains about ten whi ch showed pleochroic cores , whilst one grain had a pale greenish tint , but was not noticeably ple ochroic. Garnet was scarce. In the case of the Trias of S.W. Lancashire Mrs . Alty writes (1924), "No defin ite distinction can be drawn between t he Bunter and Keuper. . . . Such minerals as have been found in only one of these divisions are those which a re so rare as to be useless for determinati ve purposes. . ." Boulde rs of Triassic sandstone occurring in the B oulder Clay of Anglesey and believed t o have been derived from the floor of t he I rish Sea have be en examined by Mr. 1. S. D ouble, who finds a heavy mineral assemblage simi lar to that of t he Vale of Clwyd. (Double , 1928). Cheshire. The K euper at Stockport has been examined by Mrs . Alty (1926), who found that in many respects it was intermediate between the Trias of S.W . Lancashire and that of the Midlands. Staurolit e is st ill a sca rce m ineral, but ga rn et has become abun dant. Midlands . The Triassic ro cks in a wide region aroun d Bi rmingham h ave been exam ined by Dr. Fleet (1923, 1925, 1927, 1929, 1930). H ere apatite is abundant in many of th e sa m ples, and dark cored and ple och roic grains are not infrequent. The data collected by Dr. F leet are particularly inter esting, be cause he has attempted t o ob tain accurate quantitative results, and a comparison with the Y orkshire T rias should be of value . A brief summary cannot do ju stice to Dr. Fleet's work , but the following table (show ing the average percentages of t ourmaline, staurolite, ga rnet , rutile and zir con for the three divisions of the Bunter and for three types of suites foun d in the Keuper) contains the data most impor tant for the presen t discussion . KEUPER- N or mal Type A Type B Type C UPPER BUNTER PEBBLE BED LOW ER BUNTER

T. 1.5 1. 8 2·3 1.5 2 .2

1.5

S. o,I

0. 2 0 . 1

0.2 1. 8 0· 4

G. 4· 9 13 ·5 6 ·3 2 ·3 2.1 0 ·4

R.

Z.

3. 0

46 . 7 23 · :) 25 ·9 21. 7

2 ·7 4 ·9 3 .0 2.1 2 ·9

II .2

11.3

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FRAN K

SM ITHSON,

In compa ring the se figures with the data obtai ned from the Trias of Yorkshire it must be pointed out that Dr. Fl eet' s figures are percentages based up on a count of ten mineral spe cies ; and that he worked wit h panned concentrates , so that the percentages found for tourmaline, st aurolite and t o a less exten t, garnet ar e less than would have been obtain ed by using bromoform, H owever , t he Trias of York shire shows the following points of agreement with the Trias of t he Midlands : (i) Tourmalin e and rutile do not show any great variation on passing from Bunter t o Keuper. (ii) Staurolite diminishes from Bu nter t o Keu per . (iii) Zircon increases in amo unt from Bunter to Keuper . But p oints of disagreement ar e also t o be noti ced :(i) In the Midlands garnet in creases from Bunter to Keuper , whilst in Yorkshire it diminishes.

(ii) In the Midlan ds the variation of sta urolite is much less pronounced t han in Yor kshire. In t he Keu per of No ttinghamshire stauro lite was recorded by Mr. T . H. Burton (I9 I7) as very ab unda nt , and garne t as frequent. In the Bunter of the Cha rnwoo d area Dr. Bosworth found t hat stauro lite was some times very abunda nt and ga rne t wa s very abundant.

S.W. England. In the Bunter deposits extending northward s from Bu dleigh Salte rton, Dr. Thomas (I 902) foun d a rich suite of heav y minerals, some of which , e.g. garnet and cassiterite, he believed to be of local origin, whilst others, e.g. staurolite, were t houg h t to be de rived from the Armorican lan d-m ass farther south. Stauro lite forms as much as 20 per cen t. of the heavy residue (obtained by heavy liquid separatio n) in the Budleigh Salterton area . Garnet becomes ab undant far t her north, where a strea m from the west is believed to have brought detrit us from Mid-Devon. Distribution of Staurolite and Garnet. I t will be noticed that in the ab ove desc riptions much im portance has been attached to stauro lite an d ga rnet. The reason is that the st udy of t he vari able minerals is likely to lead t o t he mos t valu able results, an d amongst these no others attain such abundan ce as st aurolite and ga rne t. The m ere occurrence or absence of miner als which ar e extremely scarce in the localities where they have been recorded , affor d data of doubtful va lue either for correla tion or for palreogeographical reconstructi on .

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

BUNTER. GARNET.

None Neyer abundant Rare None None Scarce

Plentiful

Carlisle Ribble Estuary S:W. Lanes. . Vale of Clwyd \Virral Stockport .. North Yorkshire ..

None

South Yorkshire..

Abundant

Nottingham

KEUPER.

STAUROLITE.

Lossiemouth Ulster

Occasional Scare Rare Usually scarce. None

GARNET.

None

None

None Scarce Usually none

None Abundant Usually none

Fairly frequent Veryabund- Frequent dant

Fairly to yery frequent

Scarce or variable

Stratford Boring S.W. England

STAUROLITE.

?

Chamwood Birmingham District.

147

Sometimes Very very abundant abundant Fairly Abundant frequent. Rare

Abundant especially in the South

Very abundant

Abundant in part of area.

Even allowing for the fact that the records are the work of different observers, who may have had different standards of judgment and that they have not all employed the same methods of separation, the table brings out some striking features. In the Trias of Yorkshire we find a much greater contrast between the heavy mineral suite of the Bunter and that of the Keuper than in any other region; whilst the distinctions recognised in Yorkshire are not in accord with those observed in the Midlands. If we relied upon heavy mineral suites as a criterion of age we should certainly not correlate the Keuper of Yorkshire with the Keuper of Charnwood, for we might more justly compare it with the Bunter of Lancashire. The table suggests that to accept without qualification the principle of heavy mineral correlation is to reject at once the present divisions of the Trias. A further examination of the table will show that the horizontal line divides the Trias geographically into two parts: (i) the Northern area in which staurolite is scarce or absent, and garnet also is scarce or absent except in the neighbourhood of Stockport (which is on the margin of this area), and at Lossiemouth in the extreme north; and (ii) the Midland and Southern area in which both staurolite and garnet are usually characteristic minerals.

'148

FRANK

SMIT HSO N,

These data suggest t o the writer that the heavy mineral suites of the Trias are det ermined mu ch more by geographical than by stratigraphical position .* VI. SOURCES OF THE GRAINS. It is generally admitted that during Triassic times arid

conditions prevailed over t he Bri tish I sles and of a great part

PoSSIBLE DIRECTIONS OF TRANSPORT: ~

Detritus w ith staurolite . Detritus without staurolite.

~

FI G . XL-MAP S HOW ING POSS IB LE SOURCES OF T RIAS SI C SEDIMENT S .

The shaded p ortions represent t he areas of non-deposi t ion bas ed up on ]ukes-Browne's paheogeogr a ph ic map of the Keuper . The d otted areas r epresent t he regions where stauro lite is known to be common in Triassic Sediments.

of Europe. The freshness of the felspars and such heavy minerals as apatite and staurolite, both in Yorkshire and elsewhere, confirm this view. Concerning the precise conditions under which the various Triassic deposits were laid down* The writer does not wi sh to sugges t that within a li mit ed area it is not pos sib le to dlst ingui sh the horizo n o f a bed by i ts heavy mineral suite, but that co rrelation by such means is cer tainly not applica ble to the co unt ry a s a whole .

TRIASSI C SANDSTONES OF YORKSHIRE AND D URHAM.

149

whether they are true te rrestrial deposits or were laid down in lagoons or inland seas-there is much disagreement. Professor P . F . Kendall and Mr. H. E. Wroot (1924) con sider that th ese divergent views may be reconciled " for sands owing their form and general ch ar act eristics t o wind action may have co me t o rest at length in wa ter. " Indeed , whatever was the final mode of deposition of the material it is likely that a large proportion of the gra ins were carried at least par t of their journey by wind. It is likely that th e Triassic desert was visited a t inter vals by rainstorms sufficiently heavy to mak e aqueous tran sp ort an important fact or. It is by no mean s certa in that the directi on of t ra nsport by th e torrential stream s would be the sa me as the directi on of transport by wind; the form er would d ep end on the relief of the land surface and whil st it would probably remain the same for many centuries, it might change many times during the course of a geological period* ; th e latter would depend upon com plex geographical and meteorological c onditions and might be subject to seasonal variations. Under such circumstances it seems undesirable to speak of the directi on of tran sport of the material, or to seek to trace all th e detritus t o a sing le sour ce. It is likely that during at least a part of th e Triassic Period a porti on of the P ennine Carboniferous area was un dergoing erosion. Certain min erals of the Triassic rocks might be a ttributed to this source . Monazit e, zircon, rutile and garnet , for e xa mple, all occur in the Millstone Grit (Gilligan , 1919) and the erosion of this rock might well have sup plied these minerals t o the Tri as. But there are t wo heavy minerals for which it d oes not appear possible t o find an adequate local source, nam ely apatite and staurolite. Apatit e does not occur in th e Millst one Grit, and , althongh pr esent in the Coal Measur es (Lewis and R ees, 1926), the qu antit y is not sufficient t o acco unt for it s abundan ce in the Trias. The stauro lite has probably been d eri ved direct from some region containing met amorphic rocks. Indeed , the abundance of apa tite, staurolite, prismatic gra ins of t ourmaline and fresh orthoclase together with th e evidence of the compound gra ins obse rve d in thin sect ions all poin t to the d etritus having been largely derived first-hand from some region or regions of granites an d schist s. The absence of hornblende, hypersthene, etc., fr om a dep osit in which such an unstable • Prof. Kendal and Mr. Wr oot (I9 24) cite th e example of the Amu Daria (R . Ox us). .. In the time of the Greek historians the Oxus ran int o the Caspian. It has since then wandered to Lake Aral, back to the Caspian, and now drains into the Aral. Besides this great river there flow down from the mountains many s trea;ms whos~ water~ are quickly swallowed up in t he sand of the desert. In flood seasons the POlUt of ultnnate disappearance of the waters is many miles in adva nce of the farthe st point reached in dry weather, and as the flood subsides each c hannel becomes a series of pools. The erosio n going on in the mountains is so enormous, and the burden of sand and pebbles borne by the stream so great that soon the course becomes choked. The stream, thereupon, drifts o ut in a new direction and leaves its old delta dry ." The dista nce be tween the mo uth of the Amu Daria in th e Aral Sea and th e old mou th in th e Casp ian is about lour h undr ed m iles.

FRANK

S~lITHSON,

mineral as apatite is abundant, may be taken as conclusive evidence that there were no basic rocks in the regions supplying the detritus. As stated above, staurolite is abundant in the Trias of many parts of England. Dr. H. H. Thomas (1902) believed that the staurolite of the S.W. of England was derived from the schists of Normandy and Brittany. Dr. W. F. Fl eet was not able to decide upon the source of the Midland st aurolite , but states that it must be some area of igneou s and metamorphic rocks such as Scotland or Brittany. Mrs. Alty (S. W. Harris) and 1. S. Double have drawn attention to th e ab sence or rarity of staurolite in the Trias of the Irish Sea area and have assigned a northern source for the material of this ar ea. Thev believe that this basin of deposition was more or less cut off "from th e Midlands, which in their opinion derived its material from the south. For the staurolite of the Yorkshire Trias Scotland is apparently the nearest possible source, but there are three arguments against a Scottish origin . Firstly, the Triassic rock s of N.E . Scotland cont ain no staurolite; secondly , the Trias of Arran , Ulster and t he Carlisle district , all presum abl y derivin g much of their detritus from Scottish rocks, cont ain ver y lit tl e of this mineral; and thirdly , st aurolite is ab sent from the Trias in the north of Yorkshire and rather abundan t in th e south of the county. In considerin g the validity of this third ar gum ent it must be rem embered that th e Triassic sandstones in th e north of Yorkshire are assigned t o the Keuper and those exp osed in the south of th e county t o the Bunter. If this classification is correct any classification based up on variation in the amount of staurolite is falla cious; but whilst the old classification of th e Permo-Triassic rocks has been called in question by Dr. Sherlo ck (1926) and others, no new one ha s been generally accepted. The que sti on as t o wh ether the variation of the heavy mineral suites in Yorkshire is chiefly geographical or chiefly stratigraphical could, of course, be definit ely answered by examining samples from borings which passed through the Keuper down into the Bunter. Th e writer has tried to obtain such samples, but without success. Mr. J. Allen Howe has informed the writer that the Geological Survey have no samples available for examination, " one reason for the dearth of specimens being that in recent deep borings in Yorkshire the boring through Trias has been jumped, i.e. cores were not commenced until the Grit or Coal Measures was reached." It seems, therefore, that the question must be left undecided. * If the staurolite is of southern origin one might consider • Since tb e reading of th is paper tbe autbor bas been informed tbat the Geological Survey bas recentl y collected a considerable amo unt of material from boreholes at Wb eatley Hall , Hatfi eld , Crowle, Eastoft and other plac es and that it is int end ed to publish a report in due course.

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

ISI

that the apatite was derived from the south, e.g. Normandy, Brittany and the Channel Islands, where there are granites containing cored and coloured grains (Groves and Mourant, I929). There are two arguments against this contention. Firstly, the pleochroism and other properties of the Yorkshire apatite do not agree with those recorded by Dr. Groves from the Armorican granites; and, secondly, when staurolite diminishes in amount or disappears all the varieties of apatite increase in amount, suggesting a Scottish rather than a southern source. Although there is not sufficient evidence to reconstruct with certainty the geographical conditions which existed at the time when the Trias was laid down, the following conception may be put forward tentatively. Whilst the Midlands and the South-west of England were deriving detrital material from the Armorican mountain range farther south and from certain local sources, the northern part of the country received most of its material from the Caledonian ranges. In Cheshire and in South Yorkshire an intermediate zone received detritus from both sources. Jukes-Browne in his map of the geography of Bunter times shows a belt ·of non-deposition extending across the Midlands (I9II, Fig. 36). Whilst it is likely that such a belt existed there is no reason to suppose that the barrier would be sufficient to prevent the transport of material by winds from the southern region. The map for Bunter times also shows an area of non-deposition in the southern Pennine area, which probably contributed a limited amount of detritus to the neighbouring deposits.

VII. NOTE ON SOME SUPERFICIAL DEPOSITS. Where exposures of the Triassic sandstone are scarce attention has been directed to certain red sands which are evidently material from the Triassic deposits that had been re-distributed in Glacial or Post-Glacial times. At some small pits near Brayton Barf the upper beds of sand contain pebbles of Carboniferous sandstones, but the lower parts of the exposures are free from pebbles. Near Catton-on-Swale and Kirklington the red sand contains pebbles of red sandstone which yield a suite of heavy minerals similar to that of the Keuper sandstones of that neighbourhood. The heavy mineral suites of the sands strongly resemble those of the Triassic sandstones in the districts in which thev occur, except that apatite is much scarcer and that a few grains of epidote and hornblende provide evidence of the introduction of other material. At Brayton, in the Bunter area, both staurolite and garnet are abundant, but farther north where the red sands rest upon the Keuper sandstones these minerals are absent, and dolomite becomes a characteristic mineral. PROC. GEOL.

Assoc.,

VOL.

XLII.,

PART 2, 193I.

II

FRANK SMITHSON,

Beneath the very thick glacial deposits at Rawcliffe (Rockcliff) Scar, on the River Tees near Croft, there is an exposure of red sand which is probably re-distributed Trias. Its heavy mineral assemblage resembles that of the Keuper sandstones from the Tees bed, but in addition it contains a few grains of hornblende and kyanite, whilst fresh pyrite is abundant. VIII. SUMMARY. Samples of Triassic sandstone from various parts of Yorkshire and Tees-side have been used for heavy mineral separations. The residues obtained are comparatively rich in mineral species, twenty-nine being recorded. The abundance of apatite in many of the samples has allowed this mineral to be studied in detail. Examples of abnormal magnetic properties, colour, pleochroism and secondary growth have been observed. The mineral composition affords evidence of transport under arid conditions from a region or regions of acid igneous rocks and metamorphic rocks. It is probable that detritus both from the Caledonian and from the Armorican land-masses occurs in the Yorkshire Trias, while the amount of material derived from local sources must be small. It is possible to draw distinctions (which are confirmed by careful counting of the grains) between Bunter and Keuper suites in Yorkshire, staurolite and garnet being more abundant in the former than in the latter; but this distinction does not agree with the results which other workers have found in other parts of the country. A review of the published data relating to the heavy minerals of the British Trias suggests that the composition of the heavy mineral suites is determined much more by geographical position than by geological age. In conclusion, the writer wishes to express his indebtedness to Professor P. G. H. Boswell for placing slides and records at his disposal, to Mr. G. M. Davies for helpful criticism and advice upon laboratory work, to officials of the L. & N.E.R. for rock samples from Doncaster, and to the Trustees of the Dixon Fund for a grant in aid of the research. BmLIOGRAPHY AND LIST OF WORKS REFERRED TO. ALTY. Mrs. S. W. (S. W.HARRIS). 1924. The Petrography of the Triassic Sandstones of South-west Lancashire. Proc: Liverpool Geol, Soc., vol. xiv., pp. 48-64. - - - - . 1926. The Petrographic Features of Keuper Rocks from a Boring at Wilmslow, ncar Stockport, Cheshire. Proc, Liverpool Geol. Soc., vol. xiv., pp. 278-283. BOSWELL, P. G. H. 1927. The Rarer Detrital Minerals of British Sedimentary Rocks. Trans. Geol. Soc., Glasgow, vol. xviii., Part 1., p. 138.

TRIASSIC SA NDSTONES OF YORKSHIRE AND D URHAM.

IS3

BOSWORTH , T . O . 191 2. Th e 1{euper Marls around Charnwo od . Quart. J ourn . Geol: S oc., vo l. Ix v rri . , pp . 28 1-292 . BURTON, T . H . 1917 . T he Micr oscop ic Material of the Bunter Peb b lebeds of Nottingharnshire a n d its Probable So urce of Origin. Quart . J OU1'1l . Geol. S oc., v ol. lx x iii ., p p. 328-337. BRAMMALL, A . 192 8. D artmoor D etritals. P roc, Geol. A ssoc., v ol. xxxix ., pp . 27- 48. DOUBLE, 1. S. 1926 . T he Pet rograph y of the Triassic Rocks of the Val e of Clwy d . P roc, L iverpool Geol. S oc., vol. xiv ., pp. 249-262 . - - - 1928. The P etrolog y of the T riassic Boulders fr om the B oulder Clay of Anglesey . P roc, L iverp ool Geol, S oc., vol. xv., pp . 63-68 . FL EET, W . F. 1923 . N otes on T ri assic Sands ne ar Birmingham . P roc , Geol, A ssoc., v ol . xxxiv., pp . II4-rI 9. - - - - 1<)2 5 . The Chief H eavy Detrital Mineral s i n the R ocks of t he English Midl ands . Geol. Mag ., vol. Ixii ., Trias, pp , 121-124 . - - - - . 1927. The H eav y Min erals of the Keele, Enville, P ermian and L ower Triassic R ocks of the Mi d la n ds, and the Corrre lation of t h ese Strat a . Proc , Ceol. Assoc., v ol. xxxviii., pp . 1-48 . - - - 1929. Petrography of the Upper Bunter Sandstone of the Midlan ds. Pro c. B irm in gham Nat . Hist . and Phil. S oc., v ol. xv., Part VII. , pp . 213-217. - - - - . 1930. Petrography of the Lower Keuper Sandstone of the Midlands. Proc. B irmingham N at. Hist. and Phil. S oc., vol. x vi.; Part i ., pp . 13-17. FL EET, W . F ., and SMITHSON, F . 1928. On the Occurren ce o f D ark Apatite in some British R ock s . Geol, Mag., v ol. lxv., pp. 6- 8. FOX-STRANGWAYS, C., CAMERON, A . G. , a n d BARROW, G. 18 86 . The Geology of the Cou n t ry around Northallerton and Thirsk . M em , Geol , Suru, FRAZER, F . J . 192 7. In Summary Report, Geol . S tu·v., Canada . Pa rt B .. p . soB. GILLIGAN, A. 191 8. The L ow er P ermi an at Ashfi eld B ri ck a n d T ile W orks, Conisbor ou gh. P roc, Y ork s. Ceol. S oc., vol. xix ., pp . 28 9- 297. - - - - . 1919. The P etrogr aph y of t h e Mill stone Grit of Yorkshire . Quar t . J ourn . Ceol. So c., vol. lxxv ., p p. 251- 294 . G RO VES, A . \V ., a n d MOURAST, A. E . 1929 . Inclusion s in the Apatites o f some Igneou s R ocks. M in , Mug ., v ol. xxii., pp. 9 2-99 . .J UKES-BROWNE, A . J . 191 1. The Buildin g of the British I sles . 3rd edit . KE NDALL, P . F . 191 7 . In Ha ndbu ch der R egionalen Geologic, vol , ji i..; P art 1. The B ri tish I sl es . p . 187. - - - - and WROOT, H . E. 1921 . The Geology of Y orkshire, Trias and Rhaetic, pp . 286 - 29 4 . LE WIS, H. P. an d RE ES, W . J. 1926 . Some Grindstones from t he Coa l Measu res of Yorkshire. Geol, lYlag., v ol. lx iri . , p . 13-27. MACKIE, W. 1923 . The P r inc ip les that Regulate the Di stribution of H eavy Mineral s in Sedi m ent ar y Rocks, as illustrated by the Sa n dst ones of the N .E . of Scotland. T ran s. Edinburgh Geol. Soc ., v ol. xi., pp . 13 8-1 64 . - - - - . 1928 . (READ 1925) . The Heavier Accessory Minerals in t he Granites of Scotland . T rans . Edinburgh Geol, S oc., v ol. x ii. , pp. 22-4°·

154

FRANK SMITHSON,

REYNOLDS, Miss D. L. 1928. The Petrography of the Triassic Sandstones of N.E. Ireland. Geol. Mag., vol. lxv., pp. 448-473. ---1929. Some New Occurrences of Authigenic Potash Felspar , Geot, Mag., vol. lxvi., pp. 390-399. RICHARDSON, L. 1929. In Handbook of the Geology of Great Britain. SHERLOCK, R. L. 1926. A Correlation of the British Permo-Triassic Rocks. Part 1. North England, Scotland and Ireland. Proc. Geol, Assoc., vol. xxxvii., pp. 1-69. SMITHSON, F. 1928. The Heavy Minerals of the Granite and Contiguous. Rocks in the Ballycorus District (Co. Dublin). Ceol Mag., vol. lxv., pp. 12-25. ---1930. The Reliability of Frequency-Estimations of Heavy Mineral Suites. Geol, Mag., vol. lxvii., pp. 134-136. THOMAS, H. H. 1902. The Mineralogical Constitution of the Finer Material of the Bunter Pebble Bed in the West of England. Quart. Journ. Geol, Soc., vol. lviii., pp. 620-631. ---1909. A Contribution to the Petrography of the New Red Sandstone in the West of England. Quart. Journ. Geol, Soc., vol. lxv., pp. 229-45. TRAVIS, C. B. and GREENWOOD, H. W. 191 I. The Mineral and Chemical Constitution of the Triassic Rocks of Wirral. Part 1. Proc. Liverpool Geol. Soc., vol. xi., pp. II6-139. VERSEY, H. C. 1925. The Beds Underlying the Magnesian Limestone in Yorkshire, with Appendix (the Red Rock Outlier at Brearton). Proc. Yorks. Geol, Soc., vol. xx., pp. 200-214.

ILLUSTRATION OF GRAINS. Nos. 1-12.

Magnification X 195 approx.

PLATE 5. APATITE.

(Polarised light; lower nicol with conventional orientation.) I 2. Colourless grains; Bunter, Bilbrough, near York. Corroded grain; Bunter, Hensall. 3· Grain with brownish, pleochroic core, in position of minimum 4· absorption; Keuper, Button Moor. Pleochroic orange-brown grain in position of minimum. 5· absorption; opaque core; Keuper, Hurworth. Grain with core of inclusions; Keuper, Copt Hewick. 6. 7, 8. Grains showing secondary apatite; Keuper, near Cleveland, Dyke, Coatham Stob, Co. Durham. Basal grain showing hexagonal dark core; Bunter, Hamble9· ton Haugh. Dark sepia-coloured grain in position of maximum absorption; 10. Keuper, Aldborough, near Boroughbridge. Grain containing two brownish pleochroic cores in position II. of minimum absorption, and containing a negative crystal; Keuper, Hutton Moor. Pleochroic brownish "ghost" apatite, showing growth. 12. stages; Bunter, Doncaster. J

Nos. 13-19. Magnification X 160 approx. STAUROLITE. 13. Rounded grain; Bunter, Doncaster. 14. " Ragged" grain; Bunter, Bilbrough,

PROC. GEOL. Assoc., VOL. XLII. (1931).

PLATE

5

...,

2.

6

4

9 7

8

10

12.

11

18

13

15

.,

-

{

19

oJ

'( 17 14

16

HEAVY MINERAL GRAINS FROM THE TRIAS OF YORKSHIRE.

[To face p. 15+

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

I55

GARNET. 15· 16.

GREEN 17· 18. 19·

Rounded grain, pink in colour; Bunter, Doncaster. Angular grain, almost colourless; Bunter, Bilbrough. SPINEL. From Bunter, Hambleton Haugh. Grain showing acicular cavities, Keuper, Hutton Moor. From Keuper, Hurworth.

DISCUSSION. Mr. E. E. L. DIXO:- : I regret that I shall not be present this evening, as I should like to enquire which sandstones in Northern Yorkshire are referred by Dr. Smithson to the Keuper. If they are the thick mass some 800 ft. in all, above the Saliferous marls of Middlesbrough, it may be that in their respect the conflict between the classification of the Trias and the evidence of the heavy minerals, mentioned in the abstract of the paper, is more apparent than real. For, although these sandstones were referred by E. Wilson and others long ago to the Keuper, a view once supported by Dr. Sherlock, all except the highest were correlated by Goodchild with the Bunter. Further the St. Bees-Kirklinton Sandstones of the Carlisle basin, which seem to be the western equivalent of the North Yorkshire sandstones, are regarded by the speaker and his colleagues on the Survev as Bunter, together possibly with lowest Keuper. If it is in the thick sandstones of Middlesborough that the" Bunter" and" Keuper" suites of heavy minerals occur it would be interesting to learn whether the suites are mixed or successive. Dr. FLEET expressed his great appreciation of the Author's careful work. He, the speaker, had no acquaintance with Yorkshire geology, but having regard to his own researches on the Midland Trias he was keenly interested in the present paper. He agreed that difficulties arose when comparing results of different workers, but in spite of that he felt that when the whole of the Trias had been systematically dealt with petrologically, results of great interest and value would accrue. Quite apart from the idea of correlation of strata by heavy minerals when fossils were absent, it seemed to the speaker that given fairly widespread conditions of transport and deposition of material, one might expect to find corresponding upward sequences of heavy mineral suites in different parts of Britain. He asked, therefore, whether the Author had noticed any correspondence between the heavy mineral suites of Yorkshire and those found by the speaker in the Bunter and Keuper of the Midlands. Dr. R. L. SHERLOCK congratulated the Author on his clear exposition and on the amount of work done, but doubted if the number of samples taken was sufficient for so extensive an area. He enquired if the samples of Keuper were all from sandstones, for the marls have been almostignored. Also in comparing samples much will depend on whether the rocks sampled are really of Keuper or Bunter age respectively, a matter of considerable doubt. The author's discovery of the very different results obtained by panningor not panning before using bromoform was important. No doubt in his tables of comparison of results obtained by different workers the method they adopted had been considered by the author, as otherwise results would not be comparable. In the case of the Brearton rock of doubtful " Permian" or " Triassic" age the speaker noted that the samples from the red and yellow parts of the rock differed markedly; another warning of the difficulty of using the method to find the age of rocks. The speaker was not surprised to hear that the author found the differences between Keuper and Bunter in Yorkshire were in the other direction to those found in the Midlands by other workers. He thought the work done by Prof,

156

TRIASSIC SANDSTONES OF YORKSHIRE AND DURHAM.

Bosw ell on the Midford Sands , where the results from mineral grai n determination clashed with those from the fossils ,showed how little miner al grains could be relied on for a ge d et ermination s. Nevertheless, in de aling with a lmost un fossilifero us rock s, no lin e of attack must b e ne gle cted a nd the study of sediments under the mi cr oscope was to b e welcomed . T HE AUTHOR: As time w as short the au t h or r eplied b riefly to th e more im portan t points rais ed . In a ns we r to Mr. Dix on's quer y he m enti on ed that the localities fr om which samples had been coll ect ed were st a ted in the paper. The author agreed with Dr. Sherlock that it would have been d esirable to h a ve had samples from a greater number of localities" All the sampl es r ecorded were sa ndst ones ; result s from an ex amination of the m arl s were so di sappointing that they we re not con t inue d and were n ot incl ude d in the paper. W ith re gard t o the naming of the bed s as Bunter a nd K euper, the author followed the Survey m aps as a m atter of con venience. The author stated that the number of Yorkshire samples ex a mined d id not allow him to answer , D r . Fleet's question satisfactorily.* H e believed that if a suitable su ccession of bore-hole sa m ples could be obtained it might be possible to recognise s uch st a ges as Dr. F leet had observed in the Midlands.

0The un certainty of the st ratigraphical posit ion of the sampl es from Yorkshire present s an ad diti onal difficult y.