126

THE SKIDDAW GRA~ITE: A STRUCTURAL STUDY. By J. F~ N. GREEN, B.A., F.G.S.

[Read 7th JIIlIe 1918J PLATES II and 12. N a recent paper on the age of the chief intrusions of the I Lake District* the problem of the Skiddaw granite was deferred until the other intrusions had been dealt with. The conclusion arrived at was that those of Eskdale, Buttermere, St. John's Vale, Carrick Fell and Haweswater were all part of the Borrowdale suite and of Llanvirn age. This result is opposed to the views commonly held since the time of ]. C. Ward, but, if accepted, naturally gives rise to the query whether the Skiddaw intrusion may not also belong to the same epoch, owing its peculiarities merely to a greater thickness of cover. Such a query is a da.ring one. The historical resume given in the paper referred to above brings out a striking unanimity among the many competent geologists who have studied the intrusion and its aureole during the last half-century. The formidable concurrence of the names of Ward, Harker, Marr and Rastall demands that any divergence from current opinion should be supported by new and strong evidence; and this. is the more necessary inasmuch as the stratigraphical relations which assisted in the attack on Carrick Fell do not appear to extend into the Skiddaw region; nor can the occurence of a few minute grains of garnet in a sample of the Skiddaw rock be strained to an example of the mineralogical peculiarity which the 'writer believes to characterise the Borrowdale suite. The query must be answered, so far as can be seen at present, on structural grounds alone. It is therefore, with some diffidence that observations are brought forward. which, in the writer's opinion, refute current ideas on the relationships of the granite. At least they are for the most part new : and although some, and those perhaps the most important, were recorded by Ward on published Survey maps, they appear never to have been discussed or even mentioned in print. The Skiddaw granite and its metamorphism have often been described, but it will be convenient to give here a brief state­ ment of the main facts. For a more complete account reference may be made to Mr. R. H. Rastall's paper on "The Skiddaw Granite and its Metamorphism."]' • Proc, Geol, Assoc., xxviii., 1917, p. r, t Quart, Journ, Gcol, Soc., lxvi. 1910, p. r ro. ? ... THE SKIDDAW GRANITE. I -/ The granite itself is not well exposed at the surface, but has a well-developed aureole of metamorphism, rudely oval in shape, now about six miles long from north-east to south-west. In this direction it has been somewhat reduced, perhaps by as much as a mile, by the fault which bounds the south of Carrick Fell. The breadth is about three and a half miles. The in­ trusion is obviously related to an anticline running nearly north east-southwest, which brings up the Skiddaw Sandstone (pro­ bably of Lingula Flag age) in a long strip. The later rocks (Skiddaw Slates) within the influence of the granite are believed to be all Tremadoc and Arenig. The largest outcrop of the granite, about one mile by half a mile, occurs at the centre of the aureole near the core of the anticline. The outer­ most part of the aureole is incompletely re-crystallised and is characterised by chiastolite except in the most siliceous strata. The intensity of alteration is nearly related to concentric ovals round the central granite, but the effect is less con­ spicuous in the sandstone strip. CLEAVAGE. It is proposed first to deal with the relation of the intrusion to the cleavage. The distribution of the cleavage and folding in the northern part of the Lake District is synclinorial (Ward's "clea.vage­ anticline ") the main axis running in a direction about 12 degrees north of east across the northern end of Bassenthwaite Lake (see Plate II). In an ideal synclinorium, the cleavage along the main axis would be vertical, beds on either side overfolded inwards towards it and dipping outwards with cleavage parallel to the axial planes of the folds. If evenly Worn down, the stratigraphically highest beds would tend to be preserved only near the central axis. Accordingly, in the northern part of the Lake District the latest beds involvedin the folding, i.e., the BalaWatch Hill Beds, are so far only known in the neighbourhood of the synclinorial axis, though the occurrence of fragments in the Mell Fell Conglomerate* shows that they once spread widely to the south-east; but, owing to a pre-existing denuded anticlinal fold] the older beds do not follow in regular order outwards, but are asymmetrically disposed in order reverse to the usual. The Cambrian sandstone is only seen on the south-east side, where, owing presumably to superior hardness, it gives rise to various complications, one of the more important being a minor svnclinorium south of Sale Fell. The Skiddaw aureole thus lies wholly on the south-south­ east side of the central synclinorial axis, so that the pressure over all the area occupied by it came from south-south-east. • Proc. Geot. Assoc., xxix., 1918, r. 117. t Ibid., xxviii., 1917, p, 25. 128 J. F . N. G l{EE"I O N

Thisgives a test for th e date of th e granite. 1£ themetamorphism is later than th e pressure, it may be expected to have symmetrical relations to the cleavage ; but if it is older, th en the solid block of granite and hornfels would suffer to the south and eas t, but would shadow th e north-west. The distribution of the cleavage about the Skiddaw granite is shown on the map (Pl. II). To the north-west it enters a fringe 0:' the aureole half a mil e wide and , so far as observed, does not affect rocks with well-characterised cordierite. To the south-west cleavage affects nearly half the dist an ce between the outer edge of t he aureole and the .central granite; to the south it only rea ches to the Sinen Gill intrusion. But on th e south-east the cleavage cuts right through th e whole mass of metamorphosed rock t o the surface of the central granite itself. A traverse along the upper Glenderamarkin and down Blackhazel Beck reveals excell ent cleavage dipping at 60° to 50", ISO east of south, from the outer edge of the chiastolite zone to a point about 400 yards from the junction-of the granite and t he sediment arios. Its direction and dip here have been noted by Ward on the one-inch map (O.S. lOI N.E.) published in 1 8 9 0 , an d on the puhlished six -inch map, Cum berland 56, in Blackhazel Beck and in a small t ributary gill; and also near White Gill less than 200 yards from the granite. The rocks are well exposed and the cleavage can be seen weathering out all the way np the steep sides of the Blackhazel gorge. Close exa mination discovers bedd ing, intensely folded, so sharply that the cleavage often nearly coincides with it. The rock is coarsely cryst alline, presenting a high grade of metamorphism , more so than even the uncleaved rocks of Grainsgill; neverthe­ less th e cleavage planes have certainly not been " sealed up by th e re-cryst alli sati on induced by intense heat,II as has been stated . On following the heck sec tions towards the granite. cleavage still continues. There are bands of uncleaved roc k. but the greater part of the hornfels is fissile, the plan es of separat ion undulating and dipping lower and lower, till close to the granite they may be nearly horizontal. Bedding was not observed, but the separation-surfaces are similar to the clea vages, and it is hardly conceivable that the bedding should suddenly become fissile with increasing met amorphism. It would appear, there­ fore, that the structure is of the nature of rotatory strain or shearage, tending to parallelism to the surface of the intrusion as it is approached. The distribution of the cleavage and of its dips thus corresponds to that which" would he expected if the intrusion were pre­ viously in "existence ; and it has affected crystalline roc ks in a manner only explicable on the hypothesis that it is later th an the metamorphism . PROC GEOL. Assoc. , VOL. XX IX. PLATE II.

-- - Q- -- Cleaved un altered Cleaved altered Un clea ved altered Bala grits, In trusions. Skiddaw Slate. Skiddaw Sla te . Skiddaw Slate. STRUCTURAL SKETCH - :\I A P OF THESK IDDAW I{EGIO N. Scale>- -One in ch to the m ile . THE SKIDDAW GRANIT E.

This conclusion is supported by the microscopic evidence. Beginning,with the outer part of the zone of black slate with chiastolite, .' the new prismatic crystals are well-formed and no other new mineral is discernible except in places a few minute biotites. The cleavage is st rong and tends to cur ve round the chiastolites, which are without an y dire ctional relation to cleavage or bedding. When cleavage-planes approach one of the crystals, they may be seen to diverge and either to pass over it or to end up against a face. The minute mica-scales th at compose the bulk of the rock follow these curves. On passing further into the area of metamorphism , th e slate, th ough ex cellently cleaved, becomes paler and harder , and contains more hiotite with much of a mineral believed by Mr. Rastall to be ottrelite, both again without orientation; but the andalusite changes, assuming an appearance referred by Ward* to incomplete development. This explanation cannot. however, be accepted, since the cryst al faces remain clear and sha rp, th ough broken, the irregularity being due to the opening of small clefts filled with the surrounding material, and to the displacement of bits of andalusite between clefts and cracks. The face s belong to single crystals comparable in size and form to those in the outermost zone. Longitudinal secti ons of the crystals are greatly cracked and their ends sometimes com­ minuted , though the sharp crystal-faces can still be distinguished. The majority are replaced by a brightly depolarising decom­ position-product. (See Figs. I and 2, Plate 1 2) . On passing still further inwards towards the middle of th e aureole, biotite becomes coarser, ottrelite decreases, and th e pseudomorphed andalusite is torrrinto most irregul ar shapes, only traces of the crystal structure being left. Some are mere lin es of broken fragments extended along the cleavage. or pressed into an S-shape across it , as in a specimen from Dash Beck . Other minerals can often be seen to be distorted . The explanation offered of the notable differences bet ween the state of lite andalusite in the cleaved rocks of th e black outer zone and of that nearer the granite is that in the oute r zone the c1eavage confined it self to the softe r unaltered par t of the rock, flowing round the strong new crystals; but, where the shale had been so hardened and recrystallised that no great difference in resisting power remained between the andalusites and the rest of the rock , cleavage affected both, shattering the large crystals. This more uniform rigidity explains t oo the ohservation , surprising at first sight, that the cleavage, as may be seen in the Glenderaterra and Glenderamackin valleys, is more perfect and regular at some distance within the aureole th an in the outer chiastolite zone . A specimen Was sliced from an exposure onl y showing • Geology ';{o,./I,rru l .ake District; p. 10 . 130 J. F. N. GREEN ox slight cleavage just south of the great section of cleaved and folded rocks in Blackhazel Beck at 400 yards from the granite. The more highly cleaved specimens seem too weathered to provide good sections, owing to the penetration of decay along the divisional planes .The rock is a coarse graphitic cordierite-hornfels with a little andalusite. Much of the biotite and muscovite is above half a millimetre across. The cleavage is indicated by irregular brown cracks, rudely parallel to the bedding, which break the cordierite and mica, with a marked tendency to follow the cleavages of muscovite, even when a sharp turn is necessitated thereby. In connection with the parts of the aureole which show no obvious cleavage, it is as well first to consider what criteria must be applied to decide Whether or not they were cleaved before recrystallisation. The development of new minerals is often controlled by divisional planes in a rock, the reason being no doubt that material travels more readily along than across such planes. It is, therefore, necessary, to search for some geometrical relation between a mineral and a plane direction and then to determine Whether that is a direction of cleavage or bedding. Such a relation must be either the perpendicularity of a line or the parallelism of a plane in a crystal. For example, the statement that the cleavage of mica is perpendicular to the cleavage of a rock affords no presumption of a connection between the two, inasmuch as there is an infinite number of plane direc­ tions perpendicular to a given plane; but it gives rise to a suspicion that there may be some connection with a direction, perhaps that of bedding, perpendicular to the cleavage in the particular specimen examined. No orientation has been noted in the quartz, garnet, muscovite or aluminous silicates of the inner aureole. The biotite too is usually indefinite in direction, though clear parallelism to bedding is seen in some specimens from Sinen Gill. This be­ haviour of the mica may indicate, it is suggested, that whatever divisional planes existed before metamorphism Were not strongly marked. But graphite, which is a prominent constituent of most of the highly altered rocks, gives a very definite result. It occurs as minute grains, whose most frequent form is an oblate spheroid with long diameter two or three times as large as the short. The larger globules are commonly .01 to .02 mm. in greatest diameter. They vary very much in quantity with the bedding, the same slide including bands with none and bands with hundreds in the field of the microscope. (Fig. 3, Plate 12). These spheroids have invariably a distinct orientation in bulk, the longest diameters being grouped round a direction, which is equally invariably parallel to the bedding. This has been observed in every slide of the highly altered rocks which has THE SKIDDAW GRANITE. been examined. There are numerous globules out of the general direction, but the majority approach it sufficiently to enable it to be determined to a few degrees in almost any graphitic part of a section. In order to get some idea of the extent of variation, a count was made in a microscope field of average appearance in a cordie­ rite-hornfels from the River Caldew near Grainsgill, There Were1I7 spheroids in the field (besides some circular and irregular grains) of which 43 Were approximately parallel to the bedding, and 46 within 30 degrees on either side of it, leaving 28 which diverged more Widely, of which seven were nearly perpendicular to the general direction. It is inferred that while there is no evidence that any cleavage was in existence when heating took place, the rock Was probably then fissile along the bedding, i.e., was in a shaly condition. The possibility that the orientation may have been due to some other cause is, however, not to be overlooked.

FAULTING.

The only fault as yet known likely to throw light on the age of the Skiddaw granite is the Mosedale fault, which bounds the south of the Carrick Fell complex, bringing highly crystal­ line cordierite-mica-hornfels against unaltered basic igneous rocks, so that it is clearly posterior to the metamorphism. This fault has been studied in the neighbourhood of Mosedale, where it is well exposed in the slope immediately behind the village. The south part of this slope consists of vertically bedded and cleaved argillaceous Skiddaw sandstone of a pale grey tint; the north part is a coarse gabbro. On tracing the sandstone towards the gabbro along the base of the slope, it is found that at about 30 yards from the gabbro the bedding dis­ appears. The actual contact of gabbro and grey rock is well seen, and welded specimens are easily obtained. It seems to have always been assumed that the pale grey unbedded rock, which has the same tint and fine texture as the sandstone, was identical with it. In point of fact it is a granophyre, in which phenocrysts of felspar and biotile can occasionally be seen. The appoximate position of the actual fault between the sandstone and the granophyre is only determinable with difficulty by the bending over and disap­ pearance of the bedding, as the phenocrysts and granophyric structure become unrecognisable near the fault plane. The difficulty of distinguishing the rocks in the field is in­ creased by the appearance here and there of a thin banding, simulating bedding, in the granophyre near the fault. This at first sight might be thought to be a flaser structure, but actually 132 J. F.N. GREEN ON corresponds to differences of texture and appears to be fluxional in origin. There is no obvious brecciation at the fault, but near it the rocks are full of shear-planes ; and the granophyric structure, which, as pointed out to the writer by Mr. Wynfield Rhodes, visibly weathers out on joints, ofte n breaks down so that it resembles the weathered sandstone. Microscopic exam ination shows that in cer tain places the granophyre has a crumpled appearance, the quartz being broken up into fragments no longer in optical continuity, but retaining the peculiar shapes of the intergrowt hs . As, however, at these points the fe1spar is completely peri shed , it is not quite clear that t he st ructure is not original. When it is considered that the sandstone shows no contact ­ alterati on ; that there is no angular brecciati on ; that the rocks are full of shear-planes ; and that the bedding (except when cont orted near the junction) and cleavage of the Skiddaws are parallel to the junction , as is the banding, what ever its origin, of the granophyre ; it is plain that the characterist ics of the junction are those of a fracture caused by pressure, although the hade at Mosedale is sensibly vertical. It is in fact a limiting case of a reversed fault. The fault is therefore of the kind associated with th e Devonian folding, to which it is nearly parallel. It runs a little sou th of east, that is, with the st rike to the north and west of it , and wit h the axes of the min or folds developed in the meta­ morphosed rocks t o the south of it, though not wit h th e strike of the Skidda w an t icline . The cleavage at Mosedale di ps exactly parallel to it. Prima fa cie then the fault belongs to the Devonian movements. Thi s inference would, however , be refut ed if t he fault could be shown t o affect th e Carboniferous t o the eas t of it. The published one-inch map (O.S. 101 ~ .E .) indicates a faulted junction of t he Carboniferous and Ordovicians with another fault, almost at right angles to th at juncti on . continuing the line of the Mosedale fault. Both faults are in country cove red by thick drift , and when exa mined in company with Mr. Rhodes, it Was agree d that no t race whatever of them appea red on the ground. The writer has been permitted, by the kin dness of the Director of the Geological Survey, to see the working six­ inch maps on which the pu blished one-inch sheet is based , and they show plainly that both faults are of the most hypothetical nature. It is possible that a continuation northward of P rofessor Garwood's det ailed palseontological work may yet prove the existence of faulting, but the most probable solution appeared on the ground to be a gentle unf aulted syn cline of Carboniferous between the Ordovicians of Mosedale and Crev. sto ke Park , resting normall y on the older rocks . At all evente THE SKIDDAW GRANITE. 133 it is quite impossible in the present state of knowledge to rest any argument on the supposed faulting of the Carboniferous ~ and it is certain that no such fracture penetrates it as the ~reat Mosedale fault, which brings crystalline rocks well down in the Cambrians against unaltered middle Llanvirn. and has a throw of several thousand feet.

FOLDING.

The age of the Skiddaw anticline will be discussed later, when trying to set closer limits to the age of the granite. It may, however, be noted here that its axis is directed E35°N• W3SoS, whereas the axes of the minor folds more usually approach E1S0N-W1SQS, though considerable divergencies are not in­ frequent. Near the Mosedale fault, the average direction of which is E8°S-W8°N, the minor axes approach east-west, and are sensibly parallel to the fracture. At this stage it is only proposed to consider the peculiar folds seen in the cordierite-hornfels, south of the Mosedale fault. These contortions are highly developed in the tough rocks in and near the River Calder above Swineside Farm. They pitch at all sorts of angles, some as much as 75 degrees, but remain usually almost parallel to the fault. Frequently the contortions are made up of a series of straight segments, and in these cases the angles are sometimes brecciated. Thus the folds are unlike those produced in unaltered Skiddaw slate. which are those typical of incompetent rocks, but are such as are found in com­ petent rocks near the zone of fracture; and the direction of their axes suggests that they are due to jarnbing against the rigid mass of Carrick Fell. In the Lake District such minor folding is, as far as known, always of Devonian age, except in the case of the drag-folds in soft Carhoniferous beds figured by Professor Garwood" and their angular form suggests that the Skiddaws were already metamorphosed at the time of their formation. If so, the existence of the Mosedale fault may be explained by the apposi­ tion of two solid masses resistant to folding and cleavage, viz., the Skiddaw and Carrick Fell aureoles; and the rapid variations of pitch in the minor folds south of it', by the fact that the upward movement was nearly transverse to the stress.

THE SKIDDAW ANTICLINE.

The evidence above given is held to show that the intrusion is earlier than the cleavage, the Devonian faulting, and the minor folds; and is therefore older than the main crust-movements.

• Quar!. [ourn; (;'"0. Soc., Ixvilt.. 1912. Plate xlvI. Fi,. r, PROC. GEOL. Assoc., VOL. XXIX., PART 3, 1918.] II 134 J. F. N. GREEN ON In order to set any closer limit, it is necessary to establish a con­ nection with some earlier movement. The manner in which the minor folds ride obliquely on the great Skiddaw anticline suggests that the two are of different dates; and in dealing with other intrusions it was pointedout* that the position of the Watch Hill beds on the Skiddaw slate proved that the main anticlinal axis of the Lake District in a broad sense must be pre-Bala, and not, as had previously been supposed, Devonian, It can further be shown that in Bala tines, as at present,' the Skiddaw sandstone Was exposed at the surface. This rock is notably uniform; being everywhere, so far as yet investigated, an argillaceous sandstone of the silt grade, the sand-grains being normally from .02 to ,03 mm. in diameter. The largest grain yet noted in any specimen is only .2 mm. long. A great majority of the grains are quartz, though felspar can occasionally be recognised, and rarely films of heavy minerals occur.. There is much small clastic mica, commonly .1 to .2 him; long, and specks of carbonaceous matter. The amount of the last two varies rapidly with the bedding. The sandstone probably lies below the Tremadoc of Barf and corresponds to part of the_Lingula Flag of North Wales. The Watch Hill beds on the other hand, apart from the shale-bands; are predominantly coarse, variable arkoses with fragments of lava and characteristic flakes of dark Skiddaw slate. The plentiful mica is much coarser than in the Skiddaws. Sale Fell is a ridge running nearly east and west west of Bassenthwaite Lake, somewhat south of the map (Plate 2) of the Watch Hill beds annexed to the paper on the age of the chief intrusions of the Lake District. On the northern flank the Skiddaws are well exposed in inverse order, the lower slopes being darkslate, the upper grey sandstone, both dipping south to south-east. Just by the junction, Where there is a good deal of interleaving, occur, for the most part near the goo foot contour, a series of long narrow outcrops of Watch Hill beds, which have been proved for half a mile with maximum outcrops only about fifty yards wide. They are quite characteristic, containing all the usual constituents of the Watch Hill grits and their chloritic silty matrix, though the green colouring is less pro­ nounced, some of the beds immediately above Wythop Church being almost black. They are somewhat strongly, but irregu­ larly, cleaved; and it is of interest that the included flakes of carbonaceous slate are cleaved, not parallel to their length, but, whatever their orientation, parallel to the cleavage of the rock, so that they were clearly derived from a shale. They and the ~arge micas are often much distorted. Two of the three slides which have been cut contain pebbles • Proc; Geol, Assoc. xxviii., 1917, p. 26. THE SKIDDAW GRANITE. 135 of a material indistinguishable from the adjacent Skiddaw sandstone, an argillaceous silt of grade about .03 mm.'with mica flakes .1 mm. long and carbonaceous specks. The structure of Sale Fell may be explained in two ways. Either the narrow outliers are the keel of an infold, the upper part of the Fell being the inverted limb of the syncline; or the sandstone is thrust. the strips of Bala appearing under the thrustplane. Time has not been available to decide the al­ ternative by detailed mapping. The interleaving at the junction of slate and sandstone points to a fold; hut the question is immaterial to the present discussion, as the presence in the grit of pebbles of sandstone identical with that of the upper part -of the Fell proves that the Cambrian Was exposed at no great distance, and that, therefore, the Sale Fell anticline is pre-Bala. It seems a reasonable inference that the parallel Skiddaw-Gras­ moor anticline must also have heen produced at the same time. The intrusion of the granite cannot have been the cause of this uplift, as it only occupies a part of its course; but opinion seems to tend towards the view that intrusions follow surfaces of relief from pressure, so that the fold may have determined the position of the granite. However this may be, it is not easv to account for the undoubted connection between the anticline and the intrusion except on the hypothesis that de­ formation and injection were simultaneous. There is no known epoch of intrusion in the district between Bala and Arenig times, except that connected with the Borrow­ -dale suite. Indeed, apart from a few small dykes and sills, probably of Bala age, no other pre-movement intrusions have been proved. The evidence is therefore in favour of including the Skiddaw granite with the Borrowdales. Conversely, the formation of the main Lake District anticline is also linked with the Borrowdale episode. It is hardly necesary to point out that this line of reasoning has by no means the same force as that by which the pre­ Devonian age of the granite was supported. All that can be said is that such evidence as has yet been obtained points to the association of the Skiddaw granite and anticline with the Borrow-dale volcanic epoch.

SUMMARY. A summary of the general conceptions arrived at may be grven, Near the close of the Borrowdale volcanic episode. simple folds of large size Were formed at the main centre of eruption by local sagging and concomitant upward bending, similar to the movements which have taken place in recent times beneath some of the island groups of the Western Pacific. One result was the formation of the Skiddaw-Grasmoor anticline, THE SKIDDAW GRANITE. along which a diminution of subterranean pressure decided the injection of the Skiddaw granite, while a more complex series. of intrusions about Carrick Fell to the north found a zone of weakness at the junction of volcanics and shales. The whole was Worn down by upper Bala times, the sea probably advancing along the soft shales of the axial region of the anticline. There­ after a great thickness of Silurians Was deposited over the whole region. "When intense compression came in Devonian time on a regional scale. the Skiddaw shales were squeezed between the jaws of igneous rock formed by the limbs of the Ordovician anticline. Folding and crumpling proceeded outwards from the hard volcanic blocks. and the portion of the Skiddaw slates stiffened by intrusion was driven north-north-west. Between it and the Carrick Fell mass the less altered slates broke and overrode younger strata in a thrust, which at a depth nearer the zone of flow became vertical between the two resistant masses. When the crystalline rocks came together the break was oblique, not only to the Skiddaw anticline, which was moving bodily, but to the direction of movement, so that the pressure, with the consequent foldirig and cleavage, slewed round some twenty degrees in its neighbourhood. The cleavage cut through the Skiddaw aureole t o the grani te on the south-east. while the regions about Great Calva and Cockup to the north-west Were largely protected. The writer desires to thank Mr. ]. Wynfield Rhodes for assistance in the field; and Dr. H . H. Thomas and Mr. G. 5 . Sweeting for the photographs of the metamorphic rocks, repro­ duced in Plate 12.

D ESCRI PTION OF PLATE 12.

1 . Uncrushed crys tal of chiasto lite ill black slate. Glcnde rate rr a, Cumbe rland. Mag. 25· 2 . Cru shed crystal of chiastoiite 1"11 cleaved grey horn fels. (T he outli ne has been accentuated for clearness .) Ncar head of Blackhazej Beck, Cumberland. Mag. 46. 3. Graphite globules ill cordierite-mica-hornjels. The sluice , Grainsgill; Cum berland. Ma«. 140. PROC GEOL. Assoc., VOL. XXIX. PLATE 12.

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MICROPHOTOGRAPHS.