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01~ THE CRYSTALLINE SCHISTS OF THE ~ALYERI~ HILLS. 525

37. A PRELI~tlNAI~ INQUII~Yinto the GEnEsis of the CR~STALLI~ SOTrlSTS of the MALVERN HIL~. By C~r. CALI~WAr, Esq., D.Sc., F.G.S. (Read April 6, 1887.) Introduction. 1. The Materials from which the Schists were produced. Diorites, Several Varieties ; Granite ; Felsite. 2. Evidence of Pressure. Contortion of Granite-veins. 3. The Products of the Metamorphism. A. Simple Schists. Hornblende-gneiss, Mica-gneiss, Mica-schists. B. Injection-Schists. Duplex Diorite-gneiss, Granite-diorite-gneiss. 4. General Remarks. 5. Summary. INTRODUCTION. THE igneous origin of some foliated rocks was first suggested to me by the granite of Northern Donegal*. The Rev. E. Hill, F.G.S., had previously noticed t a gneissic structure in the granite of Guernsey. Mr. J. J. Harris Tea]a, F.G.S., has described s in basic rocks in the North-western Highlands ++ and at the Lizardw Schistosity in granitoid rocks has also been observed in the Alps by 1)rofessor Bonney]], F.R.S. Besides English workers, several foreign writers, both American and Continental, have declared in favour of an igneous origin for certain schists, so that the produc- tion of a parallel structure in igneous rocks may fairly be regarded as an established fact. The work which I have described in another paper (p. 517) led me *o hope that we might be able to advance a step further. The intru- sion of veins of granite in diorite, under pressure, suggested that at great depths, where pressures were at a maximum and chemical processes might be presumed to be most active, gneissic rocks of a more varied character might be produced. At the town of Galway I had seen gneisses which might have been produced in this way ; but the crystalline schists of the have furnished clear evidence of the genesis of some of the more complex gneisses, be- sides throwing additional light upon the production of the simpler schists. I am able to show that many of the schistose rocks of Malvern have an igneous origin, and I hope that the clues I have obtained will enable me in a future communication to extend my explanations to certain varieties whose genesis is at present less clearly ascertained. Quart. Journ. Geol. See. vol. xli. p. 22!. t Ibid. vol. xl. p. 404. Ibid. vol. xli. p. 133. w Geol. Mug., ~ov. 1886, p. 481. [[ Quart. Journ. Geol. See. Pres, Ad4ress, 1886. Q.ff.G.S. No. 171. 2N Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

526 DR. C. CALLAWAY ON THE GENESIS OF THE

1. THE MiTERIAI, S ?ROOf WHICH [[HE SCHISTS W-ERE PIIODUOE]), Diorites. I have been able to recognize at least four varieties of diorite in~ the schists thus formed. Medium-blaclc (No. 1).--This rock contains about equal pro- portions of hornblende and felspar, and in the mass appears nearly black. The felspar, under the microscope, often presents a cloudy appearance, and contains numerous clear microliths, both conditions indicating alteration. When the alteration is only slight, the twinning of plagioclase is visible, but this occurs only in a minority of the crystals. Coarse-blaclc (No. 2).--The hornblende is often in a greater pro- portion than in No. 1. The crystals of both minerals are larger. The felspars display similar alteration. Coarse-grey (No. 3).--The hornblende is usually about one fourth of the mass. Most of the felspar is less changed, and shows the striping of plagioclase. Medium-grey (No. 4).--The hornblende is abundant, but pale in colour. The felspars display no twinning, are cloudy, with large patches of opacite in the centre, and frequently contain microliths. This variety will be but slightly referred to in the present paper. I am not prepared to say that none of these varieties ever graduate into each other, but I have seen no evidence of a passage in any case. No. 4 is the newest, for veins of it occur in No. 3 ; and No. 3 is probably newer than Nos. 1 and 2, since at North Malvern it con- tains rounded and angular fragments of both. The occurrence of these fragments would seem to indicate a consolidation previous to the intrusion of the younger variety, and this is confirmed by sections in the quarries at North ~[alvern. A mass of No. 2 was seen to be penetrated by a variety of a lighter colour. The vein passed between irregularly shaped joint-blocks, and contained de- tached pieces of the darker kind. In another mass, a fine- grained diorite was intrusive in a coarser variety, and in like manner enclosed large blocks of the older rock. In this respect the mode of intrusion is similar to that of the granite in diorite, as described in my paper on the rocks of Galway. I have noticed, however, that in :Malvern, as well as the town of Galway, there was a more thorough welding together of tile two kinds of rock at their junctions than was observable in the Pre-Silurian granite and diorite of Connemara. Granite. I have no positive evidence of the existence of more than one variety of original granite in the Malvern Hills. This is the well- known binary compound of quartz and red orthoclase. This granite is younger than all the above-named diorites. By its association with them it gives rise to some of the most interest- ing phenomena of the region. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

CRYSTALLINE SCHISTS OF THE ~ALVERN HILLS. 527

Felsite. Near the Wind's Point, there is a well-marked band of felsite hading in accordance with the banding of the enclosing gneiss. It is compact, homogeneous, and of a pale-reddish colour. A similar rock occurs in the , and by its modifica- tion gives rise to schists.

2. ]~u OF PRESSURE. Zones of crushing, indicated by bands of breccia, are very common in the district ; but some of these are posterior to the metamorphism and do not concern us. Some direct evidence of mechanical force, resulting in schistosity, will come out in describing the intimate structure of certain rocks; but it is obviously difficult to obtain very abundant indications amongst igneous masses, where there are no beds to be contorted and faulted. There is reason to believe that the pressure is partly transformed into molecular energy, and thus changes are brought about which often mask mechanical effects. There is, however, some field-evidence for the action of enormous pressures. A good example is seen in a quarry of gneiss at the southern termination of Key's . The foliation-dip is at a moderate angle to the south-east. The rock is traversed by contorted veins of granite, running in several directions. One of these is shown in fig. 1, and gives a rough measure of the pressure.

Fig. 1.--Contorted Granite-vein in Gneiss.

Assuming the vein originally to have been straight, its length has been reduced in the proportion of about five to two. The regional pressure has acted with intensity only at intervals along the range. In the North Hill and the northern part of the :Beacon there are large masses of diorite which have undergone little mechanical change; but between the summit of eBeacon and the" Wyeh there are numerous alternations of diorite and granite displaying foliation. The long ridge between the Wych and the Wind's Point contains a great deal of black diorite 2N2 Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

528 DR. C, C&LLAWAY ON THE GENESIS O~ THE with granite-veins; but at intervals, and especially at the southern end, a banded structure has been produced. The northern part of Swinyard's Hill is composed of massive granite, while at the southern end is a flaggy gneiss, which has been produced from the granite by pressure. Other examples of the localization of pressure occur in the hills further south.

3. THE PRODUCTS OF THE METAMORPHISM. A. ~im~le Schists, or those formed from one lcind of re&. Hornblende-gneiss, formed from Diorite.--In this ease the pres- sure has rearranged the constituent minerals, so that the horn- blende and felspar lie in irregular folia. There are many grada- tions between the ordinary diorite and a gneiss in which there has been some reconstitution of the minerals, the formation of quartz being the most conspicuous result. The coarse-black diorite, at ~orth Malvern, is one variety which has been modified into gneiss. For about two thirds of the breadth, the intrusion of the vein was clearly seen; then, for a few inches, the two kinds of diorite were confusedly mingled. The remainder of the breadth, consisting only of the coarser diorite, was rudely foliated. Mica-gneiss, formed from Granite.--Near the southern end of Swinyard's Hill, on the crest of the ridge, is a very interesting case of the formation of gneiss by crushing. A narrow band, striking across the axis, has the appearance of the ordinary binary granite; but a laminated structure is very apparent even in the field. In immediate contact, forming part of the same mass, the rock is flaggy, and seams of mica appear. Then comes a break, but flaggy schists of the same general type appear in force a few hundred yards to the north. A description of microscopic slides will bring out the transition indicated. No. 281. This was taken as a typical specimen of the granite which is seen in mass at the northern part of the hill. It is the ordinary compound of orthoelase and quartz, with a little mica. Most of the felspar is suffused with a brownish tinge, probably iron- oxide, and presents a cloudy appearance. Many of the crystals also contain patches or mieroliths of clear mica, polarizing in brilliant colours. The felspar has therefore undergone partial decomposition. That this granite contains iron would appear from the analysis of the Rev. J. H. Timins% He states that the "quartzo-fclspathie" veins, which are almost certainly the granite I am discussing, furnished in three analyses iron-oxide varying between "92 and 1"52 per cent. Both quartz and felspar are some- what cracked, and iron-oxide is deposited in the cracks. Nos. 282-284.--From the locality at the southern end of the hill. No. 282. From a part of the laminated granite which appears more granitoid than the rest. The rock is excessively cracked and crushed, the cracks, which run in all directions, but predominantly * Quart. gourn. Geol. Soc. vol. xxiii, p. 362. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

CRYSTALLINE SCHISTS OF THE ~fALVERN HILLS. 529

with the parallel structure, being filled with either iron-oxide or mica. The quartz lies in wedges rather than folia. They are approximately parallel, roughly lenticular in section, and rarely continuous across the slide. Sometimes lines of quartz-grains curve round the felspars. In one part of the field an angular bit of quartz is immersed in a confused mass of quartz and felspar in granules. The parallelism of the quartz seems at some points to be determined by cracks, which are occupied by infiltrated products. The felspar has the brown cloudiness of the uncrushed granite. It is very much cracked, and where crystals are defined they are rather flattened, and often tail out to a point on each side, so as to resemble a human eye in shape. Some of the felspar, forming bands of small crystalline grains between the quartz folia, is very dirty. Mica is very small in quantity. It occurs in some of the narrow cracks in quartz, and in some of the felspar, andrarely it forms a sort of sheath to the attenuated ends of the eye-shaped crystals. No. 283. From the same piece as the last, within a few inches of it, but showing such clear lamination that it caught my eye at a distance. Under the microscope it is much more like a gneiss. The field is clearer, and there is much less cracking of the minerals. The folia of quartz are longer, thinner, and more uniform in thickness. In a few part s the quartz is traversed by longitudinal cracks, but their mode of origin is less evident. ]~[ost of the felspar is in regular folia of small crystalline grains, but there still remain a few of the larger felspars with rounded and irregular outlines. Some distinct folia of mica now make their appearance amongst the felspar, but the quantity is still small. There has evidently been much reconstruction of the minerals in this slide. No. 284. Part of the same block, but with the flaggy structure, and showing in the field dark seams of mica. The quartz-folia are still longer than in No. 283 and more regular in thickness. In some spots a thin folium bends out of its course round a crystal of felspar. The felspar is similar to the last. Seams of small granules of this mineral also are seen to curve out of the straight line round large crystals. The notable difference between this and all the preceding slides is in the much greater proportion of mica. It often occurs in regular folia between the quartz. Sometimes it forms a complete sheath to an eye-shaped felspar-crystal. In one place a felspar has been cracked obliquely across, and the crack is filled in with mica. Thus each half of the crystal forms an almond-shaped "eye," with its fringe of mica. There can be no question that this mica, which is the same in all the set of slides, has been formed out of the felspar. No. 285. From the flaggy schists to the north of the last. The structure of the rock strongly suggests a similar origin. There is more quartz, which here and there looks as if the lines separating its folia had originally been cracks. Several cracks also cut across the foliation, and these are filled in with mineral matter, which in one spot is seen to be optically continuous with a regular folium. A~ another point an elongated granule of quartz, forming par~ of Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

530 DR. C. CAI~AWAY ON TIIE GENESIS OF THE a folium, passes across a transverse crack which therefore must be older than the fohum, and presumably older than the general foliation in its completed state. Where a transverse crack passes through quartz, it sometimes vanishes for a little distance, reappearing further on, so that it is clear that the sides of the crack have come together and become welded. The occurrence of these transverse cracks, and their evident connexion with the process of metamorphism, is a piece of evidence which tends strongly to confirm the suggestions made by the structure of the folia. Mica.schist, formed from Felsite.--A very interesting section (fig. 2), about 30 feet in length, is seen in the slope at the end of tho

Fig. 2.--Passage of Felsite into Schist.

_..1::.,:,',,-- ,

1. Crushed Felsite. 2. Schistose Felsite. 3. Schists. south-eastern spur of the Raggedstone Hill. Taken in descending order, the following are the rocks observed :-- 1. Pale-reddish felsite, so jointed and crushed that it was difficult to obtain a piece large enough for a microscopic slide, and the specimen selected broke into fragments in grinding. This rock forms a band, striking across the ridge in concordance with the foliation of the underlying schists. Under the microscope the rock is seen to be entirely devitrified, and the microcrystaUine structure of a typical felsite is very apparent. Even minute fragments, which have remained entire in grinding, arc breceiated. The cracks are usually marked by iron-oxide, and, occasionally, when wider than usual, are filled by quartz or by mica. There is a rough parallelism in the structure, and this incipient foliation is some- times accentuated by a little mica. 2. A few feet below the last, and separated from it by soil. The rock is still felsite, much: sounder under the hammer than No. 1. Microscopically examined, the field is seen to be clearer, the parallel structure is more distinct, and there is a larger pro- portion of mica. This rock has evidently been porphyritic, for there are several deformed masses of quartz on the slide. One of these is eye-shaped, and tails out at each end in a stream of minute granules. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

CRYSTALLINE SCHISTS OF THE MALVERN HILLS. 531

3. About 15 feet of schistose rock, forming a band below No. 2, but continuous with it. Some of it is hardly distinguishable from the rock above, but the laminated structure is more evident. In other seams the foliated appearance is more marked, and a complete :gradation can be traced between the modified felsite and a true schist. The change is seen even on the ground in the increasing distinctness of the lamination proceeding pari passu with the growing thickness of the films of mica on the planes of fissility. These indications are entirely confirmed by the microscope. The following gradation is seen in three specimens taken from the same band, within a yard or so from each other. No. 294. Felsitic appearance in hand specimens, but slightly laminated. Under the microscope the parallel structure is seen to be due to intermittent folia of a green mica, in irregular bundles of fibres and sometimes dirty. This part of the rock also must have been a porphyry, for the slide shows several eye-shaped masses of ~crushed quartz which have caused the folia of mica to curve out of their course, forming, as it were, eyebrows to the quartz, both above and below, just as in the crashed granite described above (p. 529). Some parts of the slide display the felsitic structure, as above; but where the mica is most abundant the granules of the ground-mass are often of larger size and polarize in bright colours. Mineral differentiation would thus appear to have proceeded a stage further. No. 295. The change in the re]site is more advanced. The rock chiefly consists of elongated granules of quartz arranged in a linear manner, with mica lying between them in microliths, so as some- times to form a pa~ial sheath. Some of this mica is transparent, polarizing in bright colours. Patches of the same mica and some felspar are also present. Distinct seams of mica and felspar, parallel to the longer axes of the quartz-grains, accentuate the foliation. No. 296 is generally similar to the last. In about the middle of the slide is a very quartzose seam, in which the grains are much larger than in the previous specimens. It is not a vein, but a true folium, parallel to the rest. It passes by the gradual introduction of mica into a broad, very mieaceous band, which graduates in- sensibly into a zone displaying a structure strongly suggestive of the micro-felsite, but a few microliths of clear mica are present. Some parts of the hand-specimen also have a very felsitic look under the lens. No. 299 is from one of the several narrow micaceous bands inter- laminated with the more quartzose schist. It is an indubitable mica-schist. The mica is the white variety. It forms about half the mass, a great part of it being in distinct folia, which sometimes undulate. Some of the quartz has the same appearance as in Nos. 295, 296, the granules being more or less sheathed with mica. i few lenticular "eyes" of quartz are rather suggestive of the crushing ~of a porphyry; but on this I do not speak decisively. I am quite satisfied, from a very careful study on the ground, that these micaceous seams cannot be regarded as foreign fragments of schist ~ntangled in the crushed felsite. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

532 DR. C. CALLAWAY ON THE GENESIS OF THE

These modified felsites, now schists, form a low vertical cliff, 30 or 40 yards long, in which the rock is continuously exposed. I worked along the strike to the western end and found similar schists, some of them highly quartzose, passing occasionally into a material like a quartzite, and into felsitic rock like the first named. The different varieties were not always interbanded, but often passed into each other with some irregularity. That they all belong to the same mass I have no doubt. The rapid variations in the metamor- phism agree with the sudden changes noticed in the crushed granite. A specimen of one of the quartzose varieties, almost like a quartzite, was examined microscopically. In structure it is intermediate between a quartzite and a quartz-schist. There is very little orien- tation in the quartz, which is frequently in large granules. )Iica is in small proportion. Much of it is in clear microliths, which occasionally form a partial sheath to the quartz-granules, as in the other schists of the locality, but more frequently they have a rough orientation in one direction; occasionally they accumulate into. imperfect folia. Parallel with this foliation are several cracks, which are more or less filled in with mica and iron-oxide. Some, if not most, of the mica in the cracks is the same white variety which prevails throughout the slide. This parallel cracking, coincident~ with the foliation, is another interesting analogy between this schist, and the crushed granite, and is of course suggestive of similarity of causation.

B. Injection-Schists, or those in which the .Banded Structure is due to the Parallelism of Intrusive Veins. Two varieties of this rock are here described. DuTlex Diorite-gneiss, formed from veins of Diorite in Diorite.-- This fine-banded rock is common in one of the quarries at North ~[alvern. Parallel seams of grey granitoid diorite (No. 3) are enclosed in a black variety (probably :No. 1). The differences between the ordinary diorites and their gneissic representatives cer- tainly do not militate against my theory of the origin of the latter. The following points may be noted :- In the massive black diorite there is some epidote and chlorite and a little green biotite, but in the gneiss there is a much larger pro- portion of biotite, and most of it has a definite orientation parallel to the direction of the adjacent vein. Some h~ematite also, which is in lath-shaped forms, is similarly orientated. The felspar of the ordinary diorite is cloudy, rarely showing plagioelase-twinning, and contains numerous clear microliths. In the gneiss the felspar of this diorite is rather clearer and sometimes displays striping. Comparing the unfoliated grey diorite with the grey variety in the gneiss, there is no material difference observable. The crystalli- zation of the latter is larger, but in both cases the felspar is pre- dominantly plagioclastic, and the proportions of hornblende, biotiter epidote, and quartz are about the same. Owing to the want of continuous sections, I was unable to trace Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

CRYSTALLINE SCHISTS OF THE ~IALT]]RI~ HILLS. 535 an actual passage between the massive and the foliated diorites ; but if the former were diorites, so were the latter, and I submit that there is nothing in the parallelism of the veining inconsistent with the theory of an igneous origin. :But the description of the next variety of gneiss will throw additional light upon this question. Granite-diorite-gneiss, formed from veins of Granite in Diorite.- This is the most conspicuous gneiss in the Malvern Hills. The gneiss itself and the rocks out of which it is constructed constitute about one half of the entire mass of the range. The diorite is black and of medium grain (No. 1). The production of the banded struc- ture is well seen in the long ridge extending between the Wych and the Wind's Point. At the western quarry on the south side of the Wych there is a muss of the diorite with granite veins. It appears to pass into the gneiss of which the section chiefly consists, but junctions are obscured by d~bris. Similar rocks, in which there is the like association of massive and foliated mixtures of the diorite and the granite, are seen at intervals along the crest of the ridge to the south. At the top of the third summit the relations of the rocks are well seen. At one spot the granite is intrusive in the ordinary irregular veins, but it passes rather abruptly on the north into a rock in which the veins strike in a definite direction to the north-west, producing the banded structure of a gneiss. This rock is also well exposed about the Wind's Boint. The granite-seams in the gneiss vary considerably in thickness.. Sometimes they are continuous for yards, but frequently they are lenticular in section. They often behave like veins in their rapid attenuation and i in their branching habit. Their parallelism is by no means uniform, as they sometimes pass obliquely across the inter- banded diorite. Comparing microscopic specimens of this gneiss with slides from the unfoliated vein-structures in the Wyeh quarry, I do not hesitate to say that it would be impossible to determine which was gneiss and which was vein-structure. In both cases there is some epidote and chlorite produced in the diorite at the~ junction of the granite, and for a little distance from it, while the granite is slightly cracked. This banded gneiss is, then, a binary mixture of igneous rocks, i~ which regional pressure has produced a parallelism of the granite- veins. 4. GENERAL RE, iRKS. As this paper is strictly introductory, the evidence offered is in- complete; but it will perhaps suffice to prove that some of the ~[alvern schists are produced out of igneous rocks, and to create a presumption in favour of a similar origin for other varieties. In the formation of some of the schists, the chemical and mineral changes have been very great; but into this division of my subject I have barely entered in the present communication. Association of the Gneissic t~ocs with the Igneous masses.--It is generally true that particular varieties of gneiss and schist occur ia the vicinity of the igneous masses to which they are respectively Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

534 DR. C. CALI,AWAI r O~ TKE G~I~SIS OF THE most nearly related in mineral composition, k few examples are here given. In the North Hill we have large masses of several kinds of diorite penetrating each other in veins, and it is in this locality that we find the diorite-gneisses, simple and duplex. North of the Wych the above-named diorites are intermixed with granite, and here we have a variety of gneissie rocks of more com- plex structure. Between the Wych and Swinyard's Hill there is little besides black diorite and granite, and here we chiefly find the banded granite- diorite-gneiss. Swinyard's Hill consists largely of granite, and it is in this ridge that the flaggy quartzo-felspathic schists occur. In there are masses of coarse diorite, and the gnoissic rocks in association with them are certainly more allied in mineral composition with this diorite than with any other igneous rock of the region. Raggedstone Hill contains schists widely differing from any of the above, and here the associated igneous rocks are felsite. These associations cannot be due to accident, and even if no direct proof of actual conversion could be offered, they would be of weight in the argument. Absence of Stratif~cation.--]~xcept ~ perhaps in the Raggedstone ]:Iill, I could detect no true bedding in the crystallines of the :Malvern ]~ills. The zones of igneous and foliated rock, though they have a predominantly north-west strike, behave more like veins than strata. Where a sufficiently large surface is exposed in plan, we find the bands, whether massive or schistose, rapidly thin out. For example, on the crest of the ridge about half a mile north of the Wyeh the attenuation is usually from east to west. :Fig. 3 shows a pare of one of the exposures in this locality. Age of the t~ocIc.--I see no reason to doubt the received views as to the age of the greater part of these rocks. At the south-western extremity of the Raggedstone Hill, the Ho]lybush Sandstone rests at a low angle upon the edges of nearly vertical schists. The old rocks of the Salopian district afford confirmatory evidence. The Uriconian conglomerate of Charlton Hill contains several varieties of plutonic rocks, most of which can be matched in the Malverns, and these Uriconians are themselves older than the Longmynd series t. It is possible that the felsites and the schists formed from them are of a younger epoch. Period of Metamorphism.--The most effective pressures may have acted at more than one period ; but there is no doubt that the meta- morphism was substantially complete before Cambrian times, since it is incredible that a force producing a strike transverse to the ridge could have acted without dislocating the strike of the flanking Cambrian and Silurian strata. After further research I think it no longer necessary to make even this :slight reser~'ation.--C. C., July 20th. t Quart. Journ. Geol. Soc. vol. xlii. p. 481. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

CRYSTALLIIgE SCHISTS OF THE MALYEI%N HILLS. 535 Schistosity, whether produced before or after ConsoHdation.--The evidence I have submitted points towards the latter alternative. In some of the diorites of the North Hill we have seen that the in- trusive veins passed along planes of jointing and contained fragments of the enclosing rock, as in Galway. Then, too, the schists formed from the granite and the felsite are the effect of crushing. I prefer, however, to postpone the more adequate discussion of this question.

Fig. 3.--Plan of Vein-structure in Schistose t~oclcs.

m/~ ~',,'111~,'- I/l;',V~lll."; 7,,',',~{- ~'" q,, ,,"x .~"'

,i; t .:.,7,>) ,, '"l~t~f/lll, l)11Jl/llllllll~)'" JlYt/tttlIII lip I I~. 2 lIB/I;i!i i , "d/ltltllft/ l l~ "" ", .... '/ll~lllllllllllfl& ,~.s.~:lii~.~;,~m): .,, 7=:% n A/if~Ill//I/ill I Ih,~: , ~, -,. f, llI//l flllli l "-:'-"' - C A. Coarse granitoid rock with biotite. B. Bands composed of granite-veins, with seams of dark mica and of coarse rock (A) foliated. C. A. vehl macroscopicall,y like a fine-grained diorite. Under the microscope it is seen to consist of hornblende, two micas, epidote, and quartz, and has a rude foliation in a specimen taken at the margin. D. Black schist, chiefly mica, penetrated by small granite-veins.

1. ~[any of the gneissic and schistose rocks of the Malvern Hills were formed out of igneous masses and veins. Amongst the materials which underwent the metamorphism were several varieties of diorit% a granite, and a felsite. 2. The parallel structure has been caused by regional pressure. This conclusion is proved by the intense contortion of granite-veins, and by the mechanical effects recognized in the rocks under the microscope. 3. The products of the metamorphism are divided into (1) Simple and (2) Injection-schists, the ibrmer elaborated out of one kind of rock, the lat~er out of at least two kinds, one being intrusive in the Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016

536 ON TH~ CRYSTALLINE SCHISTS 01~ THE MALYE~N HILLS. other. The Simple Schists described are hornblende-gneiss, formed from diorite, mica-gneiss from granite, and mica-schist from felsite. The Injection-schists noticed were duplex diorite-gneiss, composed of veins of diorite in diorite, and granite~iorite-gneiss, of veins of granite in diorite. In conclusion it was observed that :-- (1) Particular varieties of gneiss and schist generally occurred in the vicinity of the igneous masses to which they were most nearly related in mineral composition. (2) No true stratification was detected, the bands of igneous and of foliated rocks thinning out rapidly, in the manner of veins. (3) The received view of the age of the greater part of the rocks was not affected by the conclusions of the Author. (4) The chief metamorphism was completed before the Cambrian epoch. (5) Some, at least, of the sehistosity had been caused subsequent to consolidation.

(For the Discussion on this paper, see p. 514.)