k 



Nov. 22, r888] 



NATURE 



9^ 



ias been so situated that any appreciable lateral move- 

 ment has been impossible ; it has yielded like a block in a 

 crushing-machine. In the latter, a differential lateral movement 

 of the particles has been possible, and it has prevailed when (as 

 in the case of an overthrust fault) the whole mass has not only 

 suffered compression, but also has travelled slowly forward. 

 Obviously, the two cases cannot be sharply divided, for the 

 crushing up of a non-homogeneous rock may render some local 

 shearing possible. Still it is important to separate them in our 

 minds, and we shall find that in many cases the structure, as 

 a whole, like ihe cleavage of a slate, results from a direct crush ; 

 while in others the effects of shearing predominate. The latter 

 accordingly exhibit phenomena resembling the effectsof a tensile 

 stress, ^laterials of a like character asmme a more or less 

 linear arrangement, the rock becomes slightly banded, and ex- 

 hibits, as has been said, a kind of fluxion structure. This 

 phrase, if we are careful to guard ourselves against mijconcep- 

 tion, is far from inappropriate. The mass gradually assumes a 

 fragmental condition under the pressure, and its particles as they 

 shear and slide under the effects of thrust, behave to some extent 

 like those of a non-uniform mass of rock in a plastic condition, 

 as, for example, a slaggy glass. But we must be on our guard, 

 lest we press the analogy too far. The interesting experiments 

 ' ich have been made on the flow of solids, and on rolled-out 



tic substances, while valuable as illustrations, represent, as 

 ems to me, a condition of things which must be of rare 

 rrence in a rock mas>, pulverized by mechanical forces only, 

 am to reason from them, ,1 must regard the rock not as a 

 ental solid — if the phrase be permissible — but as an im- 



fect fluid ; that is to say, I must consider them as illustrative 



structures in rocks which have yet to assume — not have 

 dy assumed — a crystalline condition. 



lustrations of, the effects of direct crushing in a granitoid 

 are common in the Alps. Those of a shearing crush are 

 ificently developed near the great overthrust faults in the 



hwest Highlands of Scotland. 



n the former case, where a granitoid rock has been affected 



y to a moderate extent, and the resulting rock in a hand 

 specimen would be called a gneiss without any very definite 

 mineral banding, we find that under the microscope it exhibits a 

 fragmental structure, the felspars are often somewhat rounded in 

 outline, are frequently rather decomposed and speckled with 

 minute flakes of while mica of secondary origin, and commonly 

 seem to "tail off" into a sort of stream of microlithic mica, 

 which has doubtless resulted from the destruction of felspar, the 

 residual silica making its appearance as minutely crystallme 

 quartz. The original quartz grains have been broken up, and 

 are now represented by snialler grains, often in rudely lenticular 

 a^regates, like little " inliers" of quartzite. The original flakes 

 of black mica have been tattered and torn, and now appear as 

 streaky clusters of flake'ets, often less than one-sixth the original 

 length. In extreme cases of crushing, the felspar has almost 

 disappeared ; the constituents are all reduced in size, and the rock 

 at first sight would no longer be called a gneiss, but a fine-grained 

 mica-schist. It has become extremely fi.'^^sile, and the flat faces 

 of the fragirents exhibit a peculiar sheen, as if it had received a 

 varnish of microlithic mica. In short, from a granitoid rock a 

 microcryslalline mica-schist has been produced, which, however, 

 differs markedly from the rock to which ihat name is ordinarily 

 applied. 



Let us now turn to a rock of similar nature, in which the effect 

 of .shearing is more conspicuous. I have selected a specimen, in 

 which, as in the first example above, some of the felspar still 

 reiuains in recognizable fragments. These, however, are com- 

 monly destitute of the " tail " of mica-microliths, and bear, at first 

 sight, some resemblance to the broken porphyritic felspars which 

 occur in a ihyolite. The mica, whether primary, but fragmen- 

 tal, or secondary, tends to get associated in undulating layers ; 

 the quartz also has a more uniform aspect and a more linear 

 arrangement. In the most extreme cases the felspar all but 

 disappears (though I fancy that it has here a belter chance of 

 surviving), the quartz and the mica are more and more aggre- 

 gated in definite but thin bands, and the former, when viewed 

 with crossing nicols, exhibits streaks, which, for a considerable 

 distance, are almost uniform in tint, as if its molecules under a 

 fetress definite in direction had acquired a polarity, so that groups 

 of these act upon light almost like a single crystal. 



The effects of mechanical deformation, followed by mineral 

 change, are also remarkably conspicuous in the case of pyroxenic 

 rocks. Augite, it is well known, is by no means a stable mineral, 



and under certain circumstances is readily transformed into- 

 hornblende. This occurs in more than one way without mech- 

 anical action, but of these I do not now speak. Only of late 

 years, however, has it been known that pressure can convert a 

 dolerite into a hornblende-schist. Of this, through the kindness 

 of Mr. Tcall, who first proved the occurrence of this alteration 

 in Great Britain, I can show you an example. The rock, as 

 you see, has lost the structures of a dolerite, and has assumed 

 those characteristic of many hornblende-schists. I say of 

 many, because, though the rock .is distinctly foliated, it does 

 not exhibit a conspicuous mineral banding. My own observa- 

 tions confirm those of Mr. Teall, though I have never been so 

 fortuna'e as to obtain, as he did, a complete demonstration of 

 the passage from the one rock to the other. 



It seems, then, to be demonstrated that, by mechanical de- 

 formation, accompanied or followed by molecular re-aTange- 

 ment, foliated rocks, such as certain gneisses and certain schists, 

 can be produced from rocks originally crystalline. But obviously 

 there are limits to the amount of change. The old proverb, 

 "You cannot make a silk purse of a sow's ear," holds good in 

 this case also. To get certain results, you must have begun- 

 with rocks of a certain character. So that it is often pos-ible, 

 as I believe, to infer not only the nature of the change, but also 

 that of the original rock. Hitherto we have been dealing with 

 rocks which were approximately unifoi m in character, though 

 composed of diverse materials— that is, with rocks more or less 

 granular in aspect. Suppose, now, the original rock to have 

 already acquired a definite structure — suppose it had assumed, 

 never mind how, a distinct mineral banding, the layers varying 

 in thickness from a small fraction of an inch upwards. Would 

 this structure survive the mechanical deformation ? I can give 

 an answer which will at any rate carry us a certain way. I car* 

 prove that subsequent pressure has frequently failed to obliterate 

 an earlier banded structure. In such a district as the Alps we 

 commonly find banded gneisses and banded schists, which have 

 been exposed to great pressure. Exactly as in the former case, 

 the new divisional planes are indicated by a coating of films of 

 mica, by which the fissility of the rock in this direction is in- 

 creased. The mass has assumed a cleavage-foliation. I give it 

 this name because it is due to the same cause as ordinary cleav- 

 age, but is accompanied by mineral change along the planes of 

 division; while I term the older structure stratification-foliation, 

 because so frequently, if it has not been determined by a 

 stratification of the original constituents, it is at any rate a most 

 extraordinary imitation of such an arrangement. In many cases- 

 the new structure is parallel with the old, but in others, as ir> 

 the " strain-slip " cleavage of a phyllite, the newer can be seei> 

 distinctly cutting across the older mineral banding. As an. 

 example, take a rock mainly consisting of quartz and mica. 

 Sometimes there has been a certain amount of crushing of the 

 constituents, followed by a re-crystallization of the quartz and 

 the formation of a pale-coloured mica. Sometimes, when the 

 direction of the disturbance has been at right angles to the 

 stratification-foliation, the latter is made wavy, and the mica- 

 flakes are twisted round at right angles to their original 

 position. Sometimes there has been a dragging or s-hearing of 

 the ma s, so that a considerable amount of mica has been re- 

 crystallized along the new planes of division. To put it briefly, 

 I assert, as the result of examining numbers of specimens, that 

 though in certain cases the new structure is dominant, a practised 

 eye seldom fails to detect traces of the older foliation, while ir> 

 a large number of instances it is still as definite as the stripe in 

 a slate. 



We have got, then, thus far, that pressure acting on rocks 

 previously crystallized can produce a foliation ; but when it ha» 

 acted in Palaeozoic or later times, the resulting structures can be 

 identified, and these, as a role, are distinguishable from lho«e of 

 the most ancient foliated rocks, while at present we have found 

 no proof that pressure alone can produce any conspicuous mineral 

 banding. I am aware that this statement will be disputed, but 

 I venture to plead, as one excuse for my temerity, that probably 

 few persons in Great Britain have seen more of crystalline rocks, 

 both in the field and with the microscope, than myself. So, 

 while I do not deny the possibility of a well-banded rock being 

 due to pressure alne, I unhesitatingly aflirm that this at present 

 is a mere hypothesis — an hypothesi-, moreover, w hich is attended 

 by serious difficulties. For, if we concede that, in the case of 

 maipy rocks originally granular, dynamic metamorphism has 

 produced a mineral banding, this is only on a very small scale r 

 the layers are but a small fraction of an inch thick. No one 



