Nov. 2, 1888.] 



SCIENTIFIC NEWS. 



4&3 



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

 kindness of Mr. Teall, who first proved the occurrence of 

 this alteration in Great Britain, I can show you an 

 example. The rock has lost the structures of a dole- 

 rite, and has assumed those characteristics of many 

 hornblende-schists. I say of many, because though the 

 rock is distinctly foliated it does not exhibit a conspicu- 

 ous mineral banding. My own observations confirm 

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

 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 

 deformation, accompanied or followed by molecular re- 

 arrangement, 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 possible, 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 uniform 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 de- 

 finite 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 deforma- 

 tion ? I can give an answer which will at any rate 

 carry us a certain way. I can 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 increased. The mass has 

 assumed a cleavage-foliation. I give it this name be- 

 cause it is due to the same cause as ordinary cleavage, 

 but is accompanied by mineral change along the planes 

 of division, while I term the older structure stratifica- 

 tion-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 in the " strain- 

 slip" cleavage of a phyllite, the newer can be seen dis- 

 tinctly 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 recrystallisa- 

 tion of the quartz and the formation of a pale-coloured 

 mica. Sometimes when the direction of the disturb- 

 ance 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. Some- 

 times there has been a dragging or shearing of the mas 5 , 

 so that a considerable amount of mica has recrystallised 

 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 domin- 

 ant, a practised eye seldom fails to detect traces of the 

 older foliation, while in 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 crystallised can produce a foliation ; but 

 when it has acted in Palaeozoic or later times, the result- 

 ing structures can be identified, and these as a rule arc 

 distinguishable from those 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 dis- 

 puted, but I venture to plead, as one excuse for my 

 temerity, that probably few persons in Great Britain 

 have seen more crystalline rocks, both in the field and 

 with the microscope, than myself. So while I do not 

 deny the possibility of well-banded rock being due to 

 pressure alone, I unhesitatingly affirm that this at pre- 

 sent is a mere hypothesis, a hypothesis moreover which 

 is attended by some serious difficulties. For if we con- 

 cede that, in the case of many rocks originally granular, 

 dynamic metamorphism has produced a mineral banding, 

 this is only on a very small scale ; the layers are but a 

 small fraction of an inch thick. No one could for a 

 moment confuse a sheared granite from the Highlands 

 with a Laurentian gneiss from Canada or with an unin- 

 jured Hebridean gneiss. For the former to attain to the 

 condition of the latter, the mass must have been brought 

 to a condition which admitted of great freedom of motion 

 amongst the particles, almost as much, in short, as 

 among those of a molten rock. Clearly the dynamic 

 metamorphism of Palaeozoic or later ages appear to 

 require some supplementary agency. Can we obtain 

 any clue to it ? 



An explanation of broadly banded structures was long 

 since suggested, and has recently been urged with addi- 

 tional force, which avoids some of cur difficulties. We 

 know that the process of consolidation in a coarsely crys- 

 talline rock has often been a slew one ; the constituent 

 minerals separate gradually from the magma, of which 

 sometimes so little may remain that a rock with a true 

 glassy base has been mistaken for one holocrystalline. 

 The residual and still unsolidified magma would admit 

 of a slow flowing of the mass, but there would be so 

 little of it that the crystals already individualised, though 

 altered in position by differential movements, would be 

 affeced by strains, and liable to fracture. Such a iock, 

 when finally consolidated, would exhibit many pheno- 

 mena in common with a rock modified by dynamic 

 metamorphism, but would differ in the greater coarse- 

 ness of its structure. This may prove to be the correct 

 explanation of the curious foliated and banded gabbros 

 in the Lizard district. That some crystalline rocks must 

 have passsed through this stage I am now in a position 

 to affirm, from evidence not yet published. 



Let us, however, see whether another line of investi- 

 gation may not throw some light on our difficulty. I 

 have already mentioned the effect produced by the 

 intrusion of large masses of igneous rocks upon other 

 rocks. These may be either igneous rocks already 

 solidified, or sedimentary rocks. The former may be 

 passed over, as they will not materially help us. In 

 regard to the latter, the results of contact-metamorphism, 

 as it is called, are, as we might expect, very various. 

 Speaking only of the more extreme, we find that sand- 

 stones are converted into quartzites ; limestones become 

 coarsely crystalline, all traces of organisms disappearing, 

 and crystalline silicates being formed. In clayey rocks 

 all signs of the original sediments disappear, crystalline 

 silicates are formed, such as mica (especially brown), 

 garnet, andalusite, and sometimes tourmaline; felspar, 

 however, is very rare. Fair-sized grains of quartz 



