90 



NATURE 



[Nov. 2 2, 1888 



phosed, the old structures have been obliterated, new minerals 

 iiave been developed, and these exhibit that peculiar orienta- 

 tion, that radely parallel arrangement, which is called foliation. 

 Except for this some masses are fairly homogeneous, while some 

 exhibit a distinct mineral banding which is usually parallel with 

 the other structure. These rocks are the gneisses and schists — 

 the latter term, often vaguely used, I always restrict to rocks 

 which exhibit a true foliation. In some schists the mineral 

 constituents are comparatively minute, in others they are of 

 ■considerable size. In the former case we may often venture to 

 affirm that the rock is a metamorphosed sediment ; in the latter 

 its original condition is a matter of conjecture. Rocks of the 

 former class often appear, to use no stronger word, to lie above, 

 and so to be less ancient than those of the latter, and beneath 

 that comes a coarser and more massive series, in which granitoid 

 rocks are common. In these last foliation is often inconspicuous, 

 and the rocks in consequence are not markedly fissile. 



That these rocks are older than the Cambrian can often be 

 demonstrated. Sometimes it can even be proved that their 

 present distinctive character had been assumed before the over- 

 lying Cambrian rocks were depo;ited. Such rocks, then, we 

 may confidently bring forward as types of the earth's foundation- 

 stones. As the inscriptions buried in the Euphrates Valley tell 

 us the tongue of Accad in the days prior to the coming of the 

 Semite, so these declare what then constituted the earth's crust. 

 If in such rocks we find any peculiarities of mineral compositi m 

 or structure, these may legitimately be regarded as distinctive. 

 We have only to beware of mistaking for original those which 

 are secondary and subsequently impressed. 



In other parts of the world we find rocks of like characters 

 with those above named, the age of which cannot be so precisely 

 •fixed, though we can prove them to be totally disconnected from 

 and much older than the earliest overlying stratum. To assert 

 that these rocks are contemporary with the others is obviously 

 an hypothesis which rests on the assumption that community of 

 structure has some relation to similarity of origin. I am well 

 aware that attempts have been made to discredit this. But if we 

 eliminate difficulties which are merely sophistical — those, I mean, 

 created by the use of ambiguous or misleading terms — if we 

 acknowledge those due to our limited means of investigation, 

 such as that of distinguishing a rock crushed in sitjc from one 

 composed of transported fragments — in other words, of separating 

 in every case a superinduced from a primary structure, and if we 

 allow for others due to the limitation of our instrumental and 

 visual powers, I do not hesitate, as the result of long and, I 

 hope, careful work, to assert that certain structures are very 

 closely related to the past history of a rock, and that in very 

 many instances our diagnosis of the cause from its effect is not 

 less worthy of confidence than that of an expert in pathology or 

 physiology. Resemblances of structures, different in origin, do, 

 no doubt, sometimes occur — resemblances not seldom due to 

 partial correspondence in the environments ; but in regard to 

 these it is our duty to labour patiently till we succeed in dis- 

 tinguishing them. The difficulty of the task does not justify us, 

 either in abandoning it in despair, or in sitting down, after a few 

 hasty observations, to fashion hypotheses which have no better 

 foundation than our own incompetence or idleness. 



As it is impossible in the time at my disposal to demonstrate 

 the proposition, I must assume what I believe few, if any, com- 

 petent workers will deny, that certain structures are distinctive 

 of rocks which have solidified from a state of fusion under this 

 or that environment ; others are distinctive of sedimentary rocks ; 

 others again, whatever may be their significance, belong to rocks 

 of the so-called metamorphic group. I shall restrict myself to 

 indicating, by comparison with rock structures of which the 

 history is known, what inferences may be drawn as to the 

 history of the last-named rocks, which, as I have already stated, 

 are in some cises examples of the earth's foundation-stones, 

 while in others, if they are not these, they are at any rate 

 excellent imitations. 



Let us proceed tentatively. I will put the problem before 

 you, and we will try to feel our way towards a solution. Our 

 initial difficulty is to find examples of the oldest rocks in which 

 the original structures are still unmodified. Commonly they are 

 like palimpsests, where the primitive character can only be 

 discerned, at best faintly, under the more recent inscription. 

 Here, then, is one of the best which I possess — a Taurentian 

 gneiss fron Canada. Its structure is characteristic of the whole 

 ;group ; the crystals of mica or hornblende are well defined, and 

 •commonly have a more or less parallel arrangement ; here and 



there are bands in which these minerals are more abundant than 

 elsewhere. The quartz and the felspar are granular in form ; the 

 boundaries of these minerals are not rectilinear, but curved, 

 wavy, or lobate ; small grains of the one sometimes appear to be 

 inclosed in larger grains of the other. Though the structure of 

 this rock has a superficial resemblance to that of a granite of 

 similar coarseness, it differs from it in this respect, as we can 

 i-ee from the next instance, a true granite, where the rectilinear 

 outline of the felspar is conspicuous. Here, then, is one of our 

 problems. This difference of structure is too general to be 

 without significance. What does it mean ? 



It is more difficult to obtain examples of schist of like 

 geological age, wholly free from subsequent modification. 

 Apparently the structure and composition of the rock have 

 rendered it more liable to disturbance. But those exhibited, 

 though by no means perfect examples, may serve to indicate the 

 structure of an Archaean schist, consisting mainly of quartz and 

 mica. We may take them as representative of a considerable 

 series of rocks, which are often associated in such a manner 

 as to suggest that, notwithstanding their present crystalline 

 condition, they had a sedimentary origin. Can this inference be 

 justified? 



How shall we attack this problem ? Clearly, the most hopeful 

 way is by proceeding from the known to the unknown. Now, 

 among the agents of change familiar to geologists, three are 

 admittedly of great importance ; these are water, heat, and 

 pressure. As probably almost all changes in nature, with which 

 we have to deal, have occurred in the presence of water, but 

 those due to it alone are generally superficial, I shall assume its 

 presence, and not attempt to isolate its effects. But we must 

 endeavour to ascertain the results of pressure and heat, when 

 acting singly and in combination, in modifying rocks of a known 

 character ; admitting, however, that probably while the one 

 agent has been dominant, the other has not been wholly 

 inoperative.^ 



The first effect of pressure due to great earth-movements is to 

 flatten somewhat the larger fragments in rocks, and to produce 

 in those of finer grain the structure called cleavage. This, 

 however, is a modification mainly mechanical. It consists in a 

 re-arrangement of the constituent particles, mineral changes, so 

 far as they occur, being quite subordinate. But in certain 

 extreme instances the latter are also conspicuous. From the fine 

 mud, generally the result of the disintegration of felspar, a mica, 

 usually colourless, has been produced, which occurs in tiny flakes, 

 often less than one-hundredth of an inch long. In this process, 

 a certain amount of silica has been liberated, which sometimes 

 augments pre-existing granules of quartz, sometimes consolidates 

 independently as microcrystalline quartz. Carbonaceous and 

 ferruginous constituents are respectively converted into particles 

 of graphite and of iron oxide. Here is an example of a 

 Palaeozoic rock, thus modified. It originally consisted of layers 

 of black mud and gray silt. In the former, this filmy mica has 

 been abundantly developed ; it is present also, as we might 

 expect, to some extent in the latter. Observe that the original 

 banded structure, notwithstanding the pressure, has not been 

 obliterated. Another point also demands notice. The black 

 lines in the section indicate the direction of the cleavage of the 

 rock, which is, roughly speaking, at right angles to the pressure 

 which has most conspicuously affected the district, while the 

 microfoliation, as we may call it, appears to be parallel to the 

 original bedding, and is thus anterior to the dominant cleavage. 

 The two may form parts of a connected series of movements, 

 but, at any rate, they are so far separated that the pressure 

 which produced the one, acted, roughly speaking, at right angles 

 to that which gave rise to the other, and the folia were developed 

 before they were bent and torn. 



Let us now pass on to examine the effects of pressure when it 

 acts upon a rock already crystalline. Here, obviously it is com- 

 paratively unimportant whether the original rock was a true 

 granite or a granitoid gneiss ; for at present we are only 

 concerned with the effect of pressure on a fairly granular crystal- 

 line rock. But in the resultant structures there are, as it seems 

 to me, differences which are dependent upon the mode in which 

 pressure has acted. They are divisible into two groups : one 

 indicating the result of simple direct crushing, the other of 

 crushing accompanied by shearing. In the former case, the rock 



' Heat will, of course, result from the crushing of rock. This some con- 

 s'der an important factor in metamorphism, but I have never been able to 

 find good evidence in favour of it, and believe that as a rule the rocks 

 yield too sbwly to produce any great e'.evat'on of temperature. 



