442 



SCIENTIFIC NEWS. 



[Oct. 26, 1888. 



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 

 Iobate ; small grains of the one sometimes appear to be 

 enclosed 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 see 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 the 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, con- 

 sisting 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, how- 

 ever, that probably while the one agent has been domi- 

 nant, the other has not been wholly inoperative.* 



The first effect of pressure due to great earth-move- 

 ments is to flatten somewhat the larger fragments in 

 rocks, and to produce in those of finer grain the struc- 

 ture 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 disintegra- 

 tion of felspar, a mica, usually colourless, has been pro- 

 duced, 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 con- 

 solidates independently as microcrystalline quartz. Car- 

 bonaceous 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 grey 

 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. 



* Heat will, of course, result from the crushing of rock. This 

 some consider 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 slowly to produce any great 

 elevation of temperature. 



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 conspi- 

 cuously 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 move- 

 ments, 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 comparatively 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 crystalline rock. But in 

 the resultant structures there are, as it seems to me, dif- 

 ferences 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 mass has been so situated that any 

 appreciable lateral movement 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 suf- 

 fered 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 the cleavage 

 of a slate, Tesults from a direct crush ; while in others 

 the effects of shearing predominate. The latter accord- 

 ingly exhibit phenomena resembling the effects of a 

 tensile stress. Materials of a like character assume a 

 more or less linear arrangement, the rock becomes slightly 

 banded, and exhibits, as has been said, a kind of fluxion 

 structure. This phrase, if we are careful to guide our- 

 selves against misconception, 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 con- 

 dition, as, for example, a slaggy glass. But we must be 

 on our guard, lest we press the analogy too far. The 

 interesting experiments which have been made on the 

 flow of solids, and on rolled-out plastic substances, while 

 valuable as illustrations, represent, as it seems to me, a 

 condition of things which must be of rare occurrence in a 

 rock mass, pulverised by mechanical forces only. If I 

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

 fragmental solid — if the phrase be permissible — but as 

 an imperfect fluid ; that is to say, I must consider them 

 as illustrative of structures in rocks which have yet to 

 assume — not have already assumed — a crystalline condi- 

 tio 11 - (To be continued.) 



Spurious Cod-Liver Oil. — According to Cosmos, a 

 manufacture of cod-liver oil has sprung up in Paris. The 

 raw material is not the fresh liver of the cod, but the 

 stale fish and other refuse of the Halles Centrales, the 

 manufacturers, doubtless, thinking that any oil may serve 

 if it be sufficiently nasty. 



