Part 72 (1/2)
In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous ma.s.ses of limestone are cut through so regularly by nearly vertical partings, and these are often so much more conspicuous than the seams of stratification, that an inexperienced observer will almost inevitably confound them, and suppose the strata to be perpendicular in places where in fact they are almost horizontal.[470-A]
Now these joints are supposed to be a.n.a.logous to those partings which have been already observed to separate volcanic and plutonic rocks into cuboidal and prismatic ma.s.ses. On a small scale we see clay and starch when dry split into similar shapes, which is often caused by simple contraction, whether the shrinking be due to the evaporation of water, or to a change of temperature. It is well known that many sandstones and other rocks expand by the application of moderate degrees of heat, and then contract again on cooling; and there can be no doubt that large portions of the earth's crust have, in the course of past ages, been subjected again and again to very different degrees of heat and cold. These alternations of temperature have probably contributed largely to the production of joints in rocks.
In some countries, as in Saxony, where ma.s.ses of basalt rest on sandstone, the aqueous rock has for the distance of several feet from the point of junction a.s.sumed a columnar structure similar to that of the trap. In like manner some hearthstones, after exposure to the heat of a furnace without being melted, have become prismatic. Certain crystals also acquire by the application of heat a new internal arrangement, so as to break in a new direction, their external form remaining unaltered.
Sir R. Murchison observes, that in referring both joints and slaty cleavage to crystalline action, we are borne out by a well-known a.n.a.logy in which crystallization has in like manner given rise to two distinct kinds of structure in the same body. Thus, for example, in a six-sided prism of quartz, the planes of cleavage are distinct from those of the prism. It is impossible to cleave the crystals parallel to the plane of the prism, just as slaty rocks cannot be cleaved parallel to the joints; but the quartz crystal, like the older schists, may be cleaved _ad infinitum_ in the direction of the cleavage planes.[471-A]
It seems, therefore, that the fissures called joints may have been the result of different causes, as of some modification of crystalline action, or simple contraction during consolidation, or during a change of temperature. And there are cases where joints may have been due to mechanical violence, and the strain exerted on strata during their upheaval, or when they have sunk down below their former level. Professor Phillips has suggested that the previous existence of divisional planes may often have determined, and must greatly have modified, the lines and points of fracture caused in rocks by those forces to which they owe their elevation or dislocations. These lines and points being those of least resistance, cannot fail to have influenced the direction in which the solid ma.s.s would give way on the application of external force.
Professor Phillips has also remarked that in some slaty rocks the form of the outline of fossil sh.e.l.ls and trilobites has been much changed by distortion, which has taken place in a longitudinal, transverse, or oblique direction. This change, he adds, seems to be the result of a ”creeping movement” of the particles of the rock along the planes of cleavage, its direction being always uniform over the same tract of country, and its amount in s.p.a.ce being sometimes measurable, and being as much as a quarter or even half an inch. The hard sh.e.l.ls are not affected, but only those which are thin.[471-B] Mr. D. Sharpe, following up the same line of inquiry, came to the conclusion, that the present distorted forms of the sh.e.l.ls in certain British slate rocks may be accounted for by supposing that the rocks in which they are imbedded have undergone compression in a direction perpendicular to the planes of cleavage, and a corresponding expansion in the direction of the dip of the cleavage.[471-C]
Mr. Darwin infers from his observations, that in South America the strike of the cleavage planes is very uniform over wide regions, and that it corresponds with the strike of the planes of foliation in the gneiss and mica-schists of the same parts of Chili, Tierra del Fuego, &c. The explanation which he suggests, is based upon a combination of mechanical and crystalline forces. The planes, he says, of cleavage, and even the foliation of mica-schist and gneiss, may be intimately connected with the planes of different tension to which the area was long subjected, _after_ the main fissures or axis of upheavement had been formed, but _before_ the final consolidation of the ma.s.s and the total cessation of all molecular movement.[472-A]
I have already stated that some extremely fine slates are perfectly parallel to the planes of stratification, as those of the Niesen, for example, near the Lake of Thun, in Switzerland, which contain fucoids, and are no doubt due to successive aqueous deposition. Even where the slates are oblique to the general planes of the strata, it by no means follows as a matter of course that they have been caused by crystalline action, for they may be the result of that diagonal lamination which I have before described (p. 17.). In this case, however, there is usually much irregularity, whereas cleavage planes oblique to the true stratification, which are referred to a crystalline action, are often perfectly symmetrical, and observe a strict geometrical parallelism, even when the strata are contorted, as already described (p. 470.).
Professor Sedgwick, speaking of the planes of slaty cleavage, where they are decidedly distinct from those of sedimentary deposition, declares his opinion that no retreat of parts, no contraction in the dimensions of rocks in pa.s.sing to a solid state, can account for the phenomenon. It must be referred to crystalline or polar forces acting simultaneously, and somewhat uniformly, in given directions, on large ma.s.ses having a h.o.m.ogeneous composition.
Sir John Herschel, in allusion to slaty cleavage, has suggested, ”that if rocks have been so heated as to allow a commencement of crystallization; that is to say, if they have been heated to a point at which the particles can begin to move amongst themselves, or at least on their own axes, some general law must then determine the position in which these particles will rest on cooling. Probably that position will have some relation to the direction in which the heat escapes. Now, when all, or a majority of particles of the same nature, have a general tendency to one position, that must of course determine a cleavage plane. Thus we see the infinitesimal crystals of fresh precipitated sulphate of barytes, and some other such bodies, arrange themselves alike in the fluid in which they float; so as, when stirred, all to glance with one light, and give the appearance of silky filaments. Some sorts of soap, in which insoluble margarates[472-B]
exist, exhibit the same phenomenon when mixed with water; and what occurs in our experiments on a minute scale may occur in nature on a great one.”[472-C]
FOOTNOTES:
[469-A] Geol. Trans., 2d series, vol. iii. p. 480.
[469-B] The Silurian System of Rocks, as developed in Salop, Hereford, &c., p. 245.
[469-C] Ibid., p. 246.
[470-A] Introduction to Geology, chap. iv.
[471-A] Silurian System of Rocks, &c., p. 246.
[471-B] Report, Brit. a.s.s., Cork, 1843, p. 60.
[471-C] Quart. Geol. Journ., vol. iii. p. 87. 1847.
[472-A] Geol. Obs. on S. America, 1846, p. 168.
[472-B] Margaric acid is an oleaginous acid, formed from different animal and vegetable fatty substances. A margarate is a compound of this acid with soda, potash, or some other base, and is so named from its pearly l.u.s.tre.
[472-C] Letter to the author, dated Cape of Good Hope, Feb. 20. 1836.
CHAPTER x.x.xVI.
METAMORPHIC ROCKS--_continued_.
Strata near some intrusive ma.s.ses of granite converted into rocks identical with different members of the metamorphic series--Arguments hence derived as to the nature of plutonic action--Time may enable this action to pervade denser ma.s.ses--From what kinds of sedimentary rock each variety of the metamorphic cla.s.s may be derived--Certain objections to the metamorphic theory considered--Lamination of trachyte and obsidian due to motion--Whether some kinds of gneiss have become schistose by a similar action.
It has been seen that geologists have been very generally led to infer, from the phenomena of joints and slaty cleavage, that mountain ma.s.ses, of which the sedimentary origin is unquestionable, have been acted upon simultaneously by vast crystalline forces. That the structure of fossiliferous strata has often been modified by some general cause since their original deposition, and even subsequently to their consolidation and dislocation, is undeniable. These facts prepare us to believe that still greater changes may have been worked out by a greater intensity, or more prolonged development of the same agency, combined, perhaps, with other causes. Now we have seen that, near the immediate contact of granitic veins and volcanic dikes, very extraordinary alterations in rocks have taken place, more especially in the neighbourhood of granite. It will be useful here to add other ill.u.s.trations, showing that a texture undistinguishable from that which characterizes the more crystalline metamorphic formations, has actually been superinduced in strata once fossiliferous.