Part 73 (1/2)

The total absence of any trace of fossils has inclined many geologists to attribute the origin of crystalline strata to a period antecedent to the existence of organic beings. Admitting, they say, the obliteration, in some cases, of fossils by plutonic action, we might still expect that traces of them would oftener occur in certain ancient systems of slate, in which, as in c.u.mberland, some conglomerates occur. But in urging this argument, it seems to have been forgotten that there are stratified formations of enormous thickness, and of various ages, and some of them very modern, all formed after the earth had become the abode of living creatures, which are, nevertheless, in certain districts, entirely dest.i.tute of all vestiges of organic bodies. In some, the traces of fossils may have been effaced by water and acids, at many successive periods; and it is clear, that, the older the stratum, the greater is the chance of its being non-fossiliferous, even if it has escaped all metamorphic action.

It has been also objected to the metamorphic theory, that the chemical composition of the secondary strata differs essentially from that of the crystalline schists, into which they are supposed to be convertible.[479-B]

The ”primary” schists, it is said, usually contain a considerable proportion of potash or of soda, which the secondary clays, shales, and slates do not, these last being the result of the decomposition of felspathic rocks, from which the alkaline matter has been abstracted during the process of decomposition. But this reasoning proceeds on insufficient and apparently mistaken data; for a large portion of what is usually called clay, marl, shale, and slate does actually contain a certain, and often a considerable, proportion of alkali; so that it is difficult, in many countries, to obtain clay or shale sufficiently free from alkaline ingredients to allow of their being burnt into bricks or used for pottery.

Thus the argillaceous shales and slates of the Old Red sandstone, in Forfars.h.i.+re and other parts of Scotland, are so much charged with alkali, derived from triturated felspar, that, instead of hardening when exposed to fire, they sometimes melt into a gla.s.s. They contain no lime, but appear to consist of extremely minute grains of the various ingredients of granite, which are distinctly visible in the coa.r.s.er-grained varieties, and in almost all the interposed sandstones.

These laminated clays and shales might certainly, if crystallized, resemble in composition many of the primary strata.

There is also potash in fossil vegetable remains, and soda in the salts by which strata are sometimes so largely impregnated, as in Patagonia.

Another objection has been derived from the alternation of highly crystalline strata with others having a less crystalline texture. The heat, it is said, in its ascent from below, must have traversed the less altered schists before it reached a higher and more crystalline bed. In answer to this, it may be observed, that if a number of strata differing greatly in composition from each other be subjected to equal quant.i.ties of heat, there is every probability that some will be more fusible than others. Some, for example, will contain soda, potash, lime, or some other ingredient capable of acting as a flux; while others may be dest.i.tute of the same elements, and so refractory as to be very slightly affected by a degree of heat capable of reducing others to semi-fusion. Nor should it be forgotten that, as a general rule, the less crystalline rocks do really occur in the upper, and the more crystalline in the lower part of each metamorphic series.

There are geologists, however, of high authority, who admit the metamorphic origin of gneiss and mica-schist even on a grand scale in some mountain-chains, and who nevertheless believe that gneiss has in some instances been an eruptive rock, deriving its lamination from motion when in a fluid or viscous state. Mr. Scrope, in his description of the Ponza Islands, ascribes ”the zoned structure of the Hungarian perlite (a semi-vitreous trachyte) to its having subsided, in obedience to the impulse of its own gravity, down a slightly inclined plane, while possessed of an imperfect fluidity. In the islands of Ponza and Palmarola, the direction of the zones is more frequently vertical than horizontal, because the ma.s.s was impelled from below upwards.”[480-A] In like manner, Mr. Darwin attributes the lamination and fissile structure of volcanic rocks of the trachytic series, including some obsidians in Ascension, Mexico, and elsewhere, to their having moved when liquid in the direction of the laminae. The zones consist sometimes of layers of air-cells drawn out and lengthened in the supposed direction of the moving ma.s.s. He compares this division into parallel zones, thus caused by the stretching of a pasty ma.s.s as it flowed slowly onwards, to the zoned or ribboned structure of ice, which Professor James Forbes has so ably explained, showing that it is due to the fissuring of a viscous body in motion.[480-B] Mr. Darwin also imagines the lamination or _foliation_, as he terms it, of gneiss and mica-schist in South America to be the extreme result of that process of which cleavage is the first effect.[480-C]

M. Elie de Beaumont, while he regards the greater part of the gneiss and mica-schist of the Alps as sedimentary strata altered by plutonic action, still conceives that some of the Alpine gneiss may have been erupted, or, in other words, may be granite drawn out into parallel laminae in the manner of trachyte as above alluded to.[480-D]

Opinions such as these, and others which might be cited, prove the difficulty of arriving at clear theoretical views on this subject. I may also add another difficulty. In many extensive regions experienced geologists have been at a loss to decide which of two sets of divisional planes were referable to cleavage and which to stratification; and that, too, where the rocks are of undisputed aqueous origin. After much doubt, they have sometimes discovered that they had at first mistaken the lines of cleavage for those of deposition, because the former were by far the most marked of the two. Now if such slaty ma.s.ses should become highly crystalline, and be converted into gneiss, hornblende-schist, or any other member of the hypogene cla.s.s, the cleavage planes would be more likely to remain visible than those of stratification.

But although the cause last-mentioned may, in some instances, be a ”vera causa,” as applied to gneiss and mica-schist, I believe it to be an exception to the general rule. Nor would it, I conceive, produce that kind of irregular parallelism in the laminae which belongs to so many of the hypogene rocks of the Grampians, Pyrenees, and the White mountains of North America, where I have chiefly studied them.

But it will be impossible for the reader duly to appreciate the propriety of the term metamorphic, as applied to the strata formerly called primitive, until I have shown, in the next chapter, at how many distinct periods these crystalline strata have been formed.

FOOTNOTES:

[474-A] Keilhau, Gaea Norvegica, pp. 61-63.

[475-A] Geol. Manual, p. 479.

[475-B] Phil. Trans., 1804.

[476-A] Poggendorf's Annalen, No. xvi., 2d series, vol. iii.

[476-B] See Principles, _Index_, ”Carbonated Springs,” &c.

[476-C] Hoffmann's Liparischen Inseln, p. 38. Leipzig, 1832.

[477-A] See Princ. of Geol.; and Bulletin de la Soc. Geol. de France, tom. ii. p. 230.

[477-B] See Princ. of Geol.; and Daubeny's Volcanos, p. 167.

[477-C] Jam. Ed. New Phil. Journ., No. 51. p. 43.

[478-A] Syst. of Geol., vol. i. p. 210.

[478-B] Ibid., p. 211.

[478-C] See above, pp. 327, 333.

[479-A] See Lyell, Quart. Geol. Journ., vol. i. p. 199.