Page:A Treatise on Geology, volume 1.djvu/151

 CHAP. VI. the bed of the sea, at intervals during the deposition of the slates; the dykes, of course, are of later origin, and the same is generally (we think) true of the mineral and quartz veins, but some metallic and quartzose masses are probably contemporaneous with the rocks which enclose them.

Besides these local effects of subterranean heat, the whole structure of the slaty rocks appears to be the result of a general pervading heat, operating on the argillaceous sediments so as to overcome their natural horizontal lamination, and induce a new, almost crystalline, fissility in vertical or highly-inclined planes, having one constant direction. We may consistently view this remarkable polarity of the cleavage as a characteristic effect of some very general agency, directing the results of molecular attraction: it is certain that this directive energy was never displayed in the same general way in the argillaceous rocks of later systems, though, as in Devonshire, South Wales, and the South of Ireland, local effects of this kind are seen in strata of the old red period, and in the Alps lias shales are decidedly subject to slaty cleavage. We may adopt, subject to a few exceptions, the rule first stated by Sedgwick—that the strikes of cleavage correspond to the strikes of the strata, though their inclination differs in amount, and even in direction; and this leads almost positively to the inference that the one is dependent on the other. My own observations have led me formerly to adopt the opinion that the divisional planes of slate were due to a molecular re-arrangement with polarity; another view ascribes the structure to pressure. It is certain that such pressure has operated, for shells and trilobites have been changed in form by it.