Lamination of clay-slate, Montagne de Seguinat, near Gavarnie, in the Pyrenees.
This disposition of the layers is illustrated in the accompanying diagram, in which I have represented carefully the stratification of a coarse argillaceous schist, which I examined in the Pyrenees, part of which approaches in character to a green and blue roofing slate, while part is extremely quartzose, the whole mass passing downwards into micaceous schist. The vertical section here exhibited is about 3 feet in height, and the layers are sometimes so thin that fifty may be counted in the thickness of an inch. Some of them consist of pure quartz.
The inference drawn from the phenomena above described in favour of the aqueous origin of clay-slate and other crystalline strata, is greatly strengthened by the fact that many of these metamorphic rocks occasionally alternate with, and sometimes pass by intermediate gradations into, rocks of a decidedly mechanical origin, and exhibiting traces of organic remains. The fossiliferous formations, moreover, into which this passage is effected, are by no means invariably of the same age nor of the highest antiquity, as will be afterwards explained.
Stratification of the metamorphic rocks distinct from cleavage.—The beds into which gneiss, mica-schist, and hypogene limestone divide, exhibit most commonly, like ordinary strata, a want of perfect geometrical parallelism. For this reason, therefore, in addition to the alternate recurrence of layers of distinct materials, the stratified arrangement of the crystalline rocks cannot be explained away by supposing it to be simply a divisional structure like that to which we owe some of the slates used for writing and roofing. Slaty cleavage, as it has been called, has in many cases been produced by the regular deposition of thin plates of fine sediment one upon another; but there are many instances where it is decidedly unconnected with such a mode of origin, and where it is not even confined to the aqueous formations. Some kinds of trap, for example, as clinkstone, split into laminæ, and are used for roofing.
There are, says Professor Sedgwick, three distinct forms of structure exhibited in certain rocks throughout large districts: viz.—First, stratification; secondly, joints; and thirdly, slaty cleavage; the two last having no connection with true bedding, and having been superinduced by causes absolutely independent of gravitation. All these different structures must have different names, even though there be some cases where it is impossible, after carefully studying the appearances, to decide upon the class to which they belong.[469-A]
Joints.—Now, in regard to the second of these forms of structure or joints, they are natural fissures which often traverse rocks in straight and well-determined lines. They afford to the quarryman, as Sir R. Murchison observes, when speaking of the phenomena, as exhibited in Shropshire and the neighbouring counties, the greatest aid in the extraction of blocks of stone; and, if a sufficient number cross each other, the whole mass of rock is split into symmetrical blocks.[469-B] The faces of the joints are for the most part smoother and more regular than the surfaces of true strata. The joints are straight-cut chinks, often slightly open, often passing, not only through layers of successive deposition, but also through balls of limestone or other matter which have been formed by concretionary action, since the original accumulation of the strata. Such joints, therefore, must often have resulted from one of the last changes superinduced upon sedimentary deposits.[469-C]
In the annexed diagram the flat surfaces of rock A, B, C, represent exposed faces of joints, to which the walls of other joints, J J, are parallel. S S are the lines of stratification; D D are lines of slaty cleavage, which intersect the rock at a considerable angle to the planes of stratification.
Fig. 510.
