Rocks and Their Origins

by Grenville A. J. Cole

SLATE

The relations between shale and Slate are so obvious that slate may readily be regarded as a very well-compacted mud. The clayey material in it, like that of muds, may be ordinary detritus or of volcanic origin; its colours repeat those of shales. Its essential character, however, is the possession of a "cleavage," that is, of well-developed planes of fissility, which are often inclined to those of bedding. The bedding may be indicated by bands of different coarseness or constitution, and these may show crumpling due to pressure that has been exerted on the mass. The cleavage, however, may run right across these bands, and the rock, as a rule, splits far more cleanly along the cleavage-planes than a shale does along its planes of bedding.

The early and historic observations on slaty cleavage have been excellently reviewed by A. Harker[47], who also provides an independent investigation. Reference may also be made to a later treatise by C. K. Leith[48], which contains numerous illustrations, and to a discussion by G. W. Lamplugh[49]. D. Sharpe and H. C. Sorby, between 1847 and 1853, developed the theory that rock-cleavage was due to compression in a direction perpendicular to the planes of cleavage and to expansion along them. As Harker points out, it is unlikely that the expansion balances the compression. The density of slate, about 2·7, is a good indication that the "porosity," or percentage of pore-space, has been reduced, while the mineral changes, soon to be referred to, are also in favour of greater density. C. Darwin[50] laid stress on the connexion between cleavage and the development of flaky minerals, such as micas, along the cleavage-planes, the structure ultimately passing into that known as "foliation" (see [p. 145]). H. C. Sorby urged that compression brings platy particles into parallel positions throughout the mass, so that the plates, which may consist of kaolin, mica, or chlorite, come to lie with their broad surfaces perpendicular to the direction of compression. At the same time, any constituents capable of deformation become compressed in this direction, become expanded in a direction perpendicular to it, and are themselves converted into lens-like forms or plates. T. Mellard Reade and P. Holland[51] have emphasised the part played by crystallisation at the close of the process of compression. They urge that the platy minerals, mica and chlorite, are produced during the alteration of the rock, and can spread with ease in directions perpendicular to that of compression; they thus give rise to slaty cleavage at a late stage in the deformation of the rock. These authors, it will be seen, have developed one of Darwin's principal propositions, as to the close connexion between rock-cleavage and foliation, and, in opposition to Sorby, consider the platiness of the original constituents to be of less importance.

In support of their view, in regard to the late stage at which cleavage is induced, it may be noted that the crystals of pyrite and magnetite that sometimes occur in slates and in the allied foliated schists have developed at an earlier date as knots which oppose the cleavage or the foliation[52].

Darwin observed that mineral differences sometimes occur along bands parallel with the cleavage-planes. In such cases, the difference may be largely one of grain, shearing having broken down the minerals into a finer state along certain bands of movement[53]. Shearing of the rock may occur along any of the cleavage-planes, which are superinduced planes of weakness, and parts of the slate thus slide over others, just as the mineral flakes slide over one another in the directions in which expansion of the rock is possible. Where traces of the original stratification remain, it is easy to see if rock-shearing has occurred.

Beds of different composition naturally take on cleavage in very different degrees. Sandy layers show the compression that has taken place by contorting; but they cleave very poorly, and in proportion to the amount of mud present in them. Where clayey and sandy layers alternate, and the direction of the cleavage is oblique to them, it is refracted, as it were, on passing from one layer to the other; it is more highly inclined to the bedding in the sandy layers and less so in the clayey layers. Hence a cleavage-surface forms a fold resembling the shape of an italic S as it traverses each harder bed. Harker[54] and Leith[55] discuss the cause of this from somewhat different points of view. It is probable that such cleavage-planes as develop within the hard bed are approximately perpendicular to the direction in which the compressive force acts, because there is in such beds little possibility of lateral creep of the material along the bedding-planes. In the softer layers, we have to deal, not only with a tendency towards the rotation of platy particles until their flat surfaces are perpendicular to the direction of pressure, but also with a tendency of the same particles to flow along the bedding-planes. The resultant arrangement gives rise to a cleavage nearer to the bedding-planes than that in the more sandy layers.

Sometimes, after the cleavage is established, compression folds it, just as strata may be folded. Still greater compression may obliterate it and establish a new cleavage, and all gradations towards this result are traceable. The cleavage layers, again, may be wrinkled into a series of sharp folds, thrust over in one direction, and parting may then take place along the ridges of these folds, which furnish a second series of planes of weakness in the rock. This type of separation has been styled a strain-slip cleavage, and by Leith a fracture-cleavage, in distinction from ordinary or flow-cleavage. Shearing may take place along it, and the true or flow cleavage-planes become thus broken across and faulted.

Fig. 9. Landslide of Limestone over Shale. Near Luc-en-Diois, Drôme, France. The scale is shown by the main road passing among the blocks.

Commercial slates should exhibit none of these structures that interfere with genuine cleavage. An argillaceous rock of uniform grain, compressed evenly over a considerable district, is required for successful slate-quarries. Yet all quarrymen will admit that the material varies from point to point, and that the best slate runs in "veins." Some of the coarser slates, with irregular surfaces, and with splashes of colour, such as are provided by limonite, are sought after for their picturesque effect; while slates which do not split readily enough for roofing purposes may have their use for flags, mantel-shelves, and billiard-tables.