Page:A Treatise on Geology, volume 1.djvu/144

 128 the cleavage structure, and other coarser bands have not. But the most obvious and constant marks of interrupted deposition from water traceable across cleavage planes are stripes of colour different from the mass, or thin bands of harder matter, or layers of coarser ingredients. The most perfectly cleanable slate rock, though it be almost a crystal in respect of its regular structure, shows in the quarry indubitable marks of stratified deposition; and where fine grained and coarse grained slate rocks alternate, a very common circumstance about Snowdon, the fact is perfectly obvious.

Cleavage must be viewed as a structure imposed on the rock by agencies subsequent to its accumulation as sediment. What was exactly the nature of these is a problem of some delicacy, which may be better discussed hereafter: in the mean time, the following laws have been established with respect to it. First, it is never so perfectly exhibited as in the ancient argillaceous strata; is most conspicuous among those of finest grain and most uniform nature; disappears in very coarse rocks; ranges in almost exact parallels over many square miles of country; preserves these parallels even across contorted stratification (as Plate IV. fig. 17. "Guide to Geology," edit. 3.); and mostly coincides in horizontal direction with the great axes of elevation and depression of strata in the region observed. Finally, imperfect cleavage structures are produced in argillaceous rocks of later date near trap dykes (Coley Hill, Newcastle), and near great granitic irruptions (Vale of Chamouni).

Succession of the Strata.—It is only of late years that the real nature of the proof of the stratification of slate rocks has been sufficiently understood, to permit of its application to particular districts for the purpose of constructing a section of the series of the strata. As yet only two districts in Great Britain can be considered as at all completely investigated, viz. the region of the Cumbrian Lakes, and North Wales; but they are, perhaps, the very best for the purpose that are anywhere known.

Between Skiddaw and Saddleback the base of the clay slate series is found resting on very thin mica schist and