Page:A Treatise on Geology, volume 2.djvu/131

 CHAP. VII. be repeated in other forms, but their present value is great, and they may, as he suggests, lead to practical results of value in mining operations.

We must, however, add some further details of the phenomena of cleavage, and discuss their bearing on another hypothesis, which ascribes to pressure this beautiful superposition of structure.

The occurrence of cleavage at all in any given district is in some degree dependent on the nature of the rocks therein. Still more obvious is it that perfect examples of it only occur in certain argillaceous deposits. In a country consisting of alternations of thick argillaceous beds with coarse conglomerates, hard sandstones, limestones, quartz rock, and felstone, or greenstone, we shall find the cleavage, after passing through the argillaceous bed, more or less constantly interrupted by the other strata,—through which, however, a certain fissility, occasionally twisted and otherwise modified, is often traceable.

We have also for many years observed a beautiful case of the bending of the cleavage surfaces when they pass from one bed, or part of a bed, to another bed or mineralogically different part of a bed. Mr. Sharpe also admits this fact. This bending is always in such a manner as to render the angle of intersection between the cleavage and the stratification more acute, just as sometimes happens when a mineral vein crosses obliquely a strong throw, or when strata rise with an uplifting fault. The law is the same in all cases. These phenomena deserve the utmost attention from those who speculate on the theory of cleavage.

Beyond these completely or partially interrupting layers the cleavage recurs in the next band of argillaceous rock, with planes parallel to those first observed.

There are, however, cases in which alternating beds