Page:A Treatise on Geology, volume 2.djvu/134

 120 outlines of shells and other organic remains are distorted, and their surfaces crumpled and waved. Thus the symmetrical forms of leptænæ, orthides and spiriferæ become abbreviated in one direction, and (relatively) lengthened in another, and if they were laid obliquely to the direction of cleavage they have become distorted. So the trilobites of Llandeilo appear, in some instances, much narrowed, in others much widened, and in other instances obliquely elliptical, but in every instance the result was a contraction of the space across the edges of cleavage, and what may be called a minute folding, or furrowing; in fact, a "creep," in the direction of the dip. This "creep" is such as in the case of specimens of Ogygia Buchii from Llandeilo to contract them ¼ and even ½ an inch.

We may illustrate this by a diagram. Let ss be the line of strike, and D the line of dip on the surface of the stratum. Let o be the semicircular smaller valve of an orthis, with r its radius, perpendicular to the diameter d. Let such a figure be placed in 1 with its diameter in the line of the dip, in 2 at right angles to it, and in 3 at some lesser angle, say 50° to it. Then let all the stratum be subject to compression along the line of the dip, the result will be that 1 becomes shortened diametrically, r remaining unchanged, 2 becomes shortened on the radius r, but unchanged on the diameter d, while 3 is shortened both on the line r and on the line d, and is distorted, so that r is no longer at right angles to d. If we had assumed an extension of the rock in the line D, we should have had d lengthened in 1, r in 2, both d and r in 3; 1 and 2 retaining their symmetry, and 3 being distorted in a different manner.

Hence arises distinctly the idea of pressure as a cause of cleavage; an idea which has been the subject of elaborate illustration by Mr. Sharpe.

Mr. Sharpe has given examples of elongation due to expansion in the direction of the dip of the cleavage;