When, however, we look closely into this bold and beautiful hypothesis, we find that the only analogy which exists between the physical structure of slate rocks and of crystals is this single one of cleavage. Such a coincidence might fairly give rise to the conjecture that both were due to a common cause; but there is great difficulty in accepting this as a theoretic truth. When we examine the structure of a slate rock, we find that the substance is composed of the debris of former rocks; that it was once a fine mud, composed of particles of _sensible magnitude_. Is it meant that these particles, each taken as a whole, were re-arranged after deposition? If so, the force which effected such an arrangement must be wholly different from that of crystallization, for the latter is essentially _molecular_. What is this force? Nature, as far as we know, furnishes none competent, under the conditions, to produce the effect. Is it meant that the molecules composing these sensible particles have re-arranged themselves? We find no evidence of such an action in the individual fragments: the mica is still mica, and possesses all the properties of mica; and so of the other ingredients of which the rock is composed. Independent of this, that an aggregate of heterogeneous mineral fragments should, without any assignable external cause, so shift its molecules as to produce a plane of cleavage common to them all, is, in my opinion, an assumption too heavy for any theory to bear. Nevertheless, the paper of Professor Sedgwick invested the subject of slaty cleavage with an interest not to be forgotten, and proved the stimulus to further inquiry. The structure of slate rocks was more closely examined; the fossils which they contained were subjected to rigid scrutiny, and their shapes compared with those of the same species taken from other rocks. Thus proceeding, the late Mr. Daniel Sharpe found that the fossils contained in slate rocks are distorted in shape, being uniformly flattened out in the direction of the planes of cleavage. Here, then, was a fact of capital importance,--the shells became the indicators of an action to which the mass containing them had been subjected; they demonstrated the operation of pressure acting at right angles to the planes of cleavage. [Sidenote: MECHANICAL THEORY.] The more the subject was investigated, the more clearly were the evidences of pressure made out. Subsequent to Mr. Sharpe, Mr. Sorby entered upon this field of inquiry. With great skill and patience he prepared sections of slate rock, which he submitted to microscopic examination, and his observations showed that the evidences of pressure could be plainly traced, even in his minute specimens. The subject has been since ably followed up by Professors Haughton, Harkness, and others; but to the two gentlemen first mentioned we are, I think, indebted for the prime facts on which rests the _mechanical theory_ of slaty cleavage.[A]