III.—On Cleavage and Distortion

Extract

The history of opinion upon the subject of cleavage seems to be this. Sedgwick had come to the conclusion that cleavage was a condition, “impressed on certain rocks and in certain regions by the operation of some very extensive cause, after the stratified rocks had undergone great displacement,” and he suggested that the cause might be some kind of crystallization. But Dr. Sorby discovered that the intimate structure to which cleavage is due is not crystallization, but a mechanical rearrangement of the particles of the rock: and he produced it artificially by squeezing clay containing flaky particles. Dr. Tyndall did the same thing with different substances, without introducing flaky particles. In either case it was by pressure that the result was obtained. Seeing therefore that pressure could succeed in laboratory experiments, and that cleavage in the field was ordinarily nearly at right angles to the direction in which the action of pressure might have folded the strata, it was natural to conclude that both folds and cleavage had been concomitant effects of such a pressure. And this appears to be the theory now generally adopted; and is of course in opposition to both of Sedgwick's conclusions. For my part I believe that Sedgwick was right in thinking that cleavage was produced by some very extensive cause operating after the rocks had undergone great displacement; while I agree with Sorby and Tyndall in attributing it to a mechanical action, accompanied with pressure. Nevertheless I do not think that this pressure was the same exhibition of force, that originally folded and elevated the rocks, as explained in Part IV. Thus I am in accord with each of these authors in the domain in which his authority stands highest.