Rev. 0. FisJier — On Cleavage and Distortion. 397 



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 crystalliza- 

 tion, 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 mechani- 

 cal 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. 



1 believe that the first occasion when a doubt occurred to me 

 regarding the usually received view upon cleavage was when I 

 recorded the following entry in my note-book in 1867, being in 

 company at the time with Mr. Pengelly.^ 



Although no author has, so far as I know, used the comparatively 



^ Phillips' Report on Cleavage, 1856, Brit. Assoc. Eeports, 1857, p. 373. 



2 Geological Magazine, 1884, p. 271. 



^ Since sending my MS. to the Editor, I have enquired of Mr. Pengelly, ""What 

 is the relation of the cleavage to the unconformity ? ' ' His reply is as follows : — 



"The cleavage in the section at Hope's Nose affects both sets of rocks, and it is 

 not diverted by the unconformity. The following is briefly the reading of the 

 section : — 



1st. The formation of the lower series by the deposition of triturated organic 

 exuvife ; the process being frequently intermitted, as is proved by the interstratifica- 

 tion of volcanic ash. 



2nd. By the operation of some approximately lateral force, the beds were contorted, 

 and were fractured at the point of greatest flexure. 



3rd. The uppermost beds of this lower series were exposed to the action of the 

 waves, and planed down to an approximately horizontal surface. 



4th. After this denudation, the beds of the upper series were deposited uucon- 

 formably on the lower, and are now neaiiy horizontal. 



5th. Though these processes must have absorbed a large amount of time, they 

 were all completed within one and the same division of the Devonian period, as is 

 proved by the specific identity of the numerous fossils in the two series. 



6th. After the deposition of the upper series, cleavage was set up in the entire 

 mass. 



7th. Assuming that cleavage is due to pressiu-e, there are conditions under which 

 pressure can produce contortion without cleavage, and conditions under which it can 

 produce cleavage without contortion." 



