Alfred Harher — Stages of Slaty Cleavage. 267 



a corresponding decrease of bulk, and effected by the closer packing 

 of the constituent fragments, accompanied by the expulsion of the 

 greater part of the interstitial water. Such packing, facilitated no 

 doubt by the fragments slipping over one another, would tend to 

 arrange them in vertical planes perpendicular to the direction of the 

 pressure, thus setting up a cleavage structure. The ellipsoid of dis- 

 tortion would be one in which the greatest and second axes would be 

 nearly equal. This is the case in the Devonian slates investigated by 

 Dr. Haughton. 



But there must evidently be a limit to the process of packing just 

 described, and when this was reached, since there could be no further 

 decrease of bulk, continued pressure would give rise to a vertical 

 expansion of the mass compensating the horizontal compression. 

 The movement in this second stage of the process might be justly 

 described as shearing. It would increase the greatest axis of the 

 ellipsoid of distortion and proportionately decrease the least axis. 

 At the same time it would produce a more perfect cleavage structure 

 by arranging the long and flat-shaped fragments more exactly in 

 vertical planes perpendicular to the direction of pressure. This is 

 the case in the Llanberis and Borrowdale slates observed by Sorby 

 and Sharpe. 



As a still further result, a more intense pressure would probably 

 bring about mineralogical and chemical as well as merely mechanical 

 changes, the rock losing its eminently fissile character by becoming 

 foliated. This connection between cleavage and foliation was long 

 ago pointed out by Darwin in South America. The obliteration of 

 fossil remains in such foliated rocks leaves us no means of testing 

 the precise character of the distortion they have undergone, but the 

 ellipsoid expressing such distortion would doubtless be excessively 

 elongated and flattened. 



The difference between the processes sketched above and the ideas 

 developed in Mr. Fisher's papers in this Magazine is evident. He 

 maintains, in effect, that the whole movement which produced slaty 

 cleavage in a rock was one of shearing. As regards Mr. Fisher's 

 paper in the April Number, there is one point raised, to which, as 

 he says, I have hitherto made no allusion, viz. the relation of 

 cleavage to the folding of the strata in which it occurs. Mr. Fisher 

 would argue that since the direction of the cleavage-planes seems to 

 be almost independent of the varying dip of the beds, the origin of _ 

 the cleavage structure must be posterior to the folding of the 

 strata. Granting this, however, it does not follow that the 

 two things were independent. As Mr. Fisher points out in the 

 same paragraph (p. ITT), they are two distinct modes of satisfying 

 compression, and therefore we need not expect to find them pro- 

 ceeding simultaneously. That contortion of the strata should 

 precede cleavage is a matter of no surprise, if the latter involve an 

 actual condensation of bulk while the former is a mere change of 

 position. In accordance with this we frequently find contortion 

 without cleavage, but cleavage without contortion never. In many 

 cases also there is clear evidence that contortion on a minute scale 



