176 Bev. 0. Fisher — TJte Cause of Slaty Cleavage. 



merits. In this manner a frilled schist may pass into a I'ock showing 

 schistose cleavage iu the direction of the shear, which will be 

 parallel to the folds. This would explain what Mr. Teall has told 

 me, viz. that he has seen facts in the N.W. of Scotland, which have 

 led him to suspect some connection between "frilling" and schistosity ; 

 having noticed the one apparently pass into the other within a 

 moderate horizontal distance. 



It seems that, as the shear increased, the limbs of the folds of the 

 frilling would be alternately compressed and elongated. This would 

 give rise to alternate layers, in which the materials of the harder 

 and softer original layers would be mingled in different proportions, 

 and, if further metamorphosed, these would pass into alternate 

 layers, in which the component minerals would not be similarly 

 abundant. If, however, the bedding of the layers was at right 

 angles, or at an obtuse angle, to the shear, they would never be 

 crumpled at all, but begin at once to be torn to pieces. Thus it 

 appears that even the same amount of shear, which at one locality 

 produces frilling, may in another, not far oif, produce schistosity : 

 the difference of effect depending upon different inclinations between 

 the layers and the shear at the two places. 



This kind of action might affect a schist, and convert it into a 

 frilled schist, and then back again into a schist having a different dip 

 from the original one. 



When we turn to the relation of cleavage to the structure of a dis- 

 trict, we approach a more difficult branch of the subject, but the one 

 which must, I believe, decide the mode of causation. The more 

 we learn about earth movements, the more complicated we tind them ; 

 witness the latest results of the Survey in the Eribol district. But 

 from what we do know, I think a great amount of shear at certain 

 places cannot be called improbable : and I am not prepared to 

 abandon my somewhat bold hypothesis to account for the movements, 

 by which I think cleavage might be superinduced upon already 

 highly disturbed rocks.-' 



Mr. Harker combats this with a diagram implying that, on my 

 hypothesis, we ought to meet with cleavage dipping in directions 

 nearly at right angles upon the opposite flanks of a range of hills, 

 but with none at the summit or in the plains. I however carefully 

 guarded against being supposed to assert, that the outline of a 

 district, such as I drew to explain the cause of vertical movements, 

 in any way represented the present contour ; saying that denudation 

 would have entirely changed its aspect. 



In the next place it does not follow that the settling down of a 

 recently raised tract would take place by consecutive gradations from 

 the outsides to the centre. More probably it would be great at 

 places, and little or none through intermediate spaces. Some of the 

 horizontal sections of the geological survey, carried through cleaved 

 districts, convey this idea.'^ In fact, those readjustments of equili- 

 brium, which in other cases have been accomplished by faulting, 



1 Loc. cit. p. 271. 



2 Hor. Sections, 6, 26, 27, 29, 31, 32, 40, 113, 114, 115, 118. 



