﻿Yol. 64.] QTJAI^TITATIVE METHODS TO THE STUDY OP EOCKS. 223 



of pressure, as shown by small contortions of the bedding ; yet 

 thej^ are seldom straight, being, as it were, drawn by a shaking 

 hand. They are sometimes parallel for a short distance, but 

 usually unite with or branch into each other. Although small 

 in size, these slips are sometimes so numerous that, if those in a 

 square inch of surface were united end to end, they would extend 

 to a length of fully 50 feet ; and, if the surfaces of those in a cubic 

 inch were united, they would give an area of 5 or 6 square feet. 

 It need not be wondered, then, that they play so important a part 

 in the structure of the rock ; and they prove most conclusively 

 that, when it was compressed by the force which developed the 

 cleavage, the rock did not give way as a truly-plastic substance, 

 but to a large extent yielded as though fairly hard. These slip- 

 surfaces are in fact analogous to the slip-planes in metals described 

 by Prof. J. A. Ewing & Mr. W. Rosenhain.^ 



The detailed study of these slips in different rocks is very 

 instructive. I have examined carefully all my numerous thin 

 sections of rocks allied to slates, and find that in those without 

 cleavage comparatively-long slips are absent, and even short ones 

 occur only when there is a very sudden change in the nature of 

 the rock, as, for example, from fine sand to clay. Highly-cleaved 

 thickly-bedded slates, like those of Penrhyn, show very few, if any 

 long slips. On close examination, however, of a section perpen- 

 dicular to the cleavage in the line of dip, vast numbers of those a 

 hundredth of an inch in length or less can be seen ; but they 

 are not so much developed in the line of strike. 



I have an excellent section of a purple slate from Birnam (near 

 Dunkeld), containing a band composed of a mixture of a green and 

 a colourless mineral, bent into contortions which show that there 

 has been great compression in a line perpendicular to the cleavage. 

 The above-mentioned minerals must have been formed before the 

 cleavage, since they are broken up in the most remarkable manner, 

 and the structure of the fine-grained purple slate near the junction 

 is very instructive. At the round ends of the contortions, where 

 the green band has protected the purple slate from compression, its 

 structure is almost exactly like that of an uncleaved rock ; whereas 

 alongside the contortions, where there must have been great 

 diff'erential movement, the purple slate looks like a complete mass 

 of slip -surfaces, which are only ^wo ^^ "STnTo" ^^^^ apart. In passing 

 away into the purple slate the longer slips rapidly decrease in 

 number, and there are only slips of a hundredth of an inch or so in 

 length between the constituent particles of the rock. It will thus 

 be seen that there is good evidence to prove that, when the rock 

 was greatly altered in dimensions by pressure, it was so hard that 

 the' crystalline mineral was broken up, and the purple part yielded 

 by slip-surfaces. These conclusions are well borne out by similar 



^ Phil. Trans. Roy. Soc. ser. A, vol. cxciii (1900) p. 353. 

 Q.J.G.S. No. 251. Q 



