﻿222 BE. H. C. SOEBY 0^'^ THE APPLICATION OF [^aV I908, 



Bethesda the spots had their axes distributed at nearl}^ all angles to 

 the cleavage, but at Penrhyn at angles varying from 17° to 41°, the 

 means of all my measurements being 27°. Calculating from 

 thisj and correcting accordingly, we obtain as the corrected ratio 

 l'60x^ = l'86. The mean for both quarries would thus be 

 1:1-94 and 1:7, and the length of the shortest axis 1^='275. 



The length of that in the line of strike being taken as unity, we 

 obtain the following results : — 



Before cleavage was developed -600 x 1 '00 X 1 = "60 



After cleavage bad been developed . . . •275 X 1'94 x 1 = "53 



Hence, the reduction in volume due to pressure was from 60 to 53, 

 or about 11 per cent. The most satisfactory explanation of this is 

 that, before compression, the rocks contained about 11 per cent, of 

 cavities, which were almost completely squeezed up, since the slate 

 now contains only about -24 per cent. This 11 per cent, must be 

 looked upon as only approximate, since it depends on four quantities 

 all subject to error. I consider, however, the result very satisfactory 

 in showing that, before cleavage was developed, the rock was, if 

 anything, rather more consolidated than the fine-grained beds of 

 the Coal-Measures, an inference which agrees admirably with the 

 microscopical structure described in other parts of this paper. 

 These various data seem to lead to the following conclusions :— 



CI) The material when deposited was not exactly a clay, since the axes 

 of the spots were originally t)0 : 109 : 100, instead of more nearly 

 52:100:100. 



(2) The spots were formed early in the history of the rock, before the 



deoxidizing power of some nucleus was lost. 



(3) The rock was fairly hard and consolidated. 



(4) It was subjected to enormous pressure, and so changed in dimensions 



in different directions that cleavage was developed. 



(5) The facts appear to establish conclusively the mechanical origin of 



cleavage. 



XVII. Slip-Suefac£S. 



On carefully examining some varieties of much-contorted mica- 

 schist, it is very easy to see that, in many cases, there has been an 

 irregular giving-way of the rock in a plane perpendicular to the 

 pressure, and one bent portion has slipped over another, so as to give 

 rise to a surface of partial or complete discontinuity. A similar 

 structure can be seen with the microscope in thin sections of some 

 imperfectly-cleaved slates ; and that these slip-surfaces are, as it 

 were, microscopic faults, often from a hundredth to a thousandth of 

 an inch apart, is proved by the upthrow or downthrow of very 

 thin beds, or by broken flakes of mica, the movement being a 

 hundredth of an inch or much less. 



As seen under the microscope, these slip-surfaces in section look 

 like very thin black lines, on the whole perpendicular to the line 



