ON THE DESTRUCTION OF ROCKS. 263 



parts, two successive plates can be separated. In 

 other places, it is apparent that the progress is about 

 to be completed at some future period ; the edge of 

 the scale just admitting a knife, but it being as yet 

 possible to detach only a small portion at the margin. 

 Sometimes, there is even an indication of a third 

 scale : leaving no doubt that, in a sufficient length of 

 time, the same process may be expected to take place 

 through the whole block, should the same external 

 circumstances continue to act. 



This fact would have been in no respect remark- 

 able, if the desquamation had taken place in a di- 

 rection parallel to the laminar structure of the stone. 

 But it occurs at considerable angles to that, so as to 

 produce a scale or slate, which consists of parallel 

 bands of quartz and mica : both of them remaining 

 unchanged, as in the solid rock, but easily separated, 

 in consequence of the fragility of the micaceous band. 

 It is evident therefore, that in this instance, the de- 

 squamation is, not only, not produced by the peculiar 

 structure of the rock, but is utterly independent of 

 it. Under the same circumstances, it might equally 

 be expected to occur, either in a mass of pure quartz 

 rock, or in a micaceous schist of a more simple and 

 homogeneous nature ; and, in this latter case, the 

 schist might desquamate at angles to its fissile ten- 

 dency. Such an occurrence would excite surprise ; 

 but there is no apparent reason why it might not 

 happen in either of these rocks in a separate state, as 

 it does here where they are intermixed in distinct 

 laminae. 



The peculiar circumstances of predominant ex- 

 posure to the weather on the one side of this rock 

 where the desquamation takes place, contribute to 



