25G Dr. Mac Cnlloch oti the 



leave anew surface exposed ; and on this the same desquamation 

 has again taken place, so that, in some parts, two successive 

 plates can be separated. The second plate is, however, in this 

 case, not placed under the first, but forms a lower stage or 

 step ; its upper surface, if prolonged, being the under one of 

 the first scale. In other cases, it is merely apparent that the 

 process is about to be completed at some future period ; the 

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

 possible only to detach a small portion at the edge. 



In one place it was also observed, that tliere was an incipient 

 mark, or 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 ex- 

 ternal circumstances continue to act. 



This fact would have been in no respect remarkable, if the 

 desquamation had taken place in a direction parallel to the 

 laminar structure of the stone. Where micaceous schist is of 

 such a nature that laminte of quartz are separated by others of 

 mica, the rusting of the iron in the latter, and its consequent 

 decomposition, easily permit the quartzose laminae to be de- 

 tached. But, in this case, the desquamation is at right angles, 

 or, at least, at considerable angles to the laminar structure. 

 Thus it produces 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 desquamation 

 is not only not produced by the peculiar structure of the rock, 

 but is utterly independent of it. Under the same circum- 

 stances, 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 tendency. Such an 

 occurrence might 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 laminse. 



The peculiar circumstances of predominant exposure to the 



