244 ON THE DESTRUCTION OF ROCKS. 



pient decomposition, the progress of which is thus 

 easily traced. This rock, it must be observed, is per- 

 fectly compact and tenacious, although far less so 

 than that from which it has been derived. In many 

 of these instances, where the rocks are of a porphyritic 

 character, the fact of such a partial decomposition 

 may be suspected, from the existence of cavities con- 

 taining yellow or brown day; and, even in porphyries, 

 of which the base appears to have undergone no 

 change, the crystals are sometimes reduced to powder. 

 That the compact clinkstones may be converted 

 into arenaceous claystones by this kind of decomposi- 

 tion, may be proved by the state of some of the rocks 

 in Arran; where the former are frequently found 

 covered with a crust of the latter, and where, without 

 fracture, it would not be suspected that the rock was 

 of a different character within. The same circum- 

 stance is visible in many places in Sky ; and, from 

 the sections there afforded, the decomposition of these 

 rocks can often be traced to an enormous depth. It 

 even appears that many of the pale and yellow Syenites 

 and porphyries of that island have been originally 

 blue; and that their imperfect compactness, like their 

 yellow colour, is the consequence of incipient or im- 

 perfect decomposition. I may confirm iny own re- 

 marks by the testimony of the French geologists re- 

 specting the rocks of Dornfront in Britany; where 

 greenstones are similarly found converted into clay- 

 stones, which, by a further decomposition, are resolved 

 into a species of fullers' earth. 



So great is the depth of the decomposition in some 

 of these instances, and so exactly do many of the 

 great masses of pale claystone correspond with those 

 which can be proved to result from decomposition, 

 that we are led to suspect that this may be the origin 



