ON THE DESTRUCTION OF ROCKS. 261 



the species. I have observed only, that when they do 

 occur, they are more numerous in the softer clay- 

 stones than in the Syenites, or in the rocks with a 

 base of clinkstone. If one, two or three, can be 

 detached in succession, that facility soon ceases ; 

 and, after some partial indications of future desqua- 

 mation, the rock is found to be massive, and to give 

 no marks of the future renewal of a similar process ; 

 though there can be little doubt that it will continue 

 in succession, as the preceding laminae become de- 

 tached. Such laminae generally present the same 

 average, but variable thickness, already mentioned. 

 The surfaces are also occasionally undulated or irre- 

 gular, and, in a few instances, slightly curved. From 

 preserving the tenacity of stone, it is generally sup- 

 posed that they are unchanged, and that they present 

 the natural characters of the rock to which they 

 belong. But this is not the fact ; and they will be 

 found, in this important circumstance, to differ essen- 

 tially from those laminae which pervade the interior 

 of veins, and which arise from an internal concre- 

 tionary structure. 



The deception has, in these cases, arisen from the 

 great depth already noticed, to which the rocks of 

 this family are sometimes affected by the weather 

 without losing their tenacity ; undergoing little ap- 

 parent change but that of colour. Hence the solid 

 parts of such rocks, when broken, even below the 

 exfoliating surfaces, will be found to present the 

 same appearance as the detached scales. But if a 

 deeper section be made, it will, I believe, invariably 

 be observed, that the natural state of the rock is 

 different, and that the process of exfoliation has 

 arisen from a partial decomposition. To a miscon- 



