264 ON THE DESTRUCTION OF ROCKS. 



prove, still more clearly than in many of the instances 

 formerly enumerated, that the whole of this process 

 is caused by the action of the atmosphere and the 

 rains. However mysterious it may at present appear, 

 it is the result of some chemical agencies which can- 

 not for ever be concealed. How far it may be con- 

 nected with any circumstances of the same nature 

 more generally interesting, it is impossible to foresee. 

 But like all new facts in an obscure science, it is 

 worthy of record. In multiplying the examples of 

 difficulties and obscurities, they become gradually re- 

 moved from the list of exceptions ; while the varieties 

 which are discovered in them on the comparison of 

 many examples, sometimes point out the causes 

 which have influenced the whole. 



Of some peculiar Modes of Decomposition in the 

 Rocks that have a venous and cavernous Structure. 



It was formerly remarked, when on the subject of 

 internal structure in rocks, that the existence of u 

 venous and reticulating arrangement in the parts of 

 granite and of some other rocks, was detected only 

 by the consequences of decomposition. This ap- 

 pearance is more common and more easily observed 

 in granite than in any other rock, but it also occurs 

 in gneiss and micaceous schist. Wherever it happens, 

 the surfaces exposed, either to the weather alone, or 

 to that imperceptible friction which is caused by the 

 tread of animals, the motion of water, or other 

 slighter actions, are corroded into shallow and un- 

 equal cavities ; the boundaries and forms of which 

 are determined by the casual reticulations of the 

 harder and more refractory veins by which the rock 



