She geological age of central ai^d west Cornwall. 



41 



before another was deposited ; but, as a great thickness has no 

 doubt been entirely denuded away, the over-estimate on the one 

 hand would tend to balance the under-estimate on the other. 



This vast thickness must have been lifted up by the granite, 

 and has since been denuded away — together with, we know not 

 how much of the granite itself. As, however, the protrusion of 

 the granite was certainly not later than the end of the Carbon- 

 iferous period in Devon, and probably about the same in Cornwall, 

 ample time has elapsed to allow of this large amount of 

 denudation. 



As to the time occupied by the deposition, contortion and 

 denudation of this vast thickness of strata, very little need be 

 said here. That the different series differ very greatly in age 

 is sufficiently evidenced by the fact that there was in each case 

 time enough for the great lines of pressure in the earth's crust 

 to change their direction, — at first, between the Ponsanooth 

 beds and Lower Silurians about 90°, then between them and 

 the Ladock beds 45°, and again by the time the Fowey beds 

 came to be crumpled — 45°. 



Even the deposition of such a great thickness of rocks, if not 

 interrupted, would necessarily occupy an enormous interval of 

 time, as the detrital matter would have to be worn away from 

 then existing land surfaces. 



I fully admit that many of the beds bear evidence of rapid 

 accumulation, as for instance the coarse conglomerate of the 

 Nare Point, the finer mass, 2,000 feet thick, of PorthoUand, and 

 the still finer sandstone beds with cross-bedding of Ladock. 

 But the mere production and transport of such immense 

 quantities of detrital matter must have occupied long ages, and 

 these no doubt alternated with at least equal periods when the 

 whole region was exposed only to atmospheric agents of 

 destruction. 



