150 ON THE ORIGIN OF COAL. 



have frequently subsided, but to have been again 

 elevated, so as to account for the occurrence of 

 successive seams of Coal, an elevation and a sub- 

 sidence being necessary for the formation of each 

 seam.* This is a very unlikely hypothesis, when 

 the degradation of pre-existing rocks, and the 

 conveyance of them, by the action of running 

 water, is so evident in all the deposits ; a subsi- 

 dence of the bed of the present shallow seas 

 would not necessarily require the assistance of 

 any subterranean force to regain its former level, 

 if we allow the action of currents of water charged 

 with sand and silt. 



The crust of the globe furnishes us with 

 numerous evidences of the ancient ocean, but 

 the direct evidences of absolutely dry land before 

 the commencement of the Tertiary period are 

 very few. The only instances in England that 

 I am aware of are some in the new red sand 

 stone formation, hereinafter alluded to, and the 

 Portland dirt bed, — and the latter may have been 

 more of a swamp than absolutely dry land. No 

 doubt the existence of tracts of dry land in 

 many of these remote ages, as assumed by some 



* Dr. Mantell's Medals of Creation. Vol. i. p. 98. 



