CHAP. VI.] CARBONIFEROUS PERIOD. 89 



3. Physical Conditions indicated by the Successive 

 Deposits of the Period. 



The Carboniferous rocks differ in certain respects from 

 those of the preceding and succeeding systems, and they 

 exhibit characters which can only have resulted from 

 a general uniformity of physical conditions having prevailed 

 over very large areas of the earth's surface. The same 

 general succession of deposits is met with, not only over the 

 whole of the British Islands, but over the greater part of 

 Northern Europe and of central North America. Further- 

 more, the fauna and flora of the Carboniferous rocks are 

 everywhere similar, and are everywhere persistent through- 

 out a great thickness of strata. Of the marine fossils a 

 few species are indeed confined to the lowermost beds, and 

 a few to the highest marine beds to beds, in fact, which 

 were formed when the physical conditions were undergoing 

 a change, and when the forms would necessarily be most 

 liable to variation; a few other species those, namely, 

 which were the chief contributors to the formation of the 

 limestones are naturally most abundant in those lime- 

 stone masses ; but by far the larger number of species have 

 a very great vertical range, many of them extending from 

 top to bottom of the marine series, and even appearing in 

 the essentially freshwater and estuarine strata above and 

 below, wherever the temporary prevalence of marine con- 

 ditions led to the formation of limestone. 



As regards the process of sedimentation which went on 

 in the central sea which covered the northern part of 

 England, Professor Green has given an account, from which 

 I extract the following : l " This Mediterranean sea had a 

 fringe of shallow water around its margin, and deep depres- 

 sions in its central portion. Bound its edges deposits were 



1 " The Yorkshire Coalfield," by Green and Russell, " Mem. Geol. 

 Survey," p. 24. 



