II.] ON SOME POINTS IN CHEMICAL GEOLOGY. 17 



The volcanic phenomena of the present day appear, so far as 

 I am aware, to be confined to regions covered by the more re- 

 cent secondary and tertiary deposits, which we may suppose 

 the central heat to be still penetrating (as shown by Mr. Bab- 

 bage), a process which has long since ceased in the palaeozoic 

 regions. Both normal metamorphism and volcanic action are 

 generally connected with elevations and foldings of the earth's 

 crust, all of which phenomena we conceive to have a common 

 cause, and to depend upon the accumulation of sediments and 

 the subsidence consequent thereon, as maintained by Mr. James 

 Hall in his theory of mountains.* 



* See, for an exposition of the views of Professor Hall, Essays V. and 

 VII. of the present volume. 



