ON THE ORIGIN OF COAL. 153 



localities, where such changes took place ; but 

 the cessation of volcanic agency in the first case, 

 the partial filling up of the bed of the ocean in 

 the second, or the subsidence of the surface of 

 the earth in the last, would again fit them for 

 animal life. Again and again the subsidences of 

 the bed of the ocean appear to have taken place, 

 during the formation of the Silurian groups, as 

 the successive bands of fossiliferous rocks testify. 

 Some of these being of great extent, would cause 

 the depth of the waters to be so great as to render 

 them unfitted for animal life ; whilst others might 

 for a period be so gradual as to permit the animals 

 to adapt themselves to the altered conditions, or 

 build their way up against the subsiding rocks, 

 like the Zoophytes of the present coral reefs. 



At the close of the Silurian system in North 

 Wales, an elevation or a period of repose, it is 

 difiicult to say which, of the strata appears to 

 have taken place, as few, if any traces of the old 

 red sandstone are said to be met with in the 

 north eastern counties of Wales ; but in the south 

 east of the principality, that deposit is found of 

 great thickness, gradually passing into the under- 

 lying Silurian group, thus showing that the 

 subsidence in the latter district was going on 



