DEVONIAN AND SILURIAN STRATA. 6/ 



fences, by which the carboniferous and Dyassic forma- 

 tions were evolved, are accordingly (reading from above 

 downwards) : 



From this account it is also manifest that in cases of 

 incomplete elevation, such as took place in North 

 America, the formation of the middle period is either 

 disturbed or totally omitted, and that it may depend on 

 local causes and the duration of the oscillations if, 

 as in the Russian Permian formations, corresponding to 

 the German Dyas, the boundaries of the subdivisions 

 are more or less obliterated. 



The two series of strata beneath the mountain lime- 

 stone, and reaching the depth of more than 3000 and 

 6000 metres, the Devonian and Silurian formations, 

 are the lowest, and therefore the first which clearly 

 bear the mark of their origin as marine deposits. Both 

 6 



