DEVONIAN AND SILURIAN STRATA. 



67 



boniferous and dyassic formations were evolved, are ac- 

 cordingly (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 

 tlie lowest, and therefore the first which clearly bear the 

 mark of their origin as marine deposits. Both groups 

 were formerly comprised under the name of Transition 

 rocks, or Graywacke formation. In them also sandy. 



