PALEOZOIC ROCKS AND ERA. 315 



ous. In Ohio it is in the Devonian and in the Silurian, 

 especially the latter. In California it is in the Miocene 

 Tertiary of the Coast Kange. 



Origin of Petroleum. It is probable that petroleum 

 was formed by a change of organic matter, somewhat 

 similar to that which makes coal, but from a different 

 kind of organic matter, and under different conditions. 

 Land-plants, in the presence of fresh water, form coal ; 

 while marine plants, and sometimes lower animals, in the 

 presence of salt water, form petroleum, bitumen, etc. 

 It has been observed that petroleum is often found in 

 connection with salt. 



Origin of Varieties. But, however formed in the 

 first instance, there is no doubt that the different varieties 

 or physical conditions are formed from each other by the 

 passing away of gaseous hydrocarbon. In this manner 

 light oil changes into heavy oil, and this into bitumen, 

 and finally into asphalt. Thus there are two series de- 

 rived from organic matter, the coal series and the petro- 

 leum series. By successive changes, coal passes from 

 fat-coals to bituminous, then semi-bituminous, then an- 

 thracite, and finally graphite ; petroleum from light oil 

 to heavy oil, then bitumen, asphalt, jet, and possibly 

 diamond. But the origin of diamond is uncertain. 



Fauna of the Carboniferous Age. 



As already stated, we shall take up the fauna of the 

 sub-Carboniferous and Carboniferous together ; only let 

 it be remembered that the land and fresh-water animals 

 are from the coal-measures, especially the vertebrates, 

 and the marine animals are mostly from the sub-Car- 

 boniferous. We shall touch only the most prominent 

 points. 



We have nothing characteristic to add about corals, 

 but only draw attention here to an exceedingly curious 



