PALEOZOIC ROCKS AND ERA. 313 



times a gradual purification of the air of its superabun- 

 dant moisture by increase of the size and height of con- 

 tinents, and of its superabundant C0 2 by its withdrawal 

 in many ways, but during the Coal period especially by 

 the growth of plants and the preservation of the carbon 

 as coal. In this process not only was the C0 2 removed, 

 but oxygen restored, and thus was the air prepared for 

 the use of air-breathing, hot-blooded animals, such as 

 birds and mammals, which were accordingly introduced 

 soon afterward. 



Petroleum and Bitumen. 



We take up these here, not because they are peculiar 

 to the coal-measures, for such is not the fact, but because 

 they seem to have been formed from organic matter by 

 a process similar to that of coal, and also because some 

 think they are actually formed from coal by distillation. 

 This, however, is not probable. 



If bituminous coal, or any organic matter, be heated 

 red-hot, out of contact with air, the volatile matters are 

 driven off, broken up, and recombined, and may be col- 

 lected in a great variety of forms of hydrocarbons some 

 solid, as coal-pitch; some tarry, as coal-tar ; some liquid, 

 as coal-oil; some volatile, as coal-naphtha; and some 

 gaseous, as coal-gas. Now, a somewhat similar series of 

 hydrocarbons is found in the earth and issuing on its sur- 

 face : some solid, as asphalt, Alb er tit e, Graliamite, etc. ; 

 some tarry, as bitumen ; some liquid, as petroleum ; some 

 volatile, as rock-naphtha ; some gaseous, as the gas of 

 burning-springs. It is almost certain also that these are 

 of organic origin. 



Mode of Occurrence. Petroleum occurs in the strata 

 much as water does, and the two are often associated. 

 Like water, and with water, it is found in porous and 

 fissured strata, such as sandstones and limestones, when 

 these are covered with a stratum of impermeable shale. 



