138 "WALKS AND TALKS. 



Another doctrine generally accepted is the vegetable origin 

 of the great supplies possessing commercial importance. It is 

 admitted that animal remains may be a source of petroleum 

 to a small amount. 



Again, it has been observed that every great oil-containing 

 reservoir has below it not always immediately below a for- 

 mation of the nature known as black bituminous shale. This 

 is soft, easily cut with a knife, and contains a large amount 

 of vegetable matter. Such shales are generally thought to 

 contain quantities of remains of sea-weeds. If so, they exist 

 in a comminuted and obscure state. 



Probably a majority of geologists entertain the opinion 

 that petroleum is produced from these black shales by a slow 

 spontaneous distillation, through the action of the heat in the 

 rocks. By artificial distillation, oil is readily obtained from 

 them, and little doubt is entertained that at a comparatively 

 low temperature, a slow natural distillation proceeds. 



Observation has shown that while black shales manifest a 

 predisposition to the production of oil, pure vegetable deposits 

 are more fixed. Thus, from proper coal-beds no oil proceeds ; 

 but from cannel coal and coaly shales oil is spontaneously 

 evolved, as it also is from the black shales where the vegeta- 

 ble matter has not attained a coaly condition. The mixture 

 of argillaceous matter with the vegetable material seems to 

 favor the oil-making process. 



Natural gas has an origin very similar to that of petro- 

 leum. The inflammable gas now so extensively employed as a 

 substitute for coal, is also composed, like petroleum, of car- 

 bon and hydrogen, but with a larger proportion of hydrogen. 

 It must be derived, in a similar way, from a similar source. 

 Petroleum, in fact, is generally associated with gas. It seems 

 to be composed of the heavier and more fixed compounds of 

 carbon and hydrogen containing much carbon, while gas is 

 a lighter compound with more hydrogen. Petroleum, how- 

 ever, is not a simple compound of definite composition, but a 

 mixture, apparently of many compounds the more fixed, like 

 asphalt and paraffine, being dissolved in the fluids kerosene. 



