ORIGIN OF PETROLEUM AND BITUMEN. 389 



gas and the concretion of the oil. It is hardly probable, therefore 

 that a light oil will ever be found in the California oil-region.* 



In gently-folded strata the most productive portions seem to be 

 along a line of anticline ; because there we may expect large fissures, 

 and also, perhaps, because the oil working up on the surface of water, 

 is apt to accumulate under the saddles of the strata. 



Origin of Petroleum and Bitumen. 



We have seen that the whole petroleum and bitumen series may be 

 made artificially by destructive distillation of coal. There seems also 

 to be little doubt that certain organic matters at ordinary temperature, 

 in presence of abundant moisture, and out of contact of air, will un- 

 dergo a species of decomposition or fermentation by which an oily or 

 tarry substance, similar to bitumen, is formed. In the interior of heaps 

 of vegetable substance such bituminous matter is often found. 



There are therefore two general theories of the origin of petroleum : 

 one, that it is produced by the distillation at high temperature of bitu- 

 minous coal by volcanic heat, the coal being left as anthracite ; the 

 other,, that it is formed at ordinary temperature by a peculiar decompo- 

 sition of certain organic matters. The evidence in favor of the first 

 view is the similarity between the artificial and the natural series ; the 

 objection to it is that the occurrence of petroleum seems to have no 

 necessary connection with the occurrence below of coal-seams, and also 

 that petroleum is found mostly in strata which have not been subjected 

 to any considerable heat. 



The argument for the other view is the fact that we actually find 

 fossil cavities in solid limestone containing bitumen, evidently formed 

 by decomposition of the animal matter. So, also, shales have been 

 found in Scotland filled with fishes, which have changed into bitumen. 



The most probable view seems to be that both coal and petroleum 

 are formed from organic matter, but of different kinds and under 

 slightly different conditions — that coal is formed from terrestrial vas- 

 cular plants, in the presence of fresh water, while bitumen and petro- 

 leum are formed from more perishable cellular plants and animals, in 

 the presence of salt-water. We have already noticed the frequent asso- 

 ciation of petroleum and salt.f 



According to this view, taking the composition of petroleum as 

 C n Il2n+2> the reaction by which it is formed from vegetable matter is 

 expressed in the following : 



* Some tolerably good oil has been found in California in metamorphic strata. 



f Recently the chemist Mendeljeff has revived the theory of the mineral origin of 

 petroleum. According to him, it is probably made by reaction at high temperatures of 

 vapor of water (H 2 0) on carbide of iron (F 2 C). It is hardly probable that geologists 

 will accept this view. 



