REVIEW OF GEOLOGICAL STEUCTUEE. 17 



2. The bituminous shales which underlie the oil reservoirs contain 

 from ten to twenty per cent, of combustible matter, and they are there- 

 fore the greatest repositories of the materials out of which petroleum can 

 be manufactured. They have already been successfully employed for the 

 production of oil and gis by artificial means, and they constitute the 

 source from which we must derive our illuminating oil whenever the 

 supply of natural oil shall fail. 



3. The organic matter of the bituminous shales is an unstable com- 

 pound, which is constantly undergoing spontaneous decomposition. This 

 results in the formation of water, carbonic acid, and the liquid and gas- 

 eous hydrocarbons. Whenever exposed to the air, the bituminous shales 

 soon lose all their carbonaceous matter, and even when buried in the 

 earth, especially where loosened and broken along lines of disturbance, 

 they are undergoing spontaneous distillation. 



4. Observation connects oil and gas springs directly with beds of bitu- 

 miaous shale. A line of gas and oil springs marks the outcrop of the 

 Huron shale from New York to Tennessee, and the porous rocks overlying 

 this formation, as well as the Waverly sandstones above the black Cleve- 

 land shile, are n thousands of localities charged with petroleum. The 

 underlying rocks are not so saturated. 



5. Wells bored through the strata overlying the black shales which 

 have been mentioned, never obtain any considerable quantity of oil or 

 gas below them. If, as has been claimed, limestones lying still lower 

 were the source of the oil and gas, the strata nearer them and below the 

 black shiles should be at least as highly charged as those nearer the sur- 

 face. A9 a matter of fact, no porous strata lying above limestones like 

 the Corniferous are ever reservoirs of oil, while there are almost no por- 

 oai strata immeJiately overlying black shales that are not more or less 

 impregnated. 



Prof. Dana says (Manual, page 26S) : "The oil obtained from this rock 

 (the Huron shale) is not present in it as oil, for no solvents will separate 

 it; it is producad by the heat of distillation out of carbonaceous matter 

 prisent." This statement requires qualification, as the Huron shale is 

 som3time? fjund chirgeJ not only with carbonaceous matter, but with 

 oil. In these circumstances, when broken, the stone emits a strong odor 

 of patroleum, and fragments thrown into water diffase an oily scum over 

 the surface. 



In the remarks on petroleum on pages 70 and IGO of Volume I, Geology, 



it is suggested that the difference in productiveness between the oil wells 



of Pennsylvania and Ohio, losated at the same geological horizon, is 



largely due to the facts that the strata overlying the Huron shale in Ohio 



2 



