REVIEW OF GEOLOGICAL STRUCTURE. 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 gas by artificial means, and they constitute the 
source from which we must derive our illuminating oil whenever the 
supply of natural oil shall fail. 
8. 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- 
minous 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 shale, 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 anv 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 shales should be at least as highly charged as those nearer the sur- 
face. Asa matter of fact, no porous strata lying above limestones like 
the Corniferous are ever reservoirs of oil, while there are almost no por- 
ous strata immediately overlying black shales that are not more or less 
impregnated. 
Prof. Dana says (Manual, page 268): ‘The oil obtained from this rock 
(the Huron shale) is not present in it as oil, for no solvents will separate 
it; it is produced by the heat of distillation out of carbonaceous matter 
present.” This statement requires qualification, as the Huron shale is 
sometimes found charged not only with carbonaceous matter, but with 
oil. In these circumstances, when broken, the stone emits a strong odor 
of petroleum, and fragments thrown into water diffuse an oily scum over 
the surface. 
In the remarks on petroleum on pages 70 and 160 of Volume I, Geology, 
it is suggested that the difference in productiveness between the oil wells 
of Pennsylyania and Ohio, located at the same geological horizon, is 
largely due to the facts that the strata overlying the Huron shale in Ohio 
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