Origin of (Jarbonacedus Shales. :3Go 



distillation through which the end is reached in a series of nio^|e 

 or less distinct steps ; that is, a fractional distillation results in 

 the formation of evolved products, liquid or gaseous, which are 

 slowly but constantly generated, and are, for the most part, 

 immediately liberated, rising to the surface by hydrostatic press- 

 ure. Usually we find in the shales only the niateiial out of 

 which the volatile hydrocarbons can he manufactured ; and as 

 these are rapidly dissipated when formed, hand specimens and 

 even larger exposed masses rarely show them ; but in a great 

 number of locali;ies petroleum is found saturating the shale, 

 betraying its presence by its characteiistic odor when the rock 

 is freshly broken ; ;ind it is sometimes present in such quantity 

 as to foi-m an oily film when fragments are thrown into water. 

 Porous and shattered rocks which overlie the black shales are 

 often the reservoirs which receive the evolved products of their 

 spontaneous distillation ; and here we find all the great stores of 

 petroleum which supply the extensive commei cial and industrial 

 operations based upon it. I have elsewhere discussed the genesis 

 of petroleum and carburetted hydrogen from these shales, and it 

 is not necessary to treat the subject at length here. The hydro- 

 carbons which have become so important in the economy of 

 civilization, have been attributed by some to the action of in- 

 organic causes. By others, who agree with me in ascribing 

 them to an organic source, they have been regarded as emana- 

 tions from other rocks than the bituminous shales ; but no 

 examples of the occurrence of these hydrocarbons in nature, 

 except in connection with organic substances, are known ; and 

 as a matter of fact, no considerable accumulation of petroleum 

 has been discovered except in close relationship with this group 

 of carbon-bearing rocks. They are the great repositories of the 

 materials from which the gaseous and volatile hydrocarbons can 

 be produced, and we may say the only ones. They claim our 

 interest, therefore, as the apparent source of the liquid and 

 gaseous hydrocarbons which are of economic importance ; and 

 we must not only credit them with all the benefits conferred 

 upon society by the excellent illuminator now furnished to 

 every family at so low a price, but we must look to them as the 

 source of supply of this necessity, as we may call it, when in the 

 not distant future the stores produced by nature's processes 



