366 Origin of Carhonactious Shales. 



s]iall have been exhausted, and we are compelled to manufac- 

 ture petroleum for ourselves, using nature's material, but sub- 

 stituting our quick for her slow methods. 



Either the minute division of the carbonaceous matter con- 

 tained in bituminous shales, and its distribution through a 

 preponderating mass of inorganic material, or some inherent 

 peculiarity of the plant-tissue which bas furnished it, makes it 

 more prone to spontaneous distillation than the pure and com- 

 pacted hydrocarbons which form co:il ; for the evolution of the 

 gaseous and liquid hydrocarbons from the shales is more con- 

 spicuous than from beds of coal, though noticeable in both ; 

 and tlie shales, though quite black when freshly broken, soon 

 become brown by exposure even in the cabinet. Where sub- 

 jected to the combined action of sun, air and moisture, they 

 rapidly lose the carbon at the surface, and ultimately show only 

 the ashen-grey color of their inorganic constituents. 



The occurrence of ii'on pyrites in bituminous shales may be 

 regarded as one of their charactei'istic features, and nothing is 

 more common than to find the rock along certain lines thickly 

 setwith small, often spheroidal, and sometimes beautifully crys- 

 talized, concretions of pyrite. It is also the material by which 

 organisms of various kinds, shells, bones, wood and the tissue of 

 sea-weeds, are often replaced. The origin of the pyrites is 

 probably due to sulphates, — sulphate of lime, etc., — decomposed 

 by the oxidation of organic matter. The original source of the 

 sulphur is perhaps beyond our reach, but we know that sulphates 

 are constantly present in sea water, and that sulphur exists in 

 organic combination in sea-weeds, these liberating suliohuretted 

 hydrogen sometimes abundantly in their decay. It is not at all 

 uncommon, also, to find concretions of impure carbonate of 

 lime imbedded in bituminous shales. In the Huron shale on 

 the Huron Kiver, at Monroeville, and in the same formation 

 north and east of Columbus, Ohio, such concretions are quite 

 numerous and sometimes large, — eight or ten feet in diameter. 

 They have evidently been slowly formed in place by segregation, 

 and often surround the bones of gigantic fishes [DmicMliys), 

 which have served as nuclei for the concretionary action. 

 The lime may have existed as carbonate in solution, or as sul- 

 phate which was decomposed by decaying vegetable matter. 



