564 GEOLOGY. 



The Coal. 



The general conditions under which sandstone, shale, and lime- 

 stone originate have already been stated, and need not be here repeated, 

 but in the discussion of the formations of the preceding periods there 

 has been no occasion to consider the formation of coal. From its 

 economic importance, this sort of rock has been studied with more 

 care than most others, and geologists are agreed, in a general way 

 at least, as to its mode of origin. 



Origin. — There is no doubt that the coal is of vegetable origin. 

 Except by the accumulation of vegetable matter, no way is known 

 by which such beds of carbon could be brought into existence. Fur- 

 thermore, the coal and its associated shales contain abundant remains 

 of plants, and sometimes even recognizable tree-trunks, in the form of 

 coal, and microscopic study has revealed the remarkable fact that the 

 coal itself, even the hardest anthracite, is often but a mass of altered, 

 though still recognizable, vegetable tissues. Concerning the exact 

 manner in which the beds of vegetable matter accumulated, there 

 is some difference of opinion, and concerning the conditions under 

 which it was converted into the various sorts of coal, there is still 

 more. 1 



Much of the coal is essentially pure, containing little matter of 

 any sort which was not in the plants which gave origin to it. Purity 

 does not mean freedom from ash, since mineral matter, which on 

 combustion becomes ash, is present in all plants. 2 Along with the 

 large amount, of coal which is pure, or nearly so, there is much which 

 contains some admixture of earthy matter. Where the admixture 

 •of earthy matter is small, the coal may still be used; but from poor 

 coal of this sort, there are all gradations into carbonaceous shale, the 

 percentage of carbon becoming so low as to show itself only in a black 

 color. Black shales are common associates of coal-beds. 



The purity of many coal-beds, not only locally but over great areas, 

 seems to warrant the conclusion that such beds were made of vegeta- 

 tion which grew where the coal now is. The vegetation concerned 



1 Discussions concerning the origin of coal are to be found in many of the geological 

 reports of the States where coal occurs, notably Ohio, Pennsylvania, Illinois, Iowa, 

 Missouri, and Arkansas. 



2 Many of the modern allies of the coal-plants contain as much as five per cent 

 of ash, and some as much as twenty* 



