630 GEOLOGY 



as present knowledge goes. In the Arctic lands of America, the 

 Mississippi an and Pennsylvanian are not differentiated, but one 

 or both are somewhat wide-spread. 



Thickness. The thickness of the Pennsylvanian system has a 

 wide range, but like all preceding systems of the Paleozoic, it is 

 especially thick (4,000 to 5,000 feet) in the Appalachian Mountains. 

 In the interior, the corresponding formations rarely much exceed 

 1,000 feet; but in Arkansas, the Coal Measures have been assigned 

 the remarkable thickness of more than 18,000 feet, from which it is 

 inferred that there must have been land close at hand capable of 

 supplying sediments in great quantity. This was probably the 

 axis of the Ouachita uplift. In Texas, the thickness of the system 

 ranges up to 5,000 feet, and in the west, the maximum thicknesses 

 exceed all those mentioned above, except that of Arkansas. 



COAL 



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

 stone originate have already been outlined, but there has been no 

 occasion heretofore to consider the formation of coal. From it< 

 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 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. 

 Furthermore, the coal and its associated shales contain abundant 

 remains of plants, sometimes even recognizable tree-trunks in the 

 form of coal, and microscopic study has revealed the fact that the 

 coal itself is often but a mass of altered, though still recogni/ablo 

 vegetable tissues. Concerning the exact manner in which the I MM Is 

 of vegetable matter accumulated, and concerning the conditions 

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

 is some difference of opinion. 



Much of the coal is essentially pure, containing little man- 

 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 



