966 A TREATISE ON METAMORPH1SM. 



this it is necessary to consider the relations of man to agriculture. The 

 limestone soils of the United States, and of many other parts of the world, 

 are of surprising' fertility. The historian tells how settlement followed the 

 limestone soils of the Great Valley, of the Blue Grass region of Kentucky 

 and Tennessee, and of the Upper Mississippi Valley. The world over, 

 limestone soils have unsurpassed fertility. Since the products of the soil 

 are the materials of most importance to man, the greatest geologic formation 

 from an economic point of view is limestone produced by the segregation 

 of carbon and calcium. 



segregation in carbonaceous deposits. — Besides the carbon which is segregated as 

 carbon dioxide in the lithosphere by carbonation, vast quantities have 

 been buried in the rocks in forms varying from nearly pure carbon, as 

 graphite and anthracite, to cellulose. Various estimates have been made 

 of the amount of carbon in the better-known coal beds, but so far as I know 

 no attempt has been made to estimate the amount of carbon in all the coals, 

 good and poor, in the carbonaceous shales associated with them, and in the 

 shales and other rocks not associated with coal beds. The fact that 

 carbon as hydrocarbons exists in shales has been usually overlooked. 

 Carbon forms 0.81 per cent of 78 rocks analyzed. (See p. 938.) This 

 quantity seems small, but when the enormous mass of the shales is con- 

 sidered, 438,750,000,000,000,000 metric tons, it is seen that this percentage 

 amounts to 3,553,875,000,000,000 metric tons. If this amount of carbon 

 were oxidized to C0 2 it would represent 5,470 times the amount of carbon 

 dioxide in the atmosphere, as calculated above. If it could be consumed at 

 the rate of 1,000,000,000 metric tons per year, which is more rapid than 

 the present rate of combustion of coal, it would last over 3,500,000 ) r ears. 

 This great amount of carbon, in these, as well as in some other rocks, is 

 ordinarily overlooked. It is therefore certain that the total amount of car- 

 bon contained as hydrocarbons in rocks of all kinds, from the great coal 

 seams to the sediments containing but a very small percentage, is enormous. 



Next to the limestones the economic product of vastly greater impor- 

 tance than any other is coal. Coal contains on an average 80 per cent 

 carbon, or 364 times the amount in the original igneous rocks, in which the 

 carbon amounts to only 0.22 per cent. The inestimable economic value of 

 coal is so fully appreciated that it need not be emphasized. The point to 

 be understood is that the forces and agents of geology have segregated 

 this immeasurably valuable material from a fraction of 1 per cent of 



