I30 



The National Geographic Magazine 



UNITED 5TATL5 332.919.34.1 s.T. 



Diagram Showing the Output of the 

 Coal Producing Nations 



tion under considerable pressure, but 

 only ordinary temperature. Where the 

 rocks are undisturbed, this is probably 

 an exceedingly slow process, but where 

 the rocks are upturned and broken, the 

 products of distillation find a ready 

 means of escape and the metamorphism 

 may go on at an extremely rapid rate. 

 Naturally the escaping gases are the 

 lightest hydrocarbons, and the material 

 remaining is the heavier, or fixed, car- 

 bon. 



In a general way, time is an important 

 factor in bringing about this change, and 

 consequently the older carboniferous 

 coals of the east are more highly altered 

 than the younger coals of the west. They 

 are generally converted into bituminous 

 coals, or, in the case of the eastern fields 

 of Pennsylvania and Rhode Island, the 

 intense folding, together with the devel- 



opment of joints and slaty cleav- 

 age, has converted the coal into 

 anthracite. 



If the coal is cut by dikes or 

 sheets of molten lava, as fre- 

 quently has been the case, rapid 

 alteration occurs and the coal is 

 converted into coke or anthra- 

 cite coal. Such cases occur only 

 in fields in which there has been 

 considerable volcanic activity. 

 High-grade coals may be pro- 

 duced in this manner, but gener- 

 ally only a small area is affected, 

 and consequently the results are 

 not of great commercial im- 

 portance. 



The progressive change in 

 composition is shown in the 

 diagram on page 131, which rep- 

 resents the actual chemical com- 

 position, as shown by proximate 

 analyses, from the poorest grade 

 of Texas lignite to the best qual- 

 3 RITA IN ity of Pennsylvania anthracite. 



The increasing value is shown 

 ^^^^^B by the relative proportions of 

 fixed carbon (fixed carbon is the 



Prmcipal carbon remaining after the vola- 

 tile hydrocarbons have been 

 driven off at a low heat) and the 

 decreasing amount of volatile matter and 

 moisture. In this case the fixed carbon 

 varies from 19 per cent in the lignite to 

 88 per cent in the anthracite. The vola- 

 tile matter varies inversely as the fixed 

 carbon, being greatest in the lignite and 

 least in the anthracite. The moisture also 

 diminishes in quantity from the lignite 

 to the anthracite, but the rate is not reg- 

 ular, since much of the moisture is due 

 to the conditions of sampling rather than 

 to the chemical composition of the coal. 

 The .ash is variable, depending largely 

 upon the amount of earthy matter that 

 was washed into the old swamp during 

 the growth of the coal-forming plants. 

 The presence of ash is an important fac- 

 tor in the commercial value of a coal, but 

 theoretically it forms no inherent part of 

 the coal and should not be considered. 

 In a general way, coals may be divided 



