680 THE JOURNAL OF GEOLOGY. 



J. P. Lesley, 1 in 1879, offered some interesting suggestions. 

 If the anthracite be metamorphosed bituminous coal, the change 

 might be caused by exposure to comparatively high temperature 

 at a great depth below the surface. As the temperature increases 

 one degree Fahrenheit for each fifty feet, more or less, of 

 descent, the coal under cover of a great thickness of rock could 

 not fail to be deprived of its volatile matter. He compares the 

 composition of coal from the highest available bed in western 

 Pennsylvania with that from the lowest bed in the same region, 

 and finds less volatile in that from the lower bed. As all of 

 the Paleozoic rocks thicken eastwardly, there must have been a 

 much greater pile of Coal Measures in the anthracite region than 

 in the bituminous areas, though erosion has removed the proof. 

 Necessarily then the coals of the anthracite region should show 

 less volatile than do those of the bituminous area, where the 

 pile of rocks was less thick. 



Professor Lesley suggests also that if one desire to explain 

 the origin of the anthracite by oxidation in preference to meta- 

 morphism, the conditions afford basis for such explanation, 

 since in the anthracite region the rocks are not only broken and 

 shattered by the folding, but they are made up largely of sand 

 and gravel, so that the conditions are such as to favor percola- 

 tion of water, evaporation, and consequently oxidation ; whereas, 

 in the undisturbed bituminous areas, clayey beds are in large 

 proportion and lute down the buried coals so as to prevent per- 

 colation and the rest. 



There is no possible room for doubt that bituminous coal can 

 be converted into anthracite by heat. The Galisteo, Elk Moun- 

 tain and other localities within the United States, the Hesse 

 Cassel and New Zealand areas in foreign lands, prove beyond 

 dispute that, under proper conditions, contact with molten rocks 

 suffices for the conversion. But no question of such conversion 

 is at issue here, for in Pennsylvania no dikes occur near enough 

 to the anthracite areas, or large enough even if near enough, to 



1 Lesley : In McCreath, 2d Geol. Surv. of Penn., 2d Rep. of Progress in the 

 Laboratory, etc. 1879, pp. 153, et seq. 



