196 a\tiite: relations between coal and petroleum 



This process covers the compression, the gradual dehydration, 

 Uthification and induration of carbonaceous rocks, including the 

 coals; the development of cleavage and even schistosity in the 

 deposits; and, simultaneously and progressively, the elimination 

 of the combined oxygen, the hydrogen, the nitrogen, and a part 

 of the carbon of the organic debris. By this process the peats 

 are gradually transformed to lignites, sub-bituminous coals, 

 bituminous coals, semi-bituminous coals, anthracites, and even 

 to graphites, while the associated organic muds are altered to 

 oil rocks or cannels, to carbonaceous shales, and finally to 

 graphites or graphitic slates. This alteration and reduction of 

 the carbonaceous deposits is most clearly marked by the progress 

 of the elimination of the ''volatile matter," 



The changes both in the physical features and in the chemical 

 composition of the deposit, as it is transformed from peat to 

 graphite, constitute in effect a metamorphism of the organic 

 matter. 1'' The extent of the transformation, that is, its progress, 

 varies with the extent of the dynamo-chemical action. 



The study of the coals, in many coal fields, and of different 

 ages, shows that the alteration may be produced in two ways. 

 The first is through distillation, by local heat incidental to con- 

 tact with sills, dikes or flows of molten rock. Such contact 

 metamorphism, though frequent and conspicuous, is so closely 

 confined to the near vicinity of the igneous intrusives, that its 

 effects, from a regional standpoint, are practically negligible. ^^ 

 The other dynamic cause of the alteration of the organic debris 

 is pressure. 



The study of the stages in the alteration of the coal in differ- 

 ent regions shows, in general, a close relation between the extent 

 to which the rocks have been subjected to thrust pressure meta- 

 morphism and the degree to which the mother peats and sapro- 



'" The chemical combinations actually existing in coals and petroleums of 

 different ranks and types are largely unknown. Portions of the coal leached by 

 reagents and extractions or separations from petroleums have bef>n chemically 

 identified, and products resulting from destructive distillation of both the solid 

 and the liquid fuels have been determined; but most of these probably are com- 

 pounds obtained by breaking up the huj^ly undetermined original chemical 

 combinations. 



" Bureau of Mines, Bull. 38: lUl. 1913. 



