ORIGIN OF COAL AND ITS VARIETIES. 357 



mat coal formed by the above process, and that the extreme forms, an- 

 thracite and graphite, are the result of an after-change produced by 

 heat. But some geologists go further : they believe that anthracite has 

 been changed by intense heat sufficient to vaporize the volatile matters, 

 which then condense in fissures above, as bitumen, petroleum, etc. ; 

 that, as in art, when bituminous coal is subjected to heat out of contact 

 with air, the fixed carbon is left as coke, the tarry and liquid matters 

 are condensed in purifiers, and the permanent gases collected in gasom- 

 eters ; so in Nature, when beds of bituminous coal are subjected to in- 

 tense heat in the interior of the earth, the fixed carbon is left as anthra- 

 cite, the tarry and liquid matters collected in fissures, as bitumen and 

 petroleum, while the gases escape in burning springs. The process is 

 of course slow and under heavy pressure, and therefore the residuum is 

 not spongy like coke. According to this view, anthracite and bitumen 

 are necessary correlatives. 



There can be no doubt that the graphitic and anthracitic varieties 

 of coal are always associated with folding and metamorphism of the 

 strata : 1. In the universally-folded and metamorphic Laurentian rocks 

 only graphite is found. 2. In Pennsylvania, in the strongly-folded and 

 highly-metamorphic eastern portion of the field, the coal is anthracite ; 

 while, as we go westward, and the rocks are less and less metamorphic, 

 the coal is more and more bituminous, until, when the rocks are hori- 

 zontal and unchanged, the coal is always highly bituminous. The same 

 has been observed in "Wales : anthracite is always found in metamorphic 

 regions, and the coal is more and more bituminous as the rocks are less 

 and less metamorphic. 3. Again, the anthracitic condition of coal may 

 be sometimes traced to the local effect of trap or volcanic overflows. 

 In a word, anthracite is metamorphic coal ; and, according to this view, 

 the same heat which changed the rocks has distilled away the volatile 

 matters, which may condense above, as bitumen or petroleum. 



We have given above the common view. It is partly true and partly 

 erroneous. The true view seems to be as follows : 



Anthracite may, indeed, be regarded as metamorphic coal, but it is 

 not probable that bitumen is its necessary correlative ; it is not probable 

 that the heat of metamorphism is sufficient to produce destructive dis- 

 tillation. "We have already shown (page 221) that a moderate heat of 

 300° to 400° Fahr. in the presence of water is sufficient to produce 

 metamorphism. Such a degree of heat would, doubtless, hasten the 

 process explained on page 356. The folding and erosion of the rocks, 

 and the consequent exposure of the edges of the seams, would still 

 further hasten the process, and bring about anthracitism by facilitating 

 the escape of the products of decomposition. In all coal-mines C0 2 , 

 CH 4 , and H 2 0, are eliminated now ; only continue this process long 

 enough, and anthracite and, finally, graphite is the result. We must 



