ON COAL. 151 



We have already said that bituminous coal may be considered as a 

 mechanical mixture of carbon and bitumen, and these two may easily 

 be separated by heat. Anthracite is the residue after separation, and 

 bitumen and naptha is the matter separated by distillation and con- 

 densed elsewhere. As in the gas manufactories we find bituminous 

 coal decomposed — a part remaining behind as coke, (pure carbon,) a 

 part passing off as gas and a part collecting in pipes as coal tar — so 

 in the laboratory of Nature coal beds subjected to heat give rise to the 

 same three substances ; anthracite is left behind, coal gas is dis- 

 charged into the atmosphere and bitumen collects in subterranean 

 pipes and gives rise to naptha and bituminous springs, pitch lakes, 

 &c. Thus, the enormous lake of boiling pitch in Trinidad is, proba- 

 bly, in connexion with coal strata below. If so, such coal will be left 

 in the condition of anthracite. All the strata of the earth are subject 

 to change under the influence of heat: limestones become marbles, 

 clays become slate. This change is called by geologists metamor- 

 phism. Now, the proposition is that anthracite is metamorphic coal. 

 The proofs of this proposition are as follows : 



In the first place, anthracite is never found except in regions very 

 much disturbed by igneous agency, the strata highly inclined, contorted 

 and broken ; and even in the same coal field the coal is anthracite or 

 bituminous, according as the region is more or less disturbed. Thus, 

 in eastern Pennsylvania^ where the coal strata are very much con- 

 torted and sometimes perpendicular, (fig. 9,) the coal is all anthra- 

 cite ; while in western Pennsylvania, where the strata are nearly 

 horizontal, the coal is bituminous. The actual transition of anthra- 

 cite into bituminous coal cannot be studied with advantage in Penn- 

 sylvania, because the coal strata have been carried away to such an 

 extent that only outlying patches are left ; but in Wales the same 

 seam may be traced from the bituminous to the anthracite condition ; 

 so that there can be no doubt that, in this case at least, anthracite is 

 metamorphic coal. 



Second. Anthracite is never found except in metamorphic rocks, 

 and conversely all coal contained in metamorphic strata is anthracite. 

 This universal connexion of two things proves, as it seems to me, 

 beyond doubt, their community of origin ; that they have a common 

 cause. Thus, in the lowest stratified or primary rocks, where the 

 rocks are altogether metamorphic, and even in the silurian, where a 

 less complete metamorphism is almost universal, what little coal is 

 found is always anthracite. In the coal measures we have coal both 

 bituminous and anthracitic, but the anthracite always in altered and 

 the bituminous in unaltered rocks. As we pass upward we find 

 anthracite more rare, because metamorphism is more rare and local ; 

 and when metamorphism entirely disappears in the tertiary rocks we 

 find that anthracite disappears also. 



Third. Trap dykes, as it is well known, are formed by the out- 

 breaking and outpouring of melted rock (lava) forced up through 

 the superincumbent stratified rocks, which are altered and rendered 

 metamorphic by the contact. Now, when a dyke passes through coal 

 strata the coal is alwa,ys thoroughly coked by the contact ; that is, it 

 is changed into a substance identical in chemical composition with 



