X.] BITUMENS AND PYROSCHISTS. 177 



leave, when this is removed by solvents, a residue of woody tissue. 

 (Proc. Geol. Soc. London, May, 1860.) These observations have 

 been confirmed by an eminent microscopist and chemist, whose 

 results, lately communicated to me by himself, are not yet pub- 

 lished. 



The chemical changes by which the conversion of woody tissue 

 into peat, lignite and bituminous coal is effected, are too well known 

 to be repeated here. The abstraction of variable proportions of 

 water, carbonic acid, and marsh-gas may give rise either to hydro- 

 carbons like C 24 H 8 , which represents idrialine and the basis of most 

 bituminous coals, to C 24 H 16 , which is the approximate formula of 

 the hydrocarbons of many asphalts, or to C 24 H 24 , which represents 

 petroleum. The removal of further amounts of marsh-gas, C 2 H 4 , 

 may even convert bituminous coal into anthracite, as Bischof has 

 pointed out ; and we conceive that although heat has in many cases 

 given rise to this conversion, by a subterranean coking, the change 

 may often have been the result of decompositions going on at 

 ordinary temperatures. Anthracite or nearly pure carbon, on the 

 one hand, and petroleum or carbon with a maximum of hydrogen, 

 on the other, represent the two extremes of a series of which bitu- 

 minous coals and asphalts are intermediate terms. 



Petroleum, as is well known, impregnates certain rocks, from 

 which it flows spontaneously, and the solid forms of bitumen are 

 often disseminated throughout limestones or sandstones, from which 

 they may be in part removed by heat, and more completely by sol- 

 vents such as benzole. To such rocks the term " bituminous " may 

 be correctly applied, but it is often inappropriately given to sub- 

 stances like coal and certain combustible schists, which contain 

 little or no bitumen, but yield, by destructive distillation, volatile 

 hydrocarbons, more or less resembling those obtained from asphalt 

 or petroleum. Analogous products are, however, obtained by the 

 distillation of lignite, peat, and even of wood, so that the epi- 

 thet " bituminous," applied to hydrogenous coals and combustible 

 schists, raises a false distinction, and perpetuates an error. I 

 therefore proposed some time since to distinguish these so-called 

 bituminous schists, the brandschiefer of the Germans, by the name 

 of pyroschists. This is the equivalent of the German term, and has 

 a precedent in the name of pyrorthite, given by Berzelius to a sub- 

 stance which appears to be a mixture of orthite with a combustible 

 hydrocarbonaceous matter. Pyroschists are well known to occur 

 in almost every geological group from the Cambrian to the tertiary, 

 8* L 



