TRANSACTIONS OF SECTION C. 623 



In the Franco-Belgian coal-field the coals become more and more anthracitic 

 as they pass down to greater depths ; both kinds, therefore, were of the same age 

 in formation ; in South Wales also, as already stated, the anthracite and the 

 other coals are all of one age. The squeezing, faultincr, and inversions in the 

 former field are accompanied by an alteration of the highly bituminous coals into 

 dry coals and anthracite. 



An interesting histoiical sketch of the use of anthracite, and some systematic 

 remarks on its distribution in South Wales, were given by J. P. Bevan, F.G.S., in 

 the ' Geologist,' vol. ii., 1859, pp. 7-5-80. 



The anthracite of Pennsylvania is traceable from the inner folds of the moun- 

 tain chain, where the strata have become more and more crystalline, and contain 

 graphite as well as this non-bituminous coal, westward into Ohio, where the same 

 beds consist of ordinary coal. In the eastern part of the Alleghanies the coal has 

 only 6 to 14 per cent, of volatile matter, further west 16 to 22 per cent., 30 to 35 

 per cent., and in Ohio 40 to 50 per cent. (Prestwich.) Tliis coal-field before com- 

 pression was probably 900 miles long by more than 200 broad in some places. 

 (Lyell.) 



The depression of strata by accumulated sediment above them may raise then- 

 temperature by the rise of the isogeotherms (surfaces of equal subterranean tempera- 

 ture), and they may reach a relatively high temperature. ' Mere descent to a great 

 depth, however, will not necessarily result in any marked lithological change, as 

 has been shown in the cases of the Nova-Scotian and South- Welsh coal-fields, 

 where sandstones, shales, clays, and coal-seams can be proved to have been once 

 depressed 14,000 to 17,000 feet below the sea-level, under an overlying mass of 

 rock, and yet to have sustained no more serious alteration than the partial con- 

 version of the coal into anthracite. They must have been kept for a long period 

 exposed to a temperature of at least 212° Fahr. Such a temperature would have 

 been sufficient to set some degree of internal change in progress bad any appre- 

 ciable quantity of water been present, whence the absence of alteration may per- 

 haps be explicable on the supposition that those rocks were comparatively dry.' 



Coal in contact with granite is changed into anthracite or graphite ; when in 

 contact with volcanic and trappean rocks it may become coke (columnar or other- 

 wise) or mere soot. 



Steam coal is very compact, burns with little smoke, and contains so little 

 bituminous matter that it is not liable to spontaneous combustion, whether pyrites 

 be present or not. It is an intermediate kind of coal, having more hydrocarbon 

 than any anthracite has. 



Ordinary coal, common coal, household coal, pit coal, black coal, coal proper, 

 bituminous stone coal : of this there are several sorts : — 



1. Caking coal, coking coal, bituminous coal (not really bituminous, but con- 

 taining the constituents of bitumen — 7 to 9 per cent, of hydrogen, witli carbon and 

 oxygen, or 4 to 6 per cent, of hydrogen and 6 to 8 per cent, of oxA'gen). When 

 heated, it undergoes a kind of fusion and ' cakes ' together, one piece adhering to 

 another by the soft bituminous matter into which it is mainly changed. Such 

 coals are used for coking, coke being more or less impure carbon left after the 

 hydrocarbons have been driven off". 



2. Cherry coal, or soft coal, is thinly laminated, soft, velvety, short-fractured, 

 friable. 



3. Spli7it coal (breaking off" in long ' boards,' and into fragments with angular 

 ends called 'splints' — Mushet), bone coal, hai-d coal, free-burning coal, dry coal 

 (passing into shaly, slaty, and stony coal). This is less bituminous than some of 

 the foregoing ; burns free and open (that is, without swelling and caking), with a 

 long smoky flame ; with less than G to 8 per cent, oxygen and 4 to 6 per cent, 

 hydrogen ; it is also called dry coal. The hard coal comes out in long blocks ; the 

 cherry coal in short pieces. 



Reedy coal has alternate layers of splint coal and bright coal. (Mushet.) 

 Cannel coal, or parrot coal, is compact, and varies from lustrous to a dull earthy 

 aspect ; breaks irregularly, but with a conchoidal (shell-like) fracture ; can be 



' Geikie, Textlooh, kc, 2nd edit., 1886, p. 273. 



