176 CARBONIFEROUS. 



by sandstone (features known by the name of " rock faults " 

 among the colliers), the rock undoubtedly occupies the old beds of 

 a stream and its tributaries, which became gradually silted up. 



The constancy of coals is, however, a very rernarkable fact. In 

 the case of the 'Arley INIine' in Lancashire, which is the same as 

 the Silkstone coal of Yorkshire, we have a seam that seldom 

 exceeds five feet in thickness, which originally must have spread 

 over an area of 10,000 square miles. ^ 



There is still some difficulty in accounting for the purity of coal, 

 and its general freedom from foreign materials. If, however, we 

 picture a vast alluvial plain covered with a Cryptogamic forest of 

 giant Lepidodendra and Sigillarice growing on a stiff tenacious 

 clay-soil, capable of retaining the rain-fall, then we have the con- 

 ditions suited for the rapid production of peat ; and that is the 

 purest form we know of any great accumulation of vegetable 

 matter unmixed with foreign material, freedom from which is the 

 peculiar feature of the Coal-seams.- The general purity of peat 

 is sometimes produced by the cleansing of muddy water in passing 

 through a reedy marsh. 



While most coals were certainly land-growths. Prof. Green 

 observes that Cannel Coal, and some of the small lenticular patches 

 of coal occasionally met with in shales and sandstones, were 

 formed under water. Cannel Coal occurs in lenticular patches, 

 associated sometimes with ordinary coal, but it very frequently 

 contains Fish-remains, and is often found to pass gradually 

 into carbonaceous shales. Probably the plants of which it is 

 composed were drifted into shallow ponds or lakes, and reduced 

 by soaking to a vegetable pulp. 



There are many different kinds of coal, and these are due partly 

 to the pressure and chemical changes they have undergone, and 

 partly no doubt to their original composition. The principal 

 varieties of coal are as follows : — 



Anthracite or Stone-coal, which is the most highly mineralized form 

 of coal, has a shining conchoidal fracture, and does not soil the 

 fingers. It does not ignite so readily as other kinds of coal, but 

 is almost pure carbon, containing from 87 to 94 per cent, of it, 

 and on burning leaves but little ash. It is sometimes described 

 as non-bituminous, and has probably been altered by pressure, 

 if not by the proximity of igneous rocks ; it exhibits no traces of 

 organic structure. Cul?n is a variety of Anthracite. 



Bituminous coal (ordinary house coal) is rich in hydrocarbon 

 gases, although it contains no bitumen, but it is termed bituminous 

 because it has a 'more flaming character in burning than anthra- 

 cite.' The varieties generally recognized are mostly named after 

 their application or chief properties : Free burning, steam or 



^ Hull, Coal-fields of Great Britain, edit. 4, p. 255. 



''■ Dr. H. Woodward, Geol. Mag. 1871, p. 500; P. Geo!. Assoc, ii. 241. 

 See also Carruthers, Geol. Mag. 1S69 p. 2S2 ; and E. W. Binney, Mem. Lit. 

 Soc. Manchester (2), viii. 148. 



