THK CARBONIFEROUS SYSTEM 321 



r.ituminous coals, which vary in their gas-producing qualities; and 

 lastly, there are the hard coals Steam-coal and Anthracite the 

 last differing in being smokeless when burnt. 



It is now generally agreed that the original substance of all 

 kinds of coal was vegetable matter of some kind, and that many 

 kinds owe their peculiarities to differences in the nature of the 

 vegetable matter of which they were originally composed. Thus 

 the Toula coal of Russia consists entirely of sheets of the bark of 

 the tree called Bothrodendron, and this is so little altered that, 

 when exposed to the weather, it splits up into thin layers. Other 

 coals appear to have been composed of a mixture of woody tissue 

 with leaves and fronds of fern-like plants. Some beds consist of 

 the spores or sporangia of Carboniferous plants ; thus Cannel coal 

 is mainly composed of Lycopod -spores, and some Boghead coals of 

 closely packed cellular bodies which are either spores or gelatinous 

 algae arising from their decay. 



Some other varieties of coal, however, are due to the amount of 

 chemical change which the substance has undergone since its 

 original deposition and burial in the earth's crust, as well as to the 

 varying degrees of pressure to which the strata have been subjected 

 at different places. These influences appear to have led to a con- 

 centration of carbon in the older coals, or rather to an increase in 

 the proportion of carbon to hydrogen. 



Let us next consider the conditions under which the coal-seams 

 are likely to have been accumulated, and it is evident that on this 

 question we can learn much from a study of the associated sedi- 

 ments, that is to say from the general characters and contents of 

 the whole series of measures of which the coal-seams form a part. 

 These beds include sandstones which are generally of fine grain, 

 and often show current bedding ; shales which are consolidated 

 silts, i.e. sandy or micaceous muds deposited in successive layers or 

 lainiuii* ; and clays which are more homogeneous muds. All these 

 beds usually contain only remains of plants, and sometimes of 

 insects, with bivalve shells such as Anthracosia and Carbonicola, 

 which were of fresh or brackish -water habitat. Occasionally, how- 

 ever, there are bands which contain marine shells, and from these 

 we may safely infer that the measures in which such beds occur 

 were formed in swamps or marshes liable to incursions of the sea. 



There is, in fact, no escape from the conclusion that the Lower 

 and Middle Coal-measures of the British region, and probably most 

 of those in the Continent, were formed in extensive swamps on the 

 borders of sinking land, where the area of alluvial levels, swamps, 

 and brackish lagoons was being continually enlarged at the expense 

 of the sea by the process of silting up and by the outgrowth of 



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