302' 



COAL. 



order of succession of the integrant rocky strata,, in 

 the numbers and relative proportions of these, and in 

 the numbers, thickness, succession, and qualities, of 

 the beds of coal, They must therefore be considered 

 as independent deposits, varying, as other local collec- 

 tions of strata are known to do, and from analogous 

 causes : namely from having been deposited originally, 

 by independent actions, in separate cavities. 



The strata which accompany the beds of coal, con- 

 tributing to form what is here called the series, consist 

 of sandstones, shales, limestones, and clays. The 

 sandstones are the most abundant, and the limestones 

 occur chiefly or solely in the inferior parts of the de- 

 posit ; where a species of transition seems also often 

 to take place between the proper coal series and the 

 mountain limestone beneath it. The characters of the 

 sandstone vary ; being, in some places, a conglomerate, 

 but more frequently tine ; when it is sometimes coin- 

 pact, pure, and white, at others micaceous, or argilla- 

 ceous, or ferruginous, and tender; occasionally also 

 containing pyrites, and often blackened by carbona- 

 ceous matter, or else including distinct fragments of 

 charcoal. Hence it also presents various colours. The 

 beds themselves are either massive and thick, or di- 

 vided into thinner laminae by intermediate clay or 

 shale, so as to descend even to the tenuity of roofing- 

 slate. At the lower part of the general series, this 

 sandstone is often a conglomerate, intermixed with 

 shales, to which the name of millstone grit has been 

 given in England ; and, in this, limestones also occur. 

 These resemble the inferior, or mountain limestone, 

 though commonly more bituminous, and of a blacker 

 colour. 



The shales vary much in aspect and hardness, pass- 

 ing ut length into clays equally various, and sometimes 



