of the Bocks of the Coal Formation. 147 



which agree in a remarkable manner with those obtained from the 



good coal. 



Carbon, 81-62 



Hydrogen, .... 578 



Nitrogen, 2*84 



Oxygen, ..... 9*76 



100-00 

 These analyses may then be considered as establishing the fact, 

 that the organic matter which permeates the strata of the coal-forma- 

 tion is chemically identical with coal itself; in fact, that bituminous 

 shale differs in no respect from coal, except in containing a largely 

 preponderating amount of ash, and that whatever may have been the 

 manner in which coal has been formed, bituminous shale must be 

 produced under precisely similar circumstances. The analyses ap- 

 pear, however, further to indicate the existence in the coal-field of 

 vegetable matter in two different phases of decomposition, for the 

 cannel and coarse coals contain a very much larger amount of car- 

 bon, and smaller of oxygen, than the other two, which approximate 

 very closely to one another in composition. Now the gradual de- 

 composition of vegetable matter is attended by the gradual diminution 

 of the oxygen present, carbonic acid, which for every six parts of 

 carbon carries off 22 of oxygen, being gradually evolved, and it is fair 

 to admit that the composition of the good coal and organic matter of 

 the bituminous shale, indicates a less advanced state of decomposition 

 than that of the other two ; and as it must be manifest that all are 

 exposed to the same causes of decomposition now, the difference" 

 which is observed must be due to causes operating before the deposi- 

 tion of the superincumbent strata. 



The analyses of the inorganic constituents of the different rocks 

 also afford evidence of a certain connection among the different 

 members of the formation, though it is less obvious both on account 

 of their greater complexity and the certainty of their being, in 

 some instances, mere mechanical mixtures, liable to great variations. 

 If, however, we compare only that portion of certain of these sub- 

 stances which is insoluble in acids, and which analysis shows to be of 

 constant composition, we arrive at some interesting conclusions. The 

 whole of the fire-clay, with the exception of small quantities of lime 

 and potash, is insoluble in acids. Now, the clays already known to 

 us, are definite mineral compounds, presenting an invariable compo- 

 sition. Porcelain-clay, for instance, is a hydrated sesquisilicate 

 of alumina represented by the formula 2AI2 O3 3Si O3 -|- 3H0, and 

 the same composition is found in all our purer clays, and the fire- 

 clay of the coal formation is also a definite silicate, though not iden- 

 tical with porcelain-clay, its composition, setting aside as non-essential 

 the small quantities of lime and potash, corresponding very closely 

 with the formula Alg Og 2Si O3 4- 2H0. Now, the same clay can 



k2 



