X.] ON THE ORIGIN OF COAL. 181 



from the microscopical studies of Dawson and others that, although ' 

 it is doubtless true that cellulose may yield bodies having the 

 chemical composition of bituminous coal, and even bitumens, by a 

 process of alteration such as I have described above, the chief 

 source of such coal in the older coal measures has been epidermal 

 tissues, which differ from cellulose in being much richer in carbon 

 and hydrogen. These tissues, as remarked by Dawson, " are very- 

 little liable to decay, and resist more than -most other vegetable 

 matters aqueous infiltration, properties which have caused them to 

 remain unchanged and resist the penetration of mineral substances 

 more than other vegetable tissues. These qualities are well seen 

 in the bark of our American white birch (Betula alba). It is no 

 wonder that materials of this kind should constitute considerable 

 portions of such vegetable accumulations as the beds of coal, and that 

 when present in large proportion they should afford richly bitu- 

 minous beds. All this agrees with the fact apparent on examination 

 of common coal, that the greater number of its purest layers con- 

 sist of the flattened bark of sigillariae and similar trees, just as any 

 single flattened trunk imbedded in shale becomes a layer of pure 

 coal. It also agrees with the fact that other layers of coal, and 

 also the cannels and earthy coals, appear under the microscope to 

 consist of finely comminuted particles, principally of epidermal 

 tissues, not only of the fruits and spore-cases of plants, but also 

 of their leaves and stems." 



In this connection I noticed in the same paper the chemical com- 

 position of the epidermal or cortical tissue of plants, to which the 

 name of suberin has been given, and compared it with that of the 

 spores of lycopodium, and at the same time with cellulose and with 

 forms of coal and related bodies. The nitrogen which the first two 

 mentioned bodies contain probably represents a portion of albuminoid 

 matter, which in lycopodium is considerable in amount. For the 

 purpose of comparison empirical formulas corresponding to twenty- 

 four equivalents of carbon have been calculated for these bodies, as 

 already done on page 176. We have then as follows : 



Cellulose C 24 H 2o 2 o 



Cork C 24 H 18 . 2 6 . 7 



Lycopodium . C 24 H 19 . 4 N0 5 . 6 



Peat (Vaux) . C 24 H 14 . 4 10 



Brown coal (Schrbtter) c 2 4 H i4- 3 io-6 



Lignite (Vaux) C 24 H n . 3 6 . 4 



Bituminous coal (Kegnault) . . . . . C 24 H ]0 3 . 3 



