32 



STEVENSON— FORMATION OF COAL BEDS. [April i8, 



so constant as to characterize the bed in great areas ; the composition 

 of the several divisions may be similar or in great contrast ; the rela- 

 tions of a coal deposit to the associated rocks may be so intimate as 

 to be interdependent or the association have all the appearance of 

 accident. One must study these conditions and their interrelations. 



Explanation of Terms. 

 Classification of the fossil fuels is foreign to the subject of this 

 work, belonging rather to a study of the origin of coal ; here, certain 

 familiar terms are used in a broad sense and the whole series from 

 peat to anthracite is taken to be continuous — at least, from the 

 chemist's point of view. The chemical relations existing between 

 members of the series have been expressed in many ways; but this 

 table from ]\Iuck- answers the present purpose : 



The mineral content is ignored in this comparison. The table 

 like all others, is merely a generalization and the boundaries between 

 groups are arbitrary. The passage from one to another is gradual 

 and in each the variations are extreme. 



Peat is the accumulation of vegetable matter decomposed in 

 presence of a constant supply of water and protected from access of 

 oxygen. It occurs in areas of greater or less extent on the present 

 surface or in Quaternary deposits. Plant structure is readily recog- 

 nized in the newer portions but, in the thoroughly matured peat, it 

 can be detected only by aid of the microscope. Under prolonged 

 pressure, peat may assume the appearance of typical brown coal. 

 Lesquereux," cited on an earlier page, saw peat exposed midway in 

 the valley of the Lock, where it is dug. At a little way toward the 

 hills, it is covered with 4 feet of marl and is much changed in ap- 



- F. Muck, " Die Chemie der Steinkohle," 2te Aufl., Leipzig, 1891, p. 2. 

 ^ L. Lesquereux, " Quelques recherches sur les marais tourbeux," Neu- 

 chatel, 1845, p. 95. 



