Dr. Mac Culloch on the Lignites. 207 



valuable ; as it establishes an important point, and constitutes a 

 portion of the history of these substances. 



Though the Woody lignites may retain the marks of vegetable 

 organization, they seldom possess the original form of the wood 

 whence they were derived ; although the monocotyledonous can 

 frequently be distinguished from the dicotyledonous plants. They 

 are commonly flattened, as if from the consequence of compres- 

 sion ; while in very few cases are the forms of submerged wood 

 altered in the same manner. But the essential chemical dis- 

 tinction already mentioned, generally serves to discriminate the 

 lignites from submerged wood, whatever correspondence there 

 may be in their forms and appearances. If, in practice, they 

 cannot always be easily distinguished, it is a case which often 

 occurs in mineralogy among undefined substances. In peat, essen- 

 tially, no marks of bituminization are to be found, unless from the 

 casual admixture of bitumen ; and, for this reason, the chemical 

 boundary between peat and lignite, is drawn where the commence- 

 ment of bituminization is visible in the latter. As to the lignites 

 which pass into coal, which are found in many of the stratified 

 rocks beneath the alluvia, the mineral distinction is often nothing ; 

 and they can be classed under this head, only from their geological 

 relations. 



If the boundary of the woody or organic lignites, towards peat, 

 is thus somewhat indefinite, it is far more indefinite towards coal. 

 In a merely mineral point of view, however, the most obvious 

 distinction would seem to consist in the mechanical, rather than 

 the chemical nature of the specimens ; at least, of those at the 

 immediate point of transition. In most cases, the woody or first 

 division of lignite, retains marks of the form of the tree whence 

 it was derived, though it is sometimes found in a pulverulent 

 state ; whereas coal assumes the shape of a rock, losing the vege- 

 table form, even where it still contains the remains of vegetables. 

 But there is one modification in which it is difficult to determine 

 whether the examples should be referred to coal, using that word 

 in its mineral sense, or to lignite. This is the case of trap, where 

 the specimens sometimes retain the vegetable form, though per- 



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