218 Dr. M&c Cullpph on the. Lignites. 



analogous to those which have determined the accumulation of 

 these in particular spots. 



Considering the numerous examples of ancient alluvia of this 

 nature which are known, it must appear remarkable that the 

 occurrence of lignites among those is so rare. It is difficult to 

 conceive that small portions only of the surface were covered 

 with trees at the period of these alluvial formations, and equally 

 difficult to comprehend by what means the deposition was 

 limited to so small a number of places, The analogy of similarly 

 limited deposites of animal remains, does not assist in explaining 

 it ; as, while these have a tendency to inhabit particular spots, 

 forests are limited to no place. For the present, this difficulty 

 must remain unexplained; like many others of which time is 

 every day diminishing the number. 



It may be asked, perhaps, why deposites of wood of a high 

 antiquity do not always assume the character of coal, when 

 antiquity alone has been supposed sufficient to produce the com- 

 pletely bituminized state of lignite. But, in fact, as far as relates 

 to chemical composition, there is often little difference between 

 such ancient lignites and coal ; and it even happens among the 

 more recent which belong to the alluvia, namely, in jet, that the 

 chemical nature is in no way different from that of coal. In such 

 eases, the true distinction of coal is its mechanical texture, or its 

 rocky or stony character, if such a term may be used ; and it is 

 easy to imagine how that might have been induced under the 

 peculiar circumstances under which beds of coal occur, and in 

 which some of the lignites have participated. It will also be 

 found, that many varieties of coal, marked by a peculiar fetid 

 smell, of which the nature had not hitherto been properly ex- 

 amined, owe that property to their participation in the chemical 

 nature of the imperfect lignites, still containing a portion of 

 that unchanged vegetable matter from which the fetid smell of 

 the lignites on burning is derived. Such coals are, in fact, 

 mixtures of lignite and coal, if such a method of expression may 

 be used in a case where two substances pass into each other by 

 imperceptible transitions ; the characters which belong to the 



