326 LIGNITES. 



cumstances these variations may be owing, the diffe- 

 rences, in a chemical view, are very considerable, and 

 imply either a longer exposure to the causes by which 

 the changes have been induced, or a much more ener- 

 getic action of these. Yet, considering the numerous 

 known examples of antient alluvia, the rare occurrence 

 of lignite among them is a remarkable circumstance; 

 since we cannot conceive that small portions only of 

 the surface were covered with trees at the period of 

 these deposits. Modern occurrences may perhaps 

 however explain it ; since in the numerous deposits 

 from rivers, or in the changes on sea shores, it is only 

 in a few cases that we find trees deposited in any con- 

 spicuous quantity. 



It may be asked, perhaps, why deposits of wood 

 of a high antiquity do not always assume the cha- 

 racter of coal, when antiquity alone has been sup- 

 posed sufficient to produce the completely bitumi- 

 nized state of lignite. But, in fact, there is often 

 little difference between the chemical nature of such 

 antient lignites and of coal ; as is true even of jet, 

 not always among the most antient in position. But 

 as I have already said, the true distinction of coal is 

 its mechanical texture, or its rocky, or stony, cha- 

 racter, and it is easy to imagine how that might have 

 been induced under peculiar circumstances in which 

 some of the lignites may not have participated. It 

 will also be found, that many varieties of coal, burn- 

 ing with a peculiar fetid smell, of which I first pointed 

 out the cause, derive that property from participating 

 in the chemical nature of the woody lignites; con- 

 taining a portion of that unchanged peat from which 

 the fetid smell of these lignites is derived. Such 

 coals are, in fact, mixtures of lignite and coal ; if such 

 an expression may be used where two substances pass 

 into each other by imperceptible transitions; the form 



