230 Dr. Mac Culloch on the Lignites. 



that, in those cases where wood is so situated in trap rocks that it 

 must necessarily have been decomposed by the action of the heat, 

 it has actually undergone a change of form, and has thus been con- 

 verted into an inorganic substance. This substance, however, 

 according to the direct experiments above stated, could not even 

 then have been coal, but bistre, or a mixture of that substance 

 with the superfluous charcoal of the wood. It is probable that it 

 is this substance which has produced the coal now found in trap, 

 and generally more or less connected with indications of lignite, or 

 of a vegetable structure not totally obliterated. 



Whether the long -continued action of a slow heat during the 

 tedious process of cooling such masses of trap may not have con- 

 verted such bistre into coal, is a question which, for want of power 

 to make similar experiments, cannot be determined. But, cer- 

 tainly, the action of water is not precluded, even in these cases ; 

 as is proved by the quantity of water which is found mixed with 

 the substance of trap, as well as of the stratified rocks, and which 

 is rendered abundantly certain by the formation of some of the 

 amygdaloidal nodules, by the existence of water in these cavities? 

 and by the deposition of stalactitic chalcedonies in them. 



Whether this view be conceived probable or not, it is not to be 

 doubted that, if the resinous exudation of trees may, by access of 

 moisture, be converted into retinasphaltum, and probably into 

 amber also, bistre, first produced by the action of fused trap, may 

 in a similar way be converted into coal. In either case, moisture 

 alone, not the free access of water, is equally sufficient, and, in 

 both situations, it is present. 



If I have thus shewn that lignite, considered as a mineral, is a 

 state of the vegetable matter intermediate between peat and coal, 

 and that it occupies a certain place in the progress of bituminiza- 

 tion, it is equally easy to account for the various approaches 

 which that substance makes to peat on the one hand and to coal 

 on the other ; in this last case, becoming a coal as perfect as that 

 which belongs to the proper coal series beneath the magne- 

 sian limestone. And it is also thence a natural expectation, that 

 the deposits of these bituminous matters which are deepest si- 



