216 
Dr ^lacCiilloch on Peat. 
self sufficient to answer all the required conditions. The po- 
rosity of the strata which surround coal, is generally such that 
they would not prevent the escape of the volatile matters which 
would be separated by the action of fire. The coal would 
therefore be charred in a great proportion of cases, while the 
surrounding materials would be impregnated with bitumen. 
That the former effect would take place, is proved by an occur- 
rence by no means unfrequent, which may be considered as a 
direct experiment in illustration of this question, performed by 
nature on the great scale. This occurrence is the passage of 
trap-veins through beds of coal, the effect of which is to pro- 
duce the consequences in question, notwithstanding the pressure 
of the superincumbent strata. The frequent occurrence of bi- 
tuminous shale, or of bituminous sandstone in the vicinity of 
coal, admits of an easy explanation, without the necessity of 
recurring to the action of fire ; by recollecting that the mixture 
of peat with clay and with sand, which is not unfrequent, would 
necessarily be converted into these compounds by the same 
causes which changed the pure peat into coal. Lastly, it is 
not possible to conceive, that had coal been produced by the 
fusion of peat or lignite, the forms of vegetables should have so 
often been preserved ; since, in applying artificial heat, even un- 
der pressure, all marks of organization are obliterated ; a conse- 
quence that might indeed be easily foreseen. 
There appears no reason, therefore, to attribute the peculiar 
form and structure of coal to the action of fire, but the suppo- 
sition is, on the contrary, attended by un surmountable difficul- 
ties. On the other hand, there appears no difficulty in conceiv- 
ing that these may have resulted from mere pressure. Jet and 
brown coal are unquestionably indebted to this for their compact- 
ness and form ; and there is scarcely a greater difference in this 
respect between coal and compact peat, than there is between 
jet and common submerged wood. Nor is it very difficult to 
conceive that, ^ under a state of minute division, aided possibly 
by the presence of water, coal might assume the peculiar mine- 
ral structure by which it is characterised. Under similar cir- 
cumstances, shale appears to be produced from clay, and there 
is a striking analogy between the concretionary structure of this 
rock and that of coal. 
