my 
{9 
Professor Harkness on Coal. 
There is another circumstance which in part serves to determine 
between hydrogenous coals and shales; this is the sp. gr., which, 
in the former, rarely exceeds 1:4, while in the latter the sp. gr. 
extends from 1-8——2°6, but in the case of coals which contain much 
iron pyrites this principle is scarcely applicable ; on the whole, how- 
ever, coals possess a less specific gravity than shales. 
Origin of Coal.—Having seen the nature of coal, how it 
is a compound substance—being, in part, composed of organic, 
and in part of inorganic matter—we are next led to enquire 
under what circumstances this originates; and, in order to 
arrive at any satisfactory conclusion on this matter, we have 
to take into consideration some of the peculiar features which 
occur in the strata intercalated with the coal beds; and also 
what phenomena the coal itself affords, which will assist us in our 
inferences concerning the immediate causes from whence this sub- 
stance resulted. One of the constant concomitants of coal is the 
presence of sigillarize rootlets, which are met with in great abundance 
in the floors on which this rests. The nature of these, and the 
strata in which they occur, lead to the inference that they were the 
roots of plants which flourished in an aquatic habitat, and, as we 
have already seen, there is every reason to conclude, that the plants, 
of which these formed a part, enter largely into the composition of 
coal ; soit would appear that the primary origin of this substance was 
aqueous. The conditions under which the plants, from whence the 
organic matter was derived, flourished, appear to have been rather 
of a nature akin to such swamps as were to a very great extent 
covered with water, than to anything in the form of peat-bogs; to 
such swamps as occur in the deltas of some of the larger rivers ; and 
there are many circumstances which support the opinion that the 
stems of the trees, as well as their roots, were, to a considerable ex- 
tent, covered with water. 
“With regard to the phenomena paasented by the coal itself, these, 
in some instances, are of an important nature, and afford evidence 
of considerable interest concerning the origin of this substance. 
With respect to the structure of coal, this, in some cases, is very 
manifest; but it is in general only in such coals as have not under- 
gone a very great amount of chemical changes and compression that 
we can expect to find this in its most perfect state. 
The first evidence we possess of the internal structure of coal is 
from the sections of Witham, figured in plate xi. of his work on the 
internal structure of fossil vegetables. Concerning the sections here 
given, Nos. 6, 7, 8, and 9, leave no doubt of their being portions of 
the vascular sheaths of probably a coniferous tree. But of sections 
4 and 5, there are very great doubts whether these can be strictly 
regarded as representing vegetable tissue. Of these sections, which 
are from Lancashire cannel, Witham observes, that “ the appearances 
