Professor Harkness on Coal. 67 
of being used as fuel. This definition embraces not only the nature 
of coal, but it also includes all the modifications to which fossil fuel 
is in a great measure subject. It also presents the inquiry concern- 
ing the nature_and origin of coal in three aspects,—1st, The nature 
of the plants from whence the vegetable matter constituting coal was 
derived; 2d, The changes which these plants have undergone in 
consequence of chemical action and compression; and 3d, As re- 
gards the associated mineral matter—in what manner this has become 
combined in coal, and in what respect it tends to modify the charac- 
ter possessed by this substance. 
The nature of the Plants forming Coal.—To commence with 
the nature of the plants which have furnished the vegetable’ 
matter of coal, we might be led to inquire into the flora of the 
earboniferous formation generally, as the knowledge of this flora 
has been derived principally from the deposits interstratifying the 
coal seams. Certain peculiar forms of vegetables, however, make 
their appearance in great abundance in connection with coal, and 
these appear to have supplied to a very great extent the organic 
matter entering into the composition of this material, and some 
knowledge of them may suffice so far as concerns coal. There 
is another circumstance which, when taken into consideration, ren- 
ders this subject less profuse than it would otherwise be, and this 
is, that the conditions which obtained during the growth of the 
plants which have supplied this substance seem to have been of a 
somewhat local nature, and such as furnished a habitat suitable for 
certain kinds of plants. The evidence which the coal formation 
affords tends to the conclusion that the deposits of coal itself, and 
also those with which it is immediately intercalated, are of an 
aquatic character; but this aquatic character, so far as respects coal, 
was the result of an estuary, or some modification allied thereto. We 
are, therefore, rather induced to inquire, what was the nature of the 
plants which occupied such a habitat during the coal epoch, than 
what forms of plants clothed the surface of the earth when the 
carboniferous strata were being deposited ? 
Mr Witham, in his work on the internal structure of fossil plants, 
p- 9, observes, in connection with this matter :—“ If we take it for 
granted that the coal-seams are formed by the deposition of vege- 
table matter produced either on the spot where it is now found, or 
brought from a distance, we can easily offer an explanation of the 
differences found to exist between the coal fields of England above 
alluded to (referring to the abundance of ferns in these coal mea- 
sures when compared with those of Scotland), and the Scotch basins 
in regard to the occurrence of fossil vascular cryptogamic plants, and 
their impressions. In a flat country like Northumberland, Durham, 
and Yorkshire, surrounded by mountains of no great elevation, froni 
which a supply of more perfect wood could have been obtained, the 
vast mass of carbonaceous matter deposited must have resulted from 
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