70 Professor Harkness on Coal. 
matter Dr Hooker observes, “‘ that the sigillarize were allied to lyco- 
podiacezee is evident, their tissues and seaming being very like 
those of lepidodendra, in which, however, there is but one series of 
zone of vascular tissue. In both the great mass of the woody axis 
is formed of large tubular vessels, identical in structure ; and in both 
fascicles are given off from the central mass to the scars on the cir- 
cumference, which fascicles consist of slenderer tubes than the axis 
does. In lepidodendra there are no medullary rays, the vascular zone 
being continuous, and surrounded by those slender vessels from 
which the fascicles diverge and run to the scars.’” In this form the 
cellular structure, as distinct from the vascular cylinder, prevails 
even to a greater extent than in sigillarize, as may be seen in Witham’s 
plates of the internal structure of the Lepidodendron Harcourtu. It 
would thereforeappear that the three formsof plants which enter most 
largely into the composition of coal are to a very great extent made up 
of simple cellular tissue, and that these have been succulent plants, very 
brittle in their nature, since we often find the lower portions in such 
a state as would result from the sudden snapping off of the stems ; 
and from the internal structure of sigillariee, calamites, and lepido- 
dendra, we may regard the vegetable portion of coal as being, to a 
very great extent, the cellular tissue of these forms of vegetation 
compressed and chemically altered. 
The changes produced by Chemical Action and Compression.— 
The next question which presents itself is, what are the changes 
which the vegetable matter, entering into the composition, has under- 
gone? From the various analyses of different coals, it would seem 
that the amount of change produced by chemical action has been 
very different in the several varieties of this substance. In that de- 
scription known in England under the name of cannel, and in Scot- 
land as parrot-coal, we have a greater amount of hydrogen than 
usually occurs in ordinary coals. In the caking and stone varieties 
of common coal there are also differences in the gaseous constituents, 
and in anthracite, we have a form far removed, so far as chemical 
constitution is concerned, from either of the other descriptions of this 
substance. These differences, in the nature and amount of the 
gaseous constituents, lead to the inference that they have originated 
from the various amounts of decomposition which the vegetable 
matter forming coal has undergone; and it would appear that 
the decomposition of the vegetable matter has been more or less 
affected by some influences which have in some instances checked, 
and in others advanced, the chemical changes, acting upon the vege- 
table tissue which form the organic portion of coal. If we divide 
coal into three groups, namely, cannel or parrot-coal, ordinary coal, © 
including caking and stone-coal, and anthracite, we express not 
only a difference in aspect and constituents, but likewise indicate 
that these three groups have been derived from vegetable matter 
which has undergone three different amounts of decomposition. 
With regard to the first of these groups, cannel coal, we lave in 
ey a an 
