Professor Harkness on Coal. 69 
lime. It is observable, that they are most abundant in those parts 
of the section where the swamp conditions of the production coal 
measures begin to disappear, and where drifted plants predominate 
over those which have grown in sitw; in other words, in the sand- 
stones above and below our section.” ‘‘ The prevalence of coniferous 
trees, as drift-wood, in the sandstones above and below the coal mea- 
sures, is probably to be attributed to their capability of floating for 
a long time without their becoming water soaked and sinking, though 
it may also indicate that their principal habitat was farther inland 
than the sigillariee swamps.” 
Altogether from the evidence which all the coal deposits afford, 
it would appear that coniferous trees do not enter largely into the 
composition of fossil fuel, but that this is produced from sigillariz, 
and probably also, in part, from other abundant coal plants, such 
as calamites and lepidodendra. A knowledge, therefore, of the 
internal organization of these forrns would lead us to some idea con- 
cerning the vegetation which has furnished the organic matter con- 
stituting coal. Probably the most concise and perfect account we 
possess of the internal structure of sigillariz, calamites, and lepido- 
dendra, is to be found in Dr Hooker’s observations in the 2d vol. 
part ii. of the Memoirs of the Geological Survey. Concerning the 
first of these plants, viz., sigillarize, from the examination of internal 
structure of the specimen by A. Brongniart, called by this botanist 
S. elegans, it would appear that this form consisted to a very great 
extent of cellular tissue, in the centre of which was a narrow vas- 
cular sheath, which bore but a very small proportion to the cellular 
mass in which it was enclosed; and this vascular sheath, on trans- 
verse section, appears somewhat akin to gymnospermous wood. With 
regard to sigillarie, Dr Hooker remarks,—‘ The great bulk of sigil- 
larize seems to have been inimical to the preservation of their tissue, 
the process of decay being generally effected on a grand scale in the 
substance of a plant evidently almost entirely composed of a lax 
tissue. The remains of a central column are, however, sufficiently 
obvious in the upright stems of many sigillarie; these have been 
called ‘ Endogenites,’ are scarcely two inches in diameter, and are 
generally obliquely placed in the substance of the specimens, five 
feet and upwards in girth. That this slender column represented 
all the vascular tissue of this plant I cannot doubt, from examina- 
tion of stigmariz, whose vascular column often assumes the same 
appearance.” 
Concerning the second tribe of plants which seems to enter into 
the constitution of coal, viz., calamites, we are not in a position to 
judge of their nature, since they have left no traces of their internal 
structure among the many specimens which occur in the coal mea- 
sures. From their generally compressed state they must have had 
a lax, probably succulent tissue, composed for the most part of cel- 
ular tissue, as distinct from vascular. Of the third form lepidoden- 
dra this seems to have been nearly akin to the sigillariz ; and on this 
