310 J. G. Goodchild—Formation of Coal-seams. 
proportion that the different parts of the plants bear to each other 
in different seams. As a rule, tree trunks form but a small part of 
any coal-seam; and in the rare cases where they do so occur, they 
are not found in the position of growth, or extending upwards 
through the seam, but are prostrate, and lie parallel to the bounding 
surface of the coal. The coarser portions of the organic constituents 
of coals present a mixture, in variable proportions, of the more solid 
parts of the vegetation in a fragmentary state, together with portions 
of the cellular and the vascular tissues of the plants. Even such 
constituents as these are commonly in the minority. Fronds, leaves, 
and small stems occur rather more plentifully. But the greater 
part of the recognizable organic constituents of most coal-seams 
consist of a varied assortment of finely-divided vegetable tissue, 
together with spores, spore-cases, and bodies of that general nature. 
Some parts of nearly every coal-seam commonly fail to show any 
definite structure at all. The relative proportions of these varied 
constituents differ considerably in different coals; but the peculiar 
constitution of each coal-seam as a whole remains tolerably uniform 
over large areas. Not only do the constituents of each coal-seam 
as a whole differ from those of the seams associated with it, but the 
component layers of each seam differ amongst themselves. Every 
coal-seam is seen, on a very cursory examination, to be simply an 
aggregate of carbonaceous lamine, which are ordinarily thin, and 
are occasionally of almost microscopic proportions. A close examin- 
ation of these laminze shows that they differ each from the other to 
a much greater extent than a cursory examination would lead one to 
suppose. More than that. The structural characters of each of the 
lamina, whatever its thickness, remain constant to such an extent as 
to enable one to identify that particular lamina over a large area. 
Practical coal-miners are well aware of this fact, although they may 
not always be able to point to the precise nature of the distinction 
in each case. One lamina may contain nothing but spores and 
spore-cases ; a second next it, above or below, may consist of leafy 
matter alone; a third may be characterized by the constant presence 
of mineral charcoal, representing the vascular tissues of the old 
vegetation ; a fourth may be devoid of any evident traces of organic 
structure; or another lamina of the same coal-seam may contain, 
along with its organic constituents, a variable amount of impurities 
of organic origin. And yet, however, the several lamine may differ 
among themselves, they are found to retain their own special charac- 
teristics throughout the whole of a large coal-field. Hven where the 
coal happens to be changeable, as it often is near old irregularities 
of the floor upon which it lies, the rate of change is far from being 
rapid, except, of course, where the coal is splitting up through 
interlaminations or partings of inorganic matter. 
Some coal-seams pass into shales, by a progressive increase in 
argillaceous impurities; but they rarely, perhaps almost never, 
graduate into sandstones; although some irregular and lenticular 
deposits, which clearly represent drifted vegetation, may often do so. 
Coal not uncommonly graduates into carbonaceous clay ironstone. 
