C.—GEOLOGY 55 
of ash. Quite moderate variation in the amount of fusain in the coal has 
important effects on its utility for various purposes. Practically as well 
as theoretically it is therefore important to notice that the amount of 
fusain in a seam cannot be estimated by the quantity existing in separate 
fragments or layers of sufficient size to be visible to the eye. Its most 
conspicuous quality is its friability, and it is consequently but natural to 
find much of this material in the form of microscopic fragments in the dull 
coal or durain, and even to some extent among the clarain. 
The next group of coal components may for the purpose of this review 
be designated the ‘ high-hydrogen ’’ group—the outer coatings of stems, 
leaves and spores which are characterised by an accumulation of waxes, 
fats, resins and allied substances. Leaving to the chemists the difficult 
problem of determining the precise nature of the materials forming 
these various plant-coatings, and degree of alteration to which they 
have been subjected in the coal, it is enough for us to note the characteristic 
fact that these components of the coal have a relatively high hydrogen 
content, and that they differ so strikingly in chemical properties from the 
rest of the coal that by the roughest chemical treatment the remainder 
of the coal can be broken down while the spore-coats and cuticles are 
left little altered. On this account Dr. Wheeler speaks of these com- 
ponents as the ‘ resistant plant remains.’ Extracted from the coal, these 
coatings are seen under the microscope to differ in structure little, if at all, 
from their condition in a living plant. Their obvious relative inde- 
structibility has resulted in their accumulation in the coal in quantities 
much in excess of their natural proportion—in extreme cases, even to the 
total exclusion of other materials. How greatly their presence may 
affect the composition of the coal is indicated by recent work in which 
such material has been mechanically separated from the coal and found 
to have a hydrogen content of nearly 8 per cent. 
Associated with the cuticles and spores in respect of chemical peculiari- 
ties and durability, but differing somewhat in the manner of distribution 
in the coal, are the resinous secretions of the plants. Since the resins 
were originally contained in the wood or bark, they are found largely in 
the vitrinite of the coal. On the other hand, they are often found in local 
aggregations among the more disintegrated plant debris, in such a manner 
as to suggest that the aggregates may represent the resinous content of a 
tissue which has, for the rest been almost entirely destroyed. 
There is one other distinctive ingredient in most ordinary coal seams 
characterised by its minute state of fragmentation. It forms a kind of 
paste, made up of particles of 1 or 2 microns or less in diameter. Careful 
examination of the rest of the coal substance has shown, as might be 
expected, that it consists of plant-fragments of every size from pieces of 
several inches down to the size of single plant cells. But below that size 
(say 20 to 60 microns) there is an unmistakable jump to the immensely 
more fine paste just mentioned. Obviously it represents quite a distinct 
stage in the degradation of the plant material ; and there is some indication. 
that it has distinctive chemical peculiarities. It is of very dense colour, . 
and is consequently opaque except in the thinnest sections. It is essen- 
tially characteristic of the dull coals. So far it lacks a generally recognised 
