56 SECTIONAL ADDRESSES 
title and has been merely described as the ‘ residuum.’ Dr. Stopes would 
christen it ‘ micronite.’ 
‘ Sapropelic’ Coals.—In the foregoing account of the components of 
coal I have dealt with those to be found in our normal bituminous coals of 
Carboniferous age. So far as my knowledge extends, the account does not 
require material modification in reference to the coals or lignites of other 
ages, except in so far as results from the minor structural differences (such 
as the greater proportion of true wood) in the more recent vegetation, and 
from some difference in the physical character of the materials in the 
lignitic condition. But there are other types of coal, particularly the 
cannels and bogheads, which have a widely different structure and at least 
one very different component material. There is much evidence to sup- 
port Potonié’s view that these latter coals are essentially water-deposited 
accumulations of fine plant-residues (sapropels), while the ‘ normal’ 
coals are essentially peat (‘humic’) deposits. One of the essential 
characters of a cannel is certainly the fine state of division of all its 
materials. But the feature of most pertinent interest is the occurrence 
in these sapropelic deposits, and in no others, of microscopic oil-bearing 
algze. It is well known that these were described long ago by Bertrand 
and Renault ; and also that their algal nature has been over and over 
again denied. None of the criticism did justice to Bertrand’s original 
work, but in the last few years these organisms have been carefully 
re-examined by Zalessky, by Thiessen, by P. Bertrand, and by my own 
colleague Dr. Temperley, who have not only demonstrated their structure 
in greater detail, but have established their essential similarity (and 
possible identity) with the living oil-alga Botryococcus braunii. 'This 
constituent of the cannels affords the most striking of all examples of the 
effect of original materials on the composition of the coals. There are 
cannels entirely devoid of algze, and others in which they are present in 
all proportions up to that in the best bogheads, of which they may form 
go percent. As the algal content increases, so the percentage of hydrogen 
in the coal rises, from about 6-0 per cent. in those without alge up to 
more than 12 per cent. in the purest algal bogheads. 
Types of Coal Aggregate.—I believe these are all the constituent materials 
which have so far been recognised in ordinary coals. It remains to con- 
sider how they are distributed among the various types of coal aggregate. 
With a considerable number of components it might be expected that a 
great variety of aggregates would be formed. It so happens, however, 
that the most important constituents are distinguished by widely different 
average size of the fragments in which they occur; and in coal, as in 
other fragmental rocks, size is the great sorting factor. The dominating 
component, vitrinite, exists for the most part in fragments much larger 
than those of any other component except fusain. It follows that as 
the average size of the particles in any coal aggregate decreases, so the 
proportion of vitrinite usually becomes less, while that of spores, cuticles, 
.tesins, fragmented fusain and residuum become greater. This intro- 
. duces an important element of order where there might have been almost 
complete chaos. It is a further fortunate circumstance that the physical 
character of the vitrinite—the brilliant cleavage surfaces with which 
