372 SECTIONAL TRANSACTIONS.—C. 
the importance of the presence of inorganic substances either adventitious 
or contained in the original plants and added to, or subtracted from, their 
débris during coalification. ‘These substances exert a subtle and profound 
effect in directing the course and degree of the carbonisation of decaying 
vegetable matter, the effects being purely chemical or catalytic. Mackenzie 
Taylor has shown the importance of the pH of the roof material lying above 
a potential coal seam, and it is important whether this material is calcareous 
or siliceous. 
The speaker holds the opinion that durain is derived from raw material 
differing in kind and particle size from that which formed vitrain and 
clarain. In durain the abundant ash is predominantly of the nature of clay ; 
in vitrain and clarain ash amounts to 1 or 2 per cent. only; it represents 
original plant ash, and is of an alkaline nature. Fusain has absorbed the 
bulk of its mineral contents after complete degradation, and its original 
plant ash is liable to have been leached out. The high calcium content of 
fusain is due to secondary imbibition. Anthracite should have a con- 
centration of inorganic constituents, yet its ash content is low, possibly 
as a consequence of leaching by water perhaps saturated with COg, in the 
final stages of rank raising. 
Dr. BERNARD SMITH. 
From analyses of coals published by H. G. Edmunds in 1933 (Summ. 
Prog. for 1932, Mem. Geol. Surv.), it appears that Hilt’s law is applicable 
to the Kent Coalfield. In this case the decrease in volatiles with increase 
in depth is a general rule, although an occasional seam may give a higher 
value than one immediately above it. As in South Wales, when the vertical 
distances between the seams under consideration is less than 100 yds., 
appreciable differences in volatile contents are small. ‘Taking, however, 
seven localities in the Kent field, in which the average differences in depth 
between the highest and lowest seams is about 400 yds., we find that the 
average decrease in volatiles throughout this range is 8 per cent., giving an 
average rate of 2 per cent. per 100 yds. This compares well with Prof. 
Hickling’s figure for decrease in volatiles in South Wales in the coals of 
lower rank. 
Mr. C. A. SEYLER. 
Prof. Hickling in his address had done good service by directing attention 
to recent progress in the petrological classification of the sedimentary rocks 
known as coal. In particular the speaker welcomed Prof. Hickling’s support 
of Dr. Stopes in the attempt to distinguish the rock-types from the micro- 
petrological units or macerals and to establish a systematic nomenclature. 
All coal petrologists recognise that the simple terms ‘ bright ’ and ‘ dull’ 
coal are inadequate. Another service of Prof. Hickling was to emphasise 
the fact that the effect of the nature of the plant material was reflected in 
coal mainly by the amount of hydrogen. In 1900 the speaker had classified 
coals into species and showed that these species could be grouped into genera 
according to the hydrogen. Modern microscopic research had shown that 
these genera were not merely logical categories, but consisted of individuals 
genetically related. 
By restricting the comparison to coals of the orthohydrous genus which 
were chiefly derived from lignified tissue the problem of ‘ rank ’ was greatly 
simplified. The conception of ‘ rank’ was primarily a geological one and 
we had no exact knowledge of the factors which produced the change. 
Until these were known and correlated with chemical composition no 
