Reviews—The Coals of South Wales. 223 
is the subject of the illustration, which centres are situate in some 
eases within, in others beyond, the present northern boundary of the 
coal-fieid. (See Map on p. 222.) 
Assembling the alternative hypotheses for the purpose of destructive 
criticism, Dr. Strahan notes that in taking the view ‘‘that the 
differences between anthracitic and bituminous coals in South Wales 
are mainly due to original differences of composition ’’? he was guided 
by four main considerations, the first three of which seem to be 
adequately proven by the diagrams given to show the distribution of 
iso-anthracitic lines. These four considerations may be summarized 
as follows: (1) ‘‘ Groups of seams possess a certain individuality,’’ and 
‘‘in some there are local anthracitic areas of which no evidence appears 
in others . . . but bands of the same seam may show considerable 
differences of composition’. (2) ‘‘Iso-anthracitic lines show no 
definite connexion with the faults and disturbances,” and ‘‘ though 
the strata may be vertical or even inverted and sharply folded as 
in Gower, yet the coal-seams retain the composition proper to that 
part of the coal-field”. (3) ‘‘ Anthracitization is obviously not 
connected with the existing outlines of the coal-field as determined 
by denudation,” and ‘‘ there is no connexion between anthracitization 
and depth from the present surface”’.. (4) is based upon the con- 
sideration of the proportion of inorganic ash which remains when 
the coals are combusted. ‘‘The comparative freedom of anthracitic 
coals from ash” was ‘“‘ already brought out by Mushet’s analyses” 
as long ago as 1840, but it has remained for the present generation 
to discover that when a sufficient number of Welsh coal analyses are 
assembled the percentage of ash rises ‘‘ with fairly steady gradient 
from 1 per cent at the anthracitic end to more than 6 per cent near 
the bituminous end of the scale’’. 
These four main considerations all point to an early date for the 
formation of the anthracite of South Wales, and if it be accepted 
that variation in the percentage of ash ‘‘ cannot be due to subsequent 
alteration” of the coal it must follow that anthracitization took 
place contemporaneously with the deposition of the Coal-measures. 
A fifth line of evidence, based upon the finding of certain ‘‘ true 
pebbles” of coal (remanié from the breaking up of some already 
consolidated seam) among the sandstone pebbles which form the 
conglomeratic beds interstratified with the Pennant Grits of the 
Upper Middle Coal-measures, clinches the argument. 
In the second edition of the memoir now before us the number of 
tabulated analyses has been increased to 821, which, with the 
inclusion of the results of direct determinations of the percentage 
of moisture and of the calorific value for forty-seven of the samples 
analysed, will make it by so much the more valuable to commercial 
men. The new samples have been collected during the recent 
revision of the maps of the coal-field, all under the standard 
conditions of sampling observed for those dealt with in the first 
edition. Of the 118 new analyses, some were done in the Survey 
laboratory and many by the staff of the Government laboratory : much 
credit is also due to Mr. C. A. Seyler, who, working first for and 
later in collaboration with the officers of the Geological Survey, has 
