Reviews—The Coals of South Wales. 221 
requires to be taken in hand at once. Among sedimentary rocks 
there is no class so essential to commerce as coal, and among coals 
there are none of greater national importance than are the anthracites 
of South Wales. It is therefore only to be expected that when we 
seek the best available account of the composition of beds of sediment 
and the lateral variation of those compositions from place to place, 
we should find it in a publication of H.M. Geological Survey which 
deals with the coals of South Wales. 
Ever since the days of De la Beche and Playfair (1848) the steam 
coal of the Navy has been a subject for chemical specification and 
research ; but though analyses of coals from many localities have 
accumulated, the geological data concerning the samples were 
generally so little precise that until the present century the results 
of the analyses have hardly been dealt with by geologists and do not 
seem ever to have been employed to throw light upon the natural 
history of the rocks. 
The Geological Survey Memoir on the Coals of South Wales was 
first published in 1908. Its main feature was the assembling of 
a great number of trustworthy and complete analyses of coals which 
had been collected, with proper precautions in sampling, from all the 
more important coal-seams, at localities which were distributed as 
widely as possible over the length, breadth, and thickness of the 
Coal-measures of the South Wales Coal-field. The analyses were all 
performed systematically, by standard methods, on samples collected 
by men whose business is not the buying or selling of the coal; 
and being all therefore strictly comparable with each other, they 
have proved of special value to those who are concerned with the 
commercial exploitation of the field. For the geologist, whose main 
concern is with the generalizations which emerge from the statistical 
treatment of these tabulated analyses, rather than with the figures 
themselves, it was the summarization of the analyses, each under 
the so-called “index of anthracitization”’ (a number obtained by 
dividing the percentage by weight of carbon by the percentage by 
weight of hydrogen contained in the coal), which marked the great 
advance. 
Until the memoir appeared, the view that, in South Wales, anthra- 
citization of the coals was an event contemporaneous with the 
deposition of the measures, was only one among several alternative 
hypotheses. Now, however, as the result of the field and laboratory 
work set forth in the memoir, this hypothesis has advanced to the 
foremost place, and the diagram maps, which show the iso-anthracitic 
lines for each of the more important seams or groups of seams over the 
coal-field, form an excellent demonstration of the forcefulness of the 
evidence which supports it. Iso-anthracitic lines are obtained by 
plotting, on separate maps for each coal-seam, the index of anthra- 
citization for the samples of coal analysed, each at the geographical 
locality from which it was collected, and by drawing contour-lines 
among the ‘ spot-levels’ so obtained. In each of the seams or groups 
of seams for which they are illustrated, the iso-anthracitic lines have 
come out as sweeping sub-parallel curves, which range about axes or 
centres peculiar each to the particular seam or group of seams which 
