60 SECTIONAL ADDRESSES 
varying type of the deposit, since this thesis can only be fully established 
when this is done. 
What do we know of the relation between the rank of coals and their 
distribution in the rocks ? 
The oft-quoted fact that Tertiary coals are mostly of low rank (lignites) 
and the Carboniferous ones mostly of high rank (bituminous or anthracitic) 
may be dismissed with the comment that it is in fairly obvious agreement 
with the fact that the former are for the most part contained in relatively 
thin deposits and the latter in much thicker accumulations ; while the 
formation of Tertiary anthracites under appropriate conditions sufficiently 
disposes of any suggestion that the mere lapse of time is a necessary 
factor. 
An association of high rank with areas of intense folding is well estab- 
lished, but is open to more than one interpretation. David White’s 
observation that the area of highest rank in the Pennsylvanian and 
Virginian fields does not lie on the Allegheny axis, but to one side of 
it, agrees with the distribution in South Wales and elsewhere. It may be 
held at least to show that the direct effect of movement on the coal sub- 
stance is not a prime factor. The true relation between folding and rank 
is more likely to be found in the relation of both to geosynclines. 
The fact which appears to me to establish: most clearly that change of 
rank must be caused by some geological factors is its areal distribution. 
In every coalfield where substantial variation of rank has been noted and 
examined, the rank of any given seam is found to change progressively as 
it is followed across the field. If the variation be plotted on a map by 
means of lines representing volatile content, carbon content, or any 
suitable measure of rank, these lines are found to be arranged in an 
orderly pattern and—this is the important point—the same pattern is 
repeated by each seam within the area. Wherever the rank of one seam 
is increased, there also is that of the others. Whatever cause has affected 
one has affected all. 
An attempt has been made to meet this fact without introducing 
‘metamorphic’ effects by the assumption that the patterns revealed may 
reflect the form of the original basin of deposit, it being further assumed 
that the character of the coal-peat varied progressively from the margin 
towards the centre of the basin. There are many reasons why it is difficult 
to accept this interpretation. Apart from the fact that I do not believe that 
any geologist familiar with all the circumstances could accept the actual 
patterns as related to shore-lines in this way, especially in view of the 
difficulty that the same pattern has to be repeated through a great thickness 
of deposits, there is the complete absence of any shred of evidence from 
the coals themselves that the character of the deposit did change in this 
manner, coupled with much positive evidence that it did not. 
Valuable light is shed on this matter by the detailed information con- 
cerning the composition of many of our own coal seams and their variation 
which is being accumulated by the National Fuel Survey. Many of the 
seams examined, particularly in Northumberland and Durham, show 
large changes in rank as they are followed across the coalfields. Some 
of the seams are notably ‘ bright’ coals, with a relatively high hydrogen 
