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of dirt, continues to be further and further separated from 
the upper portion by a continued increase in the thickness of 
the band, but that it also continues to decrease in the same 
direction, is worthy of note. The cause of this intercalation 
of dirt beds, together with their progressive thickening in 
a given direction, has been explained by Sir C. Lyell to have 
been a slow settling of a part of the area of the coal-bed, 
after the lower portion had been deposited, together with the 
subsequent deposition of the shales, &c, of which they 
are formed. 
Taking this to be the true explanation of their formation, 
and supposing it to have been proved that their commence- 
ment, thickening, and extension in any given coal-field is 
uniformly in one direction, that direction being towards the 
deeper parts of the particular coal-field as it now exists, we 
should probably arrive at the conclusion that their present 
conformation has arisen from a frequently recurring, but 
intermittent settling in one uniform direction, and also that 
as a consequence in that direction many of our lower coal- 
beds either exist in a divided form of thin beds or have 
completely thinned out in the deeper parts of the coal-field. 
And the deepest boring in the northern coal-field, viz., 
Backworth, seems to confirm this surmise ; for we do not find 
a single bed below the Low Main, or Hutton Seam, of 
workable thickness : I, therefore, advance it not as an opinion, 
but as a probability, that the lower coal-beds, developed in 
workable condition on the outcrop of extensive coal-fields, 
may now be found to underlie the upper beds already found 
to exist in the deeper portions of such fields. 
DIFFERENT QUALITIES OF COAL-BEDS. 
One peculiar feature in coal-beds is, their consisting of 
different layers of coal of totally different quality, and 
which can only be used for distinct purposes, part of the 
LL 
