But the
limited extent to which these strata are exposed at the surface is not merely
owing to their former denudation, but even in a still greater degree to their
having been largely covered by the New Red Sandstone, as in Cheshire, and here
and there by the Permian strata, as in Durham.
It has long been the opinion of the most eminent geologists that the coal-fields
of Yorkshire and Lancashire were once united, the upper Coal-measures and the
overlying Millstone Grit and Yoredale rocks having been subsequently removed;
but what is remarkable, is the ancient date now assigned to this denudation, for
it seems that a thickness of no less than 10,000 feet of the coal-measures had
been carried away before the deposition even of the lower Permian rocks which
were thrown down upon the already disturbed truncated edges of the coal-strata.
(Edward Hull Quarterly Geological Journal volume 24 page 327.) The carboniferous
strata most productive of workable coal have so often a basin-shaped arrangement
that these troughs have sometimes been supposed to be connected with the
original conformation of the surface upon which the beds were deposited.
Pages:
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735