60 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST, 
general distribution of the land was pretty much as it was when 
the Carboniferous Limestone was deposited, and this must have 
been occasioned by the land areas not participating to the same 
extent as the sea bottom in the slow downward movement which 
admitted of the accumulation of so many thousand feet of strata. 
This is sufficiently indicated in the case of the Central Island, 
by the great thinning out of all the strata as we approach its 
northern shore. The Coal Measures, for instance, in our Ashby 
Coalfield are about 2,500 feet thick, and probably were originally 
3,000 feet ; these as they approached the barrier ridge southward 
have thinned out in North Warwickshire within a distance of only 
12 or 14 miles to 600 feet. 
But we have independent testimony to the fact that the land of 
our Central Island was comparatively stationary whilst the sea 
bottom was subsiding, and that the amount of subsidence increased 
from the Island towards North Derbyshire. This evidence, to 
which but little or no attention has hitherto been paid, is as 
follows :— 
In our Ashby Coalfield the largest and most valuable seam is 
that known as the AZain Coal, which consists of two beds, the 
Over and the Nether Coal, with a thickness of 5 and 7 feet 
respectively. Inthe northern part of the Coalfield these coals 
are separated by as much as 60 feet of sedimentary strata, but 
when traced southwards they are found to come rapidly together, 
and, at the Moira Colliery, form a single undivided bed of about 
14 feet thick. Now we know from the conditions under which 
coal has been formed that the beds must have been laid down on 
a perfectly horizontal surface. After a sufficient thickness of peat 
to form the Nether Coal had accumulated, subsidence must have 
been commenced, which gradually increased in amount towards the 
north, and thus effectually prevented the continued growth of the 
peat bed in that direction, whilst in the south the growth was 
uninterrupted. By and by subsidence ceased, and allowed the 
forest growth which was to produce the Over Coal to spread once 
more over the whole area. In this way only can we account for 
the splitting up of a coal bed. 
