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1,000 feet, and often keep for many miles the same thickness 
and mineral character ; the sandstones of the lower part of 
the Coal-measures are never more than a few hundred feet 
thick, they often change very suddenly from a fine flag- 
stone to a coarse, massive grit, and very frequently die away 
altogether ; the sandstone of the Middle Coal-measures 
are rarely as much as 100 feet thick ; they can scarcely 
ever be said to have a distinctive character, and almost 
invariably in the course of a few miles they die out and 
are replaced by shale. The most important difference, 
however, between the topmost and middle group is the 
presence in the former of numerous workable coal beds. The 
fact that each of these required for its formation a land 
surface, shows that during the coal-measure epoch there was 
a still larger oscillation of the sea bottom than during 
Millstone- grit times ; and this is probably one reason why 
the minor subdivisions are so much more changeable. 
One more point requires notice. The Carboniferous rocks 
are conformable throughout, and, therefore, wherever we find 
one part, the whole group must originally have been there 
present. 
The last consideration proves that over the whole of the 
Carboniferous area defined at the beginning of the paper, 
the three divisions stretched originally in nearly level sheets. 
After a time, however, the beds began to be upheaved along 
a line running nearly north and south through the middle 
of the tract, and coinciding with what is now known as the 
Pennine chain, or " Backbone of England." By this means 
the level sheets of strata were bent into a long arch, but as soon 
as the ridge rose above the surface of the sea the waves attacked 
it and eat it away, stripping off the higher measures from 
the centre and laying bare the underlying Millstone- grit and 
Mountain Limestone. Thus the central part of the district 
is now formed of these lower rocks, while on the east and 
BBB 
