THE PHYSICAL GEOGRAPHY OF THE PAST. 
288 
east, we have no direct evidence to guide us, but at or near 
the latter place a series of borings through a great thickness 
of the overlying Secondary Rocks has proved the existence of 
a sandy and degenerate representative of the Mountain 
Limestone, thinning out northwards against land rising 
rapidly in that direction.* 
It can be shown by a similar line of reasoning that these 
old Carboniferous seas, which spread over the greater part 
of the southern and northern portions of our country, were 
really arms or inlets of a far larger sea which extended 
throughout the greater part of Northern Europe, far into 
Russia. Scandinavia formed part of the great northern 
continent, and from this, as well as from the south, the 
rivers were constantly bringing down into this island-studded 
inland sea their freight of sand and mud, whilst in the deeper 
and clearer portions limestone was being formed. 
As the limestone thickened, filling the hollows in the sea 
bottom, the water necessarily shallowed, and the deposits of 
sand and mud, which were originally confined to near shore, 
invaded the now shallowed areas, and gradually, though at 
first intermittently, rendered the water unfit to support the 
life of limestone-building organisms. 
We can readily understand how, by slowly alternating 
conditions, sometimes impure limestone, and sometimes mud 
and sand were deposited over the same areas. These are the 
conditions under which the Yoredule Bocks, the next in upward 
succession to the Mountain Limestone, were formed. But it 
is certain that these muds and sands, and the still coarser 
sediments of the Millstone Grit which followed them, were 
deposited in a slowly subsiding area. We have seen how 
the sandy deposits around our present coasts are laid down 
in comparatively shallow water, and it is manifestly impossible 
to explain the existence in the Carboniferous rocks of 
thousands of feet of shallow water deposits, deposits which 
could scarcely have been made in water deeper than 100 to 
200 feet, without supposing subsidence of the bottom to 
have taken place concurrently with the throwing down of the 
coarse sediment. 
That a subsidence of this kind actually did take place is 
shown by the fact that each member of the Carboniferous 
Series creeps over the edge of the deposit below it. The 
Millstone Grit, for instance, extends beyond the original 
boundaries of the Mountain Limestone, and the Coal Measures 
* Some of the rocks forming this old land have been shown by 
Professor Bonney to be identical with certain Charnwood Rocks. 
J.G.S., 1885 Proceedings, p. 48. 
