56 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
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 inter- 
mittently, rendered the water unfit to support the life of limestone- 
building organisms. 
We can readily understand how, by slowly alternating condi- 
tions, sometimes impure limestone, and sometimes mud and sand 
were deposited over the same areas. These are the conditions 
under which the Yoredale Rocks, 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 Jaid 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. 
* 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. 
