416 SIMPSON : STRATA AND DEPOSITION OF THE MILLSTONE GRITS. 
portion of Central Wales and the English Western Counties, and 
extends as far as and embraces Leicestershire. 
This sea, which covered so much of what is now the British 
Isles, stretched eastwards through the north-east of France, Belgium, 
Germany, Poland, and Russia, covering, indeed, a great part of the 
present continent of Europe, and extended toward the North Pole, 
by way of Bear Island and Spitsbergen. It appears to have been 
bounded on the south by a more or less continuous belt of land 
through France, Switzerland, Bavaria, Bohemia, and Hungary. 
It seems clear says the author,* " that the sea in which our 
Carboniferous beds were deposited was not an open sea or ocean, but 
a land-locked sea, comparable to some extent with the Mediterranean 
or Caribbean Sea of the present day ; it was probably the most 
westerly embayment of a large European sea ; its coast line formed a 
series of gulfs and promontories, and it was studded with islands of 
various sizes." 
Professor Green, in the Memoirs of the Yorkshire Coal-field,t 
says, " this Mediterranean Sea had a fringe of shallow water round 
its margin, and deep depressions in its central portion. Round its 
edges deposits were formed, mainly of mud and sand, though every now 
and then calcareous animals established themselves in sufficient num- 
bers to give rise to beds of limestone ; at a certain distance from the 
shore all the sediment sank down to the bottom, and beyond that limit 
the water was bright and clear, and the only deposit found consisted 
of accumulations of the hard, calcareous parts of marine animals, 
which are now pure limestone. In the deep hollows, the deposits of 
limestone reached a great thickness : over the ridges, which parted 
the hollows, it was not so thick," thus explaining why the Carbon- 
iferous Limestone shows such varying thicknesses in different places. 
" The growth of limestone gradually filled up the deeper parts of the 
sea, and at last the area became as shallow throughout as it had been 
originally only at its edges. The mixed deposits of sandstone, shale, 
and impure limestone which had at first been confined to the neigh- 
bourhood of the shore, now extended themselves over nearly the 
* Jukes-Browne, Building of the British Isles, pp. 131-2. 
t Geology of Yorkshire Coal-field, p. 23. 
