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Central England. Another line running nearly due north 
from Nottingham to Tjmemouth will mark the escarpment 
of the Magnesian Limestone. The country north of the first 
line and west of the second is, with the exception of the Lake 
district and the Yale of Eden, wholly occupied by rocks 
belonging to the great Carboniferous formation. "We will 
first shortly describe its general structure, and then give 
some details respecting the district immediately around us. 
The Carboniferous rocks may be grouped for our purpose 
under three heads, namely : — 
3. Coal-measures. 
2. Millstone- grit and Yoredale Rocks. 
1. Carboniferous or Mountain Limestone. 
The lowest, and, therefore, oldest of these, the Mountain 
Limestone, is a mass of pure limestone, of great but unknown 
thickness, with a few, but very few, thin layers of interbedded 
shale, and two or three beds of lava and volcanic ash, known 
as Toadstone. The latter are regularly interbedded with the 
limestone, and, therefore, must have been poured or thrown 
out from volcanoes during the time of its formation. The 
rock is often entirely made up of animal remains, shells, 
broken corals, and encrinites, and has, therefore, in all 
likelihood been formed entirely from these remains, and not 
by mechanical deposition of sediment. For many ages, in a 
sea unstained by mud, these creatures grew in plenty, 
obtaining from the water carbonate of lime for the formation 
of their hard parts, and at death these dropped to the 
bottom, where the heap gradually grew into a mass perhaps 
several thousand feet thick, which by pressure and chemical 
action has been converted into the well-known limestone of 
Derbyshire. 
At length, however, this peaceful time ended, and a new 
set of conditions came into action, under which the rocks of 
the second group were formed. The division is made up of 
