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sandstones, gritstones, and shale. Several of the sandstone 
beds are of great thickness, one reaching here and there to 
1,000 feet, and these thick beds are mostly very massive and 
coarse, sometimes conglomerates. Between the thick beds of 
sandstone are masses of shale with thinner and more irregular 
sandstones. The method of the formation of these rocks was 
somewhat as follows — In the place of the still, clear ocean 
of the Mountain Limestone times we must have a sea whose 
bottom was from time to time raised and depressed, and into 
which sand and mud was carried in large quantities. "When 
the water was shallow, sand and pebbles were rolled along 
the bottom till huge banks had been piled up to be afterwards 
consolidated into our present grits and sandstones; and 
afterwards when the sea bottom sank, finely divided mud 
was brought down, and, settling gently, formed the regularly 
bedded and laminated shales. We know nothing for certain 
of the source from whence the sediment came, but the com- 
position of some of the grit beds shews that they have been 
derived from the waste of granite, and it also seems likely 
that the feeding ground was to the north, because the beds 
get thicker as we go in that direction. For these reasons 
the Scandinavian peninsula, or a southerly extension of it, 
seems a likely country to look to. 
We may also notice that a few beds of coal are found in 
the Millstone grit, and, as coal is generally believed to have 
grown like peat, this shows that there was occasionally 
upheaval enough to bring the recently formed rocks above 
the level of the water. 
The topmost division of the Carboniferous rocks, or the 
Coal-measures, resembles in many respects the group last 
described. Like it they consist in the main of sandstones 
and shales, but the former are less coarse, massive, and 
persistent than the gritstones, the change, however, being 
gradual. Thus the gritstones are in thickness from 100 to 
