FISHES OF THE CARBONIFEROUS SYSTEM. 
The rocks which compose the Carboniferous system form three natural 
eroups, as follows: 
Permian. 
Upper Carboniferous - -- Coal Measures. 
Millstone grit. 
Middle Carboniferous .. Mountain limestone. 
Waverly. 
Lower Carboniferous. .. < Catskill (local, fresh-water deposit). 
Chemung. 
These strata, like those composing the other great circles of deposition, 
were formed by the advance, sojourn, and retreat of the sea, which left its 
record in a mechanical base, an organic center, and a mixed summit. The 
Carboniferous inundation was one of wide extent and long continuance, as 
we learn from the great thickness of the organic sediments which accumu- 
lated slowly over the bottom of those portions of the invading sea where the 
water,was pure and deep enough to form limestone. This calcareous mass 
is in Kentucky 1,200 to 1,500 feet in thickness, thinning out toward the old 
shore line, where only the upper divisions of the limestone are present— 
showing a progressive subsidence—and are finally replaced near the perma- 
nent land area by a great thickness of land wash, conglomerate sandstone, 
and shales. In the geological history of the North American continent we 
find records of three of these great inundations, viz: The Lower Silurian, 
in which the marine sediments are widely spread and 1,000 feet thick; the 
Carboniferous, just described; and the Cretaceous, in which the limestones 
are of even greater extent and thickness. 
Subsequent to the deposition of the Mountain limestone the eastern 
half of the continent was affected by great physical disturbances. These 
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