FISHES OF THE CARBONIFEROUS SYSTEM. 83 
We have seen that the Devonian rocks were deposited in a hydro- 
graphical basin, in which the water deepened until, over a large area, a sheet 
of unusually pure limestone (the Corniferous) was deposited; then the sea 
began to retire, its outline contracted, and in shallower water, where the 
land-wash exceeded in quantity the organic matter, the bituminous and earthy 
shales of the Hamilton were laid down. Finally the Devonian sea was 
drained away and its bottom became dry land. But not forlong. The flood 
soon returned as the Carboniferous sea, which caused a wide-spread and long- 
continued inundation; but the water never came to be deep and pure enough 
to form limestones in New York or along the Alleghany belt. There the 
Chemung, Catskill, and Waverly were deposited, all land-wash with ripple- 
marks from top to bottom. While the mechanical beds were being thus 
deposited along the shore, in the central portion of the interior basin lime- 
stone was precipitated, until in Kentucky and southern Illinois it was more 
than a thousand feet in thickness. The western geologists have divided this 
limestone into four parts: 
1. Chester limestone. 3. Keokuk limestone. 
2. St. Louis limestone. 4. Burlington limestone. 
The edge of this calcareous sheet now reaches northward to the middle 
of Ohio and eastward into Pennsylvania and West Virginia. As we ex- 
amine its boundaries we find that only the Chester group is there repre- 
sented, but going southward the lower members are seen to come in suc- 
cessively beneath the Chester, until in central Kentucky all are present. 
This means a gradual subsidence of the land or rise in the sea level, so that 
the area of pure water was constantly widening. Ultimately it reached 
beyond the present margin of the limestone, which has been considerably 
removed and cut up by erosion. 
The time occupied in the formation of the-Carboniferous limestone was 
enormous, since it sufficed for the accumulation of more than 1,000 feet of 
calcareous sediments formed by millions of generations of mollusks, forami- 
nifera, and other lime-secreting animals; each of which made its contribu- 
tion of shell or skeleton to the mass. During all this time shore waves, 
rains, and rivers were eroding the land and filling in the adjacent portions 
of the sea-basin with land-wash. But the progress of events was not uni- 
