306 The American Geologist. May, 1893 
had supplanted Cyrena and Margaritana, while marine saurians 
and teleost fishes multiplied and became the dominating types of 
the oceanic realm. 
The soft limestone and softer chalk of the Niobrara group are 
indicative of deeper water and remoter shores. No gross sedi- 
ments from the land reach as far as Sioux City. Not since the earlier 
part of the Dakota group had it been possible for leaves and 
twigs of forest trees to be carried into the region. It was during 
the Niobrara epoch that the subsidence reached its maximum, and 
the maximum extension eastward of the Cretaceous sea was at- 
tained. At the close of the Niobrara the upward movement of 
the land began ; the sea withdrew, and shales of the Fort Pierre 
group were deposited above the chalk from Yankton westward. 
When we recall the fact that the thi'ee groups recognized at 
Sioux City and Ponca represent the effects of continuous sediment- 
ation over a subsiding sea bottom, it will be seen that the question 
of dividing the sediments into distinct groups at all is simply one 
of convenience. Furthermore, any lines that can be drawn 
between the divisions, if divisions are to be made at all, must be 
to a large extent purely arbitrary. The upper portions of the 
Dakota merge gradually into the Fort Benton, while the Fort 
Benton group passes b}^ gradual transition paleontologically, and 
in some places lithologicall}^, into the calcareous beds of the 
Niobrara. 
Farther west, where the sea was deeper and the conditions pre- 
sumably more uniform, the distinctions between some of the 
groups cannot be maintained, and King has combined the deposits 
of the Fort Benton, Niobrara and Fort Pierre epochs under the 
single designation of the Colorado group. Hayden acquiesces in 
this arrangement in his annual report for 1874, but later in his 
report for 1877 he makes the Colorado group include only the Fort 
Benton and the Niobrara, while the two upper divisions, the Fort 
Pierre and the Fox Hills, are united under the name of the Fox 
Hills group. The Dakota group, with its coarse sandstones and 
leaves of forest trees is still recognized as a distinct division. 
And this leads to another consideration that is of wide-reaching 
importance in the correlation of synchronous geological deposits. 
The sandstones at the base of the Dakota group, near Sioux City, 
owe their physical and even their paleontological characters to 
conditions prevailing near the shore. As the bottom subsided 
