The Niobrara (hulk. — Calvin. 145 
tions that made the Niobrara chalk possible were brought to 
an end. The swarming life that furnished the organic skel- 
etons of which the chalk is constructed was unable to maintain 
existence under the changed environment. 
It will be noted that in the Sioux river region the condi 
tions that gave us successively the Dakota, Benton, Niobrara 
and Pierre deposits passed one into the other by practically 
imperceptible gradations. While in the Black Hills the tran- 
sition from the Dakota sands to the Benton shales is very ab- 
rupt, along the Sioux river the transition is so gradual that 
any line of separation would seem to be purely arbitrary. 
Indeed it might seem as if any lines dividing a system of 
strata that have resulted from a process of continuous depo- 
sition under very gradually changing conditions must be more 
or less arbitrary. In the Sioux river region the Niobrara beds, 
however, stand out distinctly and sharply defined from all the 
rest both lithologically and micropaleontologically, and if di- 
visions are recognized, these must rank as a separate forma- 
tion. The features that distinguish this group of strata from 
the Dakota-Benton below and the Pierre above depend upon 
the fact that when the subsidence that affected the bottom of 
the Cretaceous sea and the adjacent shores was at. or below, a 
certain stage, mechanical sediments failed, and the absence 
of such sediments favored the enormous expansion of certain 
types of microscopic life endowed with power to protect their 
■soft protoplasmic bodies with shells of carbonate of lime. 
The dead skeletons of successive generations of such organ- 
isms, unmixed with the grosser products of land erosion, con- 
stitute practically the only sediments that accumulated during 
the Niobrara phase of the Cretaceous. It is upon the nature 
of these skeletons and their mode of aggregation that the veiv 
unusual characteristics of the rocks belonging to thisparticu 
lar sta^e depend. 
Characters of the deposits in different localities 
compared. 
As has already been said, the Niobrara deposits are not 
uniformly chalky; neither have they had everywhere exactly 
the same origin. They differ in these respects at different lo- 
calities, and. to some extent, at different levels in the same 
-exposure. Along the Sioux river certain portions of the For 
