The Niobrara Chalk. — Calvin. 159 
it in the deposits east of the Sioux river. Specimens of Buli- 
mina are not rare, and Polystouiella, Cristellaria, and Frondi- 
cularia, occur occasionally at the western localities named, 
but farther east they are so far unknown. On the other hand, 
there are some genera peculiar to the region east of the Sioux, 
but it would scarcely add anything to the force of the facts 
already stated to enumerate them. 
Probable cause op the differences observed. 
Is it possible to assign any cause for the observed differ- 
ences in the foraminiferal faunas of the eastern and western 
portions of the Sioux river region? Remember that from the 
beginning of the Dakota age to near the close of the Niobrara 
the region was practically one of progressive subsidence. 
There are evidences that we cannot now discuss of numerous 
oscillations of level, but on the whole the tendency of the sea 
bottom and the adjacent lands was downward. During the 
culmination of the peculiar conditions characterizing the 
Niobrara, the sea was clear and had considerable depth, but 
not abysmal, over the present sites of Yankton and Saint 
Helena ; while farther east the waters were shallower and 
may sometimes have been polluted by rock detritus to a slight 
extent during the upward phases of oscillation, or during pe- 
riods of excessive rainfall. All the evidence suggests a clear 
peaceful sea with its bottom gradually sloping away from the 
shore to only moderate depths. Upon the bottom of this sea 
the textularians nourished, while floating near the surface 
were the younger individuals of the globigerine forms already 
noticed. Either floating in the water or resting upon the 
bottom were the peculiar coralline plants of which the bodies 
called respectively coccoliths and rhabdoliths were constitu- 
ent parts. 
Now, the differences noted between the textularians at 
Saint Helena and those at Sioux City and Auburn are in some 
way connected with differences in physical conditions at the 
points mentioned. The only differences thai are readily sug- 
gested are the differences in the depth and in the amount of 
earthy sediments over different portions of the area. 
No one can tell how such slight differences of environment 
would react on the simple living matter of Textularia, bul 
that they did affect it profoundly becomes obvious upon 
