The Niobrara Chalk. — Calvin. 141 
The Niobrara stage of the Upper Cretaceous is well repre- 
sented along the Missouri from the mouth of the Niobrara 
river to the mouth of the Big Sioux. East of the Sioux, beds 
of the same stage are found at various points in Iowa as far 
eastward as Auburn in Sac count}-,* while fossils distributed 
through the drift indicate the former existence'of Cretaceous 
strata at points many miles farther east than any localities 
where they are now known to occur in place. \ 
Areai. Distribution. 
The general distribution of the Niobrara deposits, it need not 
be said in this presence, covers an area reaching from western 
Iowa to the Rocky mountains, while north and south it 
stretches from Texas to Manitoba and probably northward to 
the Arctic ocean. It is not rny intention to extend the dis- 
cussion of the formation so as to cover more than a very small 
fraction of this magnificent area. I purpose rather to limit 
myself to some of the characteristics of the Niobrara chalk 
exhibited in the somewhat restricted region lying between the 
mouth of the river from which the formation takes its name 
and the most eastern exposure of the beds at present known, 
near Auburn, Iowa. Within these rather narrow limits all the 
typical phases of the Niobrara formation are perfectly exem- 
plified, and ample facilities are afforded for the satisfactory 
study of its composition. Furthermore, it will be remembered 
that the shore line of the Niobrara sea passed through Iowa, 
and during the culmination of the conditions that gave us tin- 
peculiar deposits of this portion of geological time the sea 
margin w T as probably not very far east of Auburn. Indeed it 
may be shown that the shore line was subject to many move- 
ments of advance and retreat, and that during the early part of 
the Niobrara age the sum of the movements was eastward and 
the ocean gradually encroached upon the land: while later 
the reverse was true and the waters retreated toward the 
west. For these reasons the region to which the discussion is 
limited affords facilities lor determining the effects of vary- 
ing depths of water and varying degrees of proximity to the 
shore upon the composition and origin of its sedimentary de- 
•■<'. It. Keyes, Proceedings Iowa Acad. Sci., Vol. I. pari IV, n 25 
1894. 
fC. A. While. Proceedings A. A. A. s.. Vol. 21, \>. 187, 1872. 
