146 Tin American Geologist. September, 1894 
niation consist of thin- bedded, brittle, shelly limestones with 
chalky partings between the lamina-. In such cases the 
crowded valves of Tnoceramus problematicvs leave no doubt 
as to the principal source of the material composing tin- 
stmata. Near Ponca, Nebraska, the Inoceramtis-bearing beds 
are more compact than on the Sioux, and they are associated 
with heavier and more typical beds of chalk. A little farther 
up the Missouri, at Saint Helena, there are no Inoceramus 
beds. Traces of occasional valves may be found here and 
there in the uniformly massive layers that rise one above the 
other in a vertical wall of almost snowy whiteness: hut there 
is nothing corresponding to the immense aggregations of indi- 
viduals that we find farther east. There are some small 
colonies of Ostrea congesta, but, on the whole, niollusk shells 
make up a very insignificant part of the deposit. The thick- 
ness of the chalk at this point is about 90 feet, and it rests on 
a foundation of Benton shales that rise to a night of 40 feet 
above the level of the water in the river. From Saint Helena 
to the mouth of the Niobrara the exposures of chalk on both 
sides of the Missouri present about the same general appear- 
ance. At Yankton. South Dakota, the chalk is used on a large 
scale in the manufacture of Portland cement. The beds 
worked for this purpose, embracing a thickness of about 60 
feet, lie geologically above those seen at Saint Helena, but 
there are no essential differences in macroscopic characters to 
be noticed. 
Palkontoi.oi.i or the Chalk. 
It is aside from the purpose of this paper to discus- the 
general paleontology of the chalk, in the region we are de- 
scribing, with any approach to fullness. A fewgeneral state- 
ments, indicating in a broad way the biological relations of 
the deposit, are all that "an be undertaken. Vertebrate 
remains are not common, and the few that are known belong 
to tishes. The sharks were represented by the genera Ptycho- 
dus, Otodus, and Lamna. and of each there are indications of 
but a single species. Pony fishes were present, but by no 
means numerous. Among invertebrates the only forms at all 
conspicuous are Inoceramus probh maticus and Ostrea congestd. 
The former flourished best in the eastern half of our area, the 
Ostrea is most common west of Ponca. Put while form- large 
