14'2 The American Geologist. September, 1894 
posits. In the eastern part of the region we may look for 
marginal beds of Niobrara age, and farther west we should 
find contemporaneous beds that accumulated off shore in the 
clear open sea. 
Physical ( IHARACTERISTICS. 
The Niobrara sediments are unique among the geological 
formations of the Northwest. Where typically developed they 
are wholly calcareous, or nearly so, and yet they are altogether 
unlike the limestones that arc so common and so characteris- 
tic a feature of the geology of the upper part of the Mississ- 
ippi valley. They lie indeed in massive strata, varying from 
six inches to more than two feet in thickness, as do some of 
the limestones, but the material is chalky in appearance and 
correspondingly soft in texture. The color of freshly ex- 
posed surfaces varies from white through shades of gray and 
yellow. In some instances the weathered surfaces become 
reddish owing to the final oxidation of the small amount of 
iron which the beds contain. The material composing what 
anyone studying the region under consideration would call 
the typical deposits, may be excavated with pick and spade: 
it may be carved with the pocket knife; the massive blocks 
into which the layers are readily quarried may be shaped with 
the saw with greater ease even than if they were blocks of 
wood; the materia] is so soft, indeed, that it cannot be han- 
dled without soiling the fingers ; it may be used for writing 
on the blackboard ; the mechanic might use it on his chalk 
lines: in a word, these typical beds to which our attention 
will be chiefly directed are nothing more nor less than chalk. 
It is not to be understood, however, that all the Niobrara 
deposits of our somewhat circumscribed region are chalky. 
Some would have to be described as soft calcareous shales, 
while others are made up of thin-bedded, fissile limestone, 
sometimes more or less earthy and impure, but composed 
chiefly of valves of Tnoceramus problematicus cemented to- 
gether. It is only the beds that represent what we may call 
the perfect and ideal product of the conditions that prevailed 
during Niobrara time that are massive, soft and chalky. 
Stratigraphical axd Bathtmetrical Relations. 
Allow me to traverse familiar ground long enough to say 
that the Niobrara chalk is a part of the Missouri Cretaceous 
