210 The American Naturalist. [Mareh, 
these peculiar veins of chalcedony resemble both in appear- 
ance and structure a huge septaria, except that the veins in the 
former are much smaller in comparison with the area of the <3 
intervening spaces than in septarie. A. singular fact in 
connection with the position of areas exhibiting this peculiar 
structure is also thought to afford in itself evidence in favor 
of this view of the origin of the veins. These veins occur 
only in certain localities of limited area. Any single locality 
is never more than afew square milesin extent. These small 
and isolated areas, wherever it has been possible to determine 
their relation to the underlying beds, have been found to have — 
been deposited in an eroded valley or depression of the latter. 
Such depressions would afford a natural basin for the waters, — 
held mechanically by the clays, and would prevent a rapid — 
drainage during their deposition. 
STRATIGRAPHICAL POSITION OF THE TITANOTHERIUM BEDS. 
Tur UNDERLYING BEDS. 
The Titanotherium beds are underlaid by various older for- 
mations, from the Laramie to early Paleozoic or perhaps 
Archean. In most places they rest unconformably upon the 
eroded surface of some member of tħe Cretaceous, most fre- 
quently the Fort Pierre shales. At the mouth of Beaver creek, 
a tributary of White river, near the Nebraska and Dakota 
State line, the writer in 1886 observed the Titanotherium beds- 
resting upon the eroded surface of the chalk of Cretaceous No. | 
3, so well developed in central and western Kansas. This is 
believed to be a more northwestern extension of these beds 
than had then been reported. ; 
THE TITANOTHERIUM “BEDS. 
The Titanotherium beds may be divided into Lower, Middle, 
and Upper beds, each characterized by different forms % — 
the Titanotheriide. Accepting a total thickness of 180 feetas 
the maximum thickness of these beds, 50 fect of this would 
belong to the Lower, 100 feet to the Middle, and 30 feet to the 
Upper beds. The different forms of Titanotheriide, especia 
