FORMING THE LANDS OF JUDITH RIVER. 133 
session at the present time, to construct a complete general section. The immense area 
occupied by this basin is shown on a geological map* published in the Proceedings of the 
Academy of Natural Sciences, June, 1858. Even yet it has not been fully explored, only 
the south-eastern and north-western boundaries being known by actual observation. I 
have traced its south-eastern outlines as they overlap the Cretaceous strata from the Mis- 
souri to the Black Hills, up the Yellow Stone River as far as the mouth of the Big Horn, 
but its northern and western limits are as yet unknown. In a former paper I estimated 
the area occupied by this basin at about 60,000 square miles, and at the same time ex- 
pressed the opinion that when more fully examined, this estimate would be found too low, 
and I am now satisfied that it will be found to cover a much larger surface. It is a very 
interesting feature in the geology of Nebraska, that within the limits of the same territory 
there should be found such remarkable deposits with some characters in .common, 
but so far as we know, entirely independent of each other. These basins may be charac- 
terized briefly as follows: 
Ist.—Bad Lands of the Judith; fresh water and estuary deposit; strata composed of 
friable or indurated sands, clays, and very impure earthy lignite; contains estuary, fresh 
water and land shells, with much silicified wood and a few leaves of dicotyledonous trees; 
chiefly remarkable for its peculiar saurian fauna. It is the upper portion of this deposit 
that seems to possess the estuary character. 
2d.—Great Lignite Basin; also composed of loose sands and indurated layers, with many 
arenaceous and argillaceous concretions disseminated throughout the deposit; is chiefly 
remarkable for the beauty and extent of its fossil flora, only the lowest beds exhibiting 
an estuary character, gradually passing up into purely fresh water strata. It contains 
many beds of lignite, more or less pure, varying from one inch to seven feet in thickness, 
and in the vicinity of the lignite are found great quantities of silicified wood. 
3d.—Tertiary Basin of White River; light and flesh-coloured indurated clays and grits, 
with many calcareous and argillaceous concretions; remarkable for its Mammalian and 
Chelonian fauna. This deposit is purely fresh water or lacustrine, and the few species of 
Mollusca which have been obtained from it, belong to the same genera and the same types 
as those living in the tributaries of the Missouri at the present time. The only indications 
of vegetable remains are a few fragments of silicified wood. 
The Molluscous fossils of the Lignite Basin, though in many instances belonging to the 
same genera with those occurring in the White River deposit, are of quite different types. 
“Tt is an interesting fact, that the most nearly allied living representations of many of 
* Explanations of a Second Edition of a Geological Map of Nebraska and Kansas. Proceedings Academy Na- 
tural Sciences of Philadelphia, June, 1858. 
VOL. x1.—18 
