100 ON THE GEOLOGY AND NATURAL HISTORY 
Feet. 
1. Coarse-grained gray sandstone, very compact and hard at summit, but becoming more friable at 
base, . 0 . : . ; 0 . 0 . : . : . c 5 100 to 150 
2. A variable bed of yellowish gray loose sand and clay, with here and there a small seam of lignite, and 
some argillo-caleareous concretions, : 0 . : . : 0 5 . 100 
3. Dull reddish argillaceous lignite, 18 inches. 
4, Gray ferruginous silicious grit, becoming yellow on exposure, with numerous argillo-silicious concretions, 
some gray, others of a reddish color. This bed contains bones of vertebrate animals and estuary shells, 
as Cyrena, &., . . c : ; 0 c : : c 0 6 3 0 . 30 to 40 
Bed 4 of the above local section rests directly upon Cretaceous formation No. 5, and 
is of an estuary character. Wherever the superincumbent strata are denuded away, the 
surface of this bed is covered with fragments of bones belonging to some huge sauroid or 
manatoid animal. ‘These bones are scattered very abundantly over the country about the 
sources of the Moreau, Grand, Cannon-ball, and Little Missouri rivers, and are for the 
most part much waterworn and uncharacteristic. On Cherry creek this bed attains con- 
siderable thickness, fifty to one hundred feet, capped with large ledges of sandstone, con- 
taining a species of Cyvena in large numbers. Near the head of the Little Missouri, an 
important stream, which takes its rise near the northern base of the Black hills and run- 
ning a northeastern course empties into the Missouri river near long. 102°, we find another 
species of mollusc, Cerithiwm Nebrascensis, which further confirms the estuary character of 
this deposit. We can thus trace this lignite formation to the northern base of the Black 
hills, and observe its edges overlapping the Cretaceous beds. But to return to Thunder 
Butte. The summit of this hill is capped with a thick layer of sandstone, the exact posi- 
tion of which I could not determine satisfactorily. A similar rock is seen capping the 
hills at Fort Union, also in many localities along the Yellowstone, and may hold the same 
position in the series. On the eastern side of the hill, the bed of sandstone presents a per- 
pendicular face, from which have been detached large fragments of the rock, that have 
fallen on a terrace below. ‘This terrace is about half the way up the hill, and about one 
hundred yards broad at its widest part, and thickly scattered over it are huge masses of a 
compact bluish sandstone, which look at a distance like large granite boulders. 
The extent of country occupied by this great basin I have estimated at about four 
hundred miles in length from east to west, and one hundred and fifty from north to south, 
or about sixty thousand square miles. We thus arrive at an approximate idea of the 
immense area covered by these lignite beds, though I am satisfied that future explorations 
will show that the above estimate is much too small. 
I have thus given a brief and imperfect sketch of this great basin, comprising all the 
important facts in my possession at this time. The local sections will, I think, show the 
