110 ON THE GEOLOGY AND NATURAL HISTORY 
thickness of this deposit, which extends a considerable distance up the Platte, Loup fork, 
and Niobrara rivers, and passes imperceptibly down into the Pliocene grits, which have 
revealed so many extinct mammalian remains. That a large portion of this formation is 
synchronous in age with what we have in this chapter termed the Drift deposit I have no 
doubt. A great thickness of waterworn pebbles often underlies the yellow marl, and the 
two deposits seem often to pass into each other by almost imperceptible gradations. In- 
deed, in many places the drift beds vary very greatly in thickness, and at others alternating 
with the marl, proving quite clearly that there is a distinct geographical difference in the 
character of the deposit, rather than a difference of age. 
The fossils peculiar to the yellow marl deposit are quite numerous in species. Mingled 
with the remains of most of the living mammals of the plains are those of extinct quadru- 
peds, as Mastodon, Elephant, &c., with large quantities of fluviatile and terrestrial molluscs 
of the genera Helix, Limnea, Physa, Paludina, Pupa, Planorbis, Succinea, Amnicola, Cy- 
clas, &c., for the most part identical with species living in the vicinity at the present time. 
One species, Succinea lineata (W. G. Binney), was first found fossil in the marl, but has 
since been procured in a living state from around some small lakes in the sandhills on 
Loup fork. Others will doubtless yet be found living when the living molluscous fauna 
of the Upper Missouri is more thoroughly investigated. 
3d. Erratic Block Deposit. 
I have given this name to a superficial deposit of rocks or boulders, which forms quite a 
conspicuous feature in many portions of the Northwest. It is formed of nearly or quite 
unworn rocks from all the formations in that country, but more especially from the meta- 
morphic and palwozoic series. That it is more recent than the two deposits already alluded 
to is evident from the fact that it overlies them both wherever they are exposed. It seems 
to be very similar in its character, though on a much smaller scale, to that of the “ Erratics”’ 
of New England, but the rocky masses are not usually so large, seldom exceeding four or 
five tons in weight. These rocks are far more numerous on the north side of the Missouri 
river, thinning out and becoming smaller in a southwesterly direction until they entirely 
disappear. But on the north side of the river, in Dakota and Minnesota, the whole surface 
of the country may be said to be covered to a greater or less extent with these erratic 
blocks. The hills that border the river opposite Fort Pierre are paved with them for 
miles, so that a person could step from block to block. Sometimes they form zones or 
belts with a southeasterly and northwesterly range. Near Fort Pierre these belts are from 
half a mile to a mile in width, on the surface of which are scattered very thickly these 
angular masses, while an intervening belt of land occurs whose surface is nearly destitute 
