OF THE UPPER MISSOURI, 129 
posits is superimposed, so that near some of the mountain elevations I have found it diffi- 
cult to draw the line of separation, no apparent physical break occurring in the sediments. 
Will not these statements go far to show that the estuary deposits ushered in the dawn 
of the Tertiary epoch, and induce the belief that they belong to the first part or Eocene 
period? ‘This point is an important one to establish, on account of its bearing upon the 
history of the physical development of our western continent. 
The estuary deposits soon lose their marine and brackish character and gradually pass 
up into the true Lignite strata, of purely freshwater origin, thence by a slight discordancy 
into the Wind river valley beds, which give evidence of being an intermediate deposit be- 
tween the true Lignite and White river Tertiary beds. ‘Then come the White river bone 
beds, which pass up into the Pliocene of the Niobrara by a slight physical break, and the 
latter are lost in the Yellow Marl or Loess deposits. I have estimated the entire thickness 
of Tertiary rocks in the Northwest at from five to six thousand feet, and their interest will 
be appreciated when I venture to suggest that by thorough investigation they will doubt- 
less reveal, step by step, in a most remarkably clear manner, the history of the physical 
growth and development of the central portion of this continent. I shall treat this subject 
more fully in a future paper, and would refer to the forthcoming Report of Capt. Raynolds 
for the details of the facts sustaining my opinions. 
We have no evidence, so far as I know, of long-continued deep-water deposits in the 
West, until far up in the Cretaceous period. If we examine the Potsdam sandstone we 
shall find that where it reaches its greatest force, the lower portion is composed of an 
aggregation of quartz pebbles cemented with silicious matter, and as we pass upward we 
find it arranged in thin layers, quite compact, with fucoidal markings, ripple-mark, &c. 
Everywhere are most abundant examples of oblique lamine of deposit, and ripple- and 
wave-markings—evidences of shallow waters. 
During the long period that elapsed between the deposition of the earliest part of the 
Silurian epoch and the commencement of the Carboniferous, we have reason to believe 
that dry land prevailed over a large portion of the West. The Carboniferous epoch com- 
mences with thin layers of arenaceous deposits, gradually passing up into homogeneous 
silicious and calcareous beds. The latter are never more than from twenty to fifty feet in 
thickness, and then the arenaceous sediments begin again to predominate, and all the 
proofs of shallow as well as turbulent waters are shown. We then pass up through the 
red arenaceous deposits and Jurassic beds, and find no rocks that indicate deep reaies de- 
position. Cretaceous formation No. 1 commences in many places with a considerable 
thickness of an aggregation of water-worn pebbles, passing up into _ oa — of 
arenaceous and argillaceous sediments, with thick beds of sandstone with Ss 
and oblique laminz, then these indications gradually cease in No. 2, and through Nos. 2, 3 
VOL. XII.—17 
