
CRETACEOUS AND TERTIARY—GENERAL ARRANGEMENT. 143 
; ness, of fine unctuous clay, with much carbonaceous matter, and contain- 
ing also crystals of pyrites and selenite, and impressions of fish scales: 
The western outcrops of No. 4 on the upper Missouri, and along the 
Rocky Mountains, as far south at least as Colorado, appear to maintain, 
to a great extent, the character which it presents in the east. 
348. The beds which I have included in the Pembina Mountain group, 
appear to belong to this part of the series. Their character has already 
been fully described. They extend; in all probability, on the forty-ninth 
parallel, from the escarpment of Pembina Mountain, nearly to the edge 
of the Lignite Tertiary formation, on the 102nd meridian. In the great 
scarcity of organic remains, they resemble the typical series as developed 
in the original locality near Fort Pierre,* and from the obscurity of their 
stratigraphical relations in the Pembina Mountain region, their position 
might still remain a doubtful question, but for their precise lithological 
resemblance to beds, north, and west of this place, the horizon of which 
can be defined by paleontological evidence, and superposition. The only 
part of the Pembina Mountain series, in which septarian nodules form a 
striking feature, is that shown at the foot of the escarpment, and near 
the base of the sections there exposed. 
349. Where these rocks are again brought to view south of the Wood 
Mountain Tertiary plateau, their general appearance is somewhat different 
from that of those of the Pembina Mountain sections. They are, as a 
whole, more crumbling and earthy, but extensive layers in them still so 
closely resemble the eastern rocks, as to be lithologically indistinguish- 
able from them. These harder layers do not appear to be confined to any 
particular horizon, but occur in different localities, sometimes near the 
top, sometimes nearer the base. The septarian nodules are now found at 
several different levels in the section, characterizing narrow zones, and it 
is almost exclusively in connection with them that fossils are preserved, 
the intermediate beds being nearly as barren as the clay-shales of Pem- 
bina Mountain, but always clearly distinguishable from the overlying 
Tertiary rocks. 
350. After passing over a great plain based on Tertiary rocks, the 
4th group again appears upturned around the flanks of the Buttes, and 
still preserving—though now within a hundred miles of the base of the 
Rocky Mountains—its old characters, the only real change being the in- 
troduction of a few arenaceous layers, smal]! in thickness and extent, but 
* Geological Report Yellowstone and Missouri Expedition, p. 40. 

