108 GEOLOGY. 
The green and gray shales which immediately overlie the sandstones are nearly homogeneous 
for 30 or 40 feet, when thin bands of bituminous shaly limestone begin to be introduced. 
Those continue to increase in number and thickness till, about 100 feet above the base, they 
entirely replace the shales. From this point to the summit of the series alternations of both 
recur, but the limestones predominate. 
The fossils of the shales are very fragmentary and imperfect, but those of the limestones are 
numerous and well preserved. At the crossing of the Canadian the most abundant fossil is 
the well known Inoceramus problematicus, (I. mytiloides,) so common in the Cretaceous rocks of 
both America and Europe, 
This occurs in such numbers that its flattened valves quite cover the surfaces of some of the 
strata. On the Upper Missouri this species of Inoceramus is found in the Middle and Upper 
Cretaceous rocks, forming Nos. III and IV of Meek and Hayden’s Nebraska section. This 
confirms the conclusion, drawn from their own fossils, in reference to the relative age of the 
sandstones and shales of the Cretaceous mesa at the Moqui villages, viz: that they are the 
equivalents of Nos. I and II of Meek and Hayden’s section. 
In New Mexico I was unable to draw any satisfactory line of separation between the different 
portions of what I have called Lower Cretaceous strata, nor are the upper members of this great 
formation sufficiently developed on our route to be susceptible of intelligible subdivision. I 
have, therefore, made of the Cretaceous rocks two great groups, upper and lower, which are 
readily distinguishable both by their lithological characters and fossils. This classification is 
not advanced as any substitute for a different one adopted elsewhere, but merely as an expres- 
sion of the stratigraphical development of the Cretaceous formation along our line of survey. 
Messrs. Meek and Hayden have apparently found in their subdivisions of the strata of this 
epoch in Nebraska tangible and distinctive characters, yet, as we have seen, it is probable 
some of them have only a local value. 
Beside the fossils already mentioned, I found in the limestones at the crossing of Red Fork 
of the Canadian a small Arca, a Gryphea, and fish teeth, all too imperfect for accurate deter- 
mination. 
The section here exposed is peculiarly interesting, as it includes so much of both groups of 
Cretaceous rocks and their point of junction. As it is directly on the line of the great Santa 
Fé road, it is to be hoped that it will be more carefully examined by those who have more 
time at command, and a larger collection of fossils made than I obtained. 
From Red Fork to Point of Rocks we were constantly upon the Upper Cretaceous forma- 
tion, here composing high and rounded hills. ‘‘Point of Rocks’’ is a bald headland of trap, 
the terminus of a lava flood poured from a neighboring cluster of volcanic mountains, appa- 
rently belonging to the Raton group. Its perpendicular faces are the result of erosion, and 
show how much the aspect of this region has been modified by aqueous agency. 
From ‘‘ Point of Rocks’ to the Cimarron the surface rock is generally the Lower Cretaceous 
sandstone, though in some localities a sheet of trap overlies and conceals it. 
On the left bank of Cottonwood creek a limited area is occupied by strata of nearly white 
tufaceous limestones, probably of fresh water origin, and an outlier of the great Tertiary basin 
of the Arkansas. At the crossing of McNees’s creek an exposed surface of soft Cretaceous 
sandstone is covered with the names of travellers who have stopped there. At Cedar Spring, 
Cold Spring, Upper Cimarron Spring, ‘‘Hole in the Rock,’’ &c., as well as at many inter- 
vening points, the same stratum is exposed. 
CIMARRON CROSSING TO PAWNEE FORK. 
‘ss traversing this interval (200 miles) we followed down the valley of the Cimarron three 
days, then crossed the ‘‘ Jornada’’ to the Arkansas, which we descended to the ‘ Dry Road,”’ 
and then crossed by way of the Crow creeks to Pawnee Fork. In this part of our route we 
