a: ra 2 ie 
324 F.V. Hayden on the Geological formations of 
the Jurassic periods. I have carefully examined these rocks for 
hundreds of miles and have never yet detected any organic re- 
mains, animal or vegetable, in them 
The Jurassic beds, as revaalea along the mountains, possess 
peculiar and marked lithological characters, so 
eughe them by the fossils in one locality we can trace them 
: ¢ areas, They were first shown to exist in the west 
in i the form. of azone engirdling the Black Hills, They here 
attain a thickness from 200 to 300 feet at least, and from the 
beds in this locality alone hae fossils enough been collected of 
such unmistakable Jurassic types as to prove their existence 
beyond a doubt. But these beds have also been shown, since 
they were first made known in the Black Hills, to be exposed 
along the margins of the Big Horn and Wind River mountains 
near Red Buttes, on North Platte and in numerous localities in 
the Laramie Plains, and westward to Fort Bridger ; so numerous 
are the species now ‘known from the west and so close are the 
affinities of most of them to well known Jurassic types that it 
is not necessary for me in this place to detail the evidence in 
support of that statement 
t is sufficient to remark, that the Jurassic system is quite 
plainly represented along t the margins of the different ranges of 
mountains north of lat. 42°, but proceeding southward from Deer 
creek on the North Platte, the Jurassic beds diminish in force 
until near Cache la Poudre it becomes doubtful whether they 
are es atall. At this point there is a thin bed, perhaps 
20 to 50 feet in thickness, of greenish gray arenaceous mar. 
ertying the red beds which seem to occupy the place of the 
Jurassic. This seems to thin out more and more as we proceed 
southward toward the Arkansas. From Deer creek 100 miles 
north of Fort Laramie to Denver, a distance of 400 miles, I 
have searched in vain for any organic remains in the rocks which 
Aged to represent the Jurassic period of the Black Hills, 
ig Horn and Wind River mountains. In the Red beds or 
cA ioe Triassic, no organic remains have been found north 
of the Arkansas and they do not differ much lithologically in 
their southward extension except that they seem to be much 
thicker and more gypsiferous northward. In the far neni the 
niferous rocks are in many localities 500 to 1500 feet in 
thickness, and even as far south as the Red Buttes the massive 
—— imest one wit a with tri CO 
to “ete rege and are ¢ quite distinct from the red or a) 
s. But as we proceed southward from this point the Car- 
us limestones seem to lose their usual lithological char- 
stirs an there ds re prevail. At the headof Pole creek on the 
eastern margin and in the Laramie Plains west, the Carbonifer- 
