about the head of the Missouri, Gc. 237 
V. Jurassic Rocks. : 
These rocks are everywhere revealed, oe the red de- 
posits just Atta and possessing an equal geographical exten- 
sion. Their fullest development and most fossiliterous condition 
Along the northeastern slope of the Big Horn mountains, this 
group of rocks presents its a appearance of grey and whitish 
calcareous and arenaceous layers, with indurated somewhat vari- 
egated beds of more or less laminated mars, containing in great 
abundance Cais densus, Pentacrinus asteriscus, a new species 
of Sar Pecien 
Red Buttes we 6 find a fair development of these beds ie. 
di ho fossils, but as we proceed southward toward Long’ 
Peak, the intercalated laminated marls disappear and be rene 
ormation seems to be reduced to a thickness of fifty to one hun- 
dred feet, with very few fossils. Along the southwest side “of the 
ig Horn mountains and the northeast side of the Wind River 
mountains we have a thickness of Jurassic rocks from eight 
hundred to one thousand feet containing organic remains in the 
greatest abundance. Crossing the Wi nd Ri ver mountains we 
observed the strata corresponding to those upon the eastern side 
with B. densus, Ostrea, &c. Returning to the eastern slope at 
the sources of the Missouri we see occasional indications of their 
7 
cae, Ginarke that the older fossiliferous beds dies pass 
beneath the more recent Cretaceous and Tertiary deposits and 
Seupy a greater or less area underneath the prairie country east 
of the ‘divide’ of the Rocky Mountains. I have made this in- 
ference from the fact that where any elevations occur the com- 
Plete series of fossiliferous beds are exposed around the axis of | 
posit That I may be not misunderstood by those geolo- os 
's who have colored large areas Triassic and Jurassic on pa oo 
eal maps of the West, I would say, that I have never se ‘no 
any of the older fossiliferous rocks from the Potsdam to the. lu ie 
Tassic Inclasive, ees except in narrow outeroppi 
the margins of the mountain elevations. The 
*« arenaceous deposits are exposed over about the same area, 
he * “yee Jurassic form a zone never more than one-fourth of a 
_— miles in width. 
Sci.—Szconp Series, VoL. Xx], No. o2.—akancs, 1861. : 
