SECTION IV., 1883. [ 148 ] TRANS. Roy. Soc. CANADA. 
V.—WNote on the Triassic of the Rocky Mountains and British Columbia. 
By GEORGE M. Dawson, DS. F.G:8. 
(Read May 25, 1883.) 
The Triassic rocks of the West present themselves under two very dissimilar aspects, 
their eastern part consisting of red beds, chiefly sandstones, while on the Pacific slope the 
character of their material is much more varied; they include numerous fossils, and are 
evidently truly marine in origin. The most complete comparison of these two classes of 
deposits has been made by Clarence King in the 40th parallel region. The red beds are 
there found participating in the Rocky Mountain uplift, but have originally passed com- 
pletely across the position of this range, and extend westward to the Wahsatch Mountains 
(longitude 112°) which here constituted the western shore of the sea in which they were 
deposited. The rocks are described by King as consisting generally of sandstones, the 
upper half being always of lighter colours than the lower, and, intercalated more or less 
with beds of dolomite and gypsum. The lower part of the series is usually from brick 
to vermilion red, the upper part pale red and buff. The dolomitic and gypsum beds are 
local in character, but the latter sometimes reach forty feet in thickness of pure calcic 
sulphate. 
~ In the Rocky Mountains, in this latitude, the Triassic is from 300 to 1000 feet in 
thickness but, on approaching the Wahsatch shore, thickens to 2000 or 2500 feet, and holds 
some conglomerates. Fossils are almost completely wanting. 
In many other districts of the western States and Territories, the Triassic beds are 
developed with similar characters. As far east as the Black Hills of Dakota, they are 
described by Professor N. H. Winchell as maintaining a thickness of over 300 feet, and hold- 
ing great quantities of white gypsum. They have been observed by Dr. Hayden in the 
mountains at the head waters of the Missouri, and in addition to the deposits of gypsum 
are in places impregnated with salt. 
Returning to the 40th parallel region, and passing westward from the Wahsatch range, 
no Triassic beds are met with till longitude 117° 30’ is reached, at a distance of nearly 
300 miles. The rocks of this period are there found to be represented by the Star Peak 
and Koissats groups of King, the former and upper subdivision consisting of fossiliferous 
limestones, with quartzites and slates, the latter of quartzites, argillites and porphyroids, 
the whole with an aggregate thickness of over 16,000 feet. Marine fossils are very 
abundant in some parts of the Star Peak subdivision, and are almost precisely similar in 
forms with those of the St. Cassian and Hallstadt beds of the Alps. The term Alpine 
Trias has consequently often been used in speaking of those rocks. 
The red beds of the Rocky Mountain region clearly point to the conditions of deposi- 
tion found in a shallow body of water, more or less completely shut off from the ocean 
or only in occasional and brief connection with it, while, for the most part, the sediments 
of the Nevada Triassic are, as unmistakably, such as might be produced under ordinary 
marine conditions in greater or less proximity to a coast line. 
