GEOLOGICAL SKETCH. 15 
Trias and those of New Mexico and Sonora, and go far to prove that all 
our Triassic rocks which have yet yielded plants belong to the uppermost 
division of the system. 
In New Mexico there are at least two thousand feet of sandstones and 
shales belonging to the Trias beneath the strata which contain the fossil plants 
at the copper mines. Immediately above the latter lie the sandstones of 
the Dakota group, the basal member of the Cretaceous system as repre- 
sented in that region; so that we have proof that these plant beds form the 
extreme upper part of the Trias. The lower beds of sandstone and the con- 
glomerate which forms the base of the series in New Mexico and Arizona 
may represent the lower portions of the Trias in the Old World, but unfor- 
tunately no fossils have yet been obtained from them. 
Many writers upon the Triassic beds of the West have ealled the whole 
formation Jura-Trias, either under the impression that both systems were 
represented in the group, or as a matter of precaution in case this should 
be found to be true. There seems, however, to be no good reason for think- 
ing that the series of rocks which I have described represents the Jurassic 
of Europe. Another set of beds overlying the Triassic and underlying 
the Dakota sandstones occur in Utah, Colorado, and Wyoming, and are 
proved by their fossils to be Jurassic. But these beds wedge out toward 
the south, and I have been unable to find any traces of them south of En- 
chanted Springs, near the lower line of Colorado. They consist of— 
(1) Gray earthy limestone with marine Jurassic mollusks, best shown in 
Wyoming and Utah. 
(2) Light sandy and gypsiferous strata which succeed the limestone to- 
ward the south, and 
(3) Alternations of reddish sandstones and shales—fresh-water beds, con- 
taining unios and saurian bones—the Atlantosaurus beds of Marsh. 
On this series rest the Dakota sandstones, and below it are the red sand- 
stones and conglomerates of the Trias. 
