MESOZOIC EOKMATIOI^^S. 
9 
the vertical sides of the valley ; the Triassic beds forming the western wall, 
with the marl below, and a very heavy bed of hard sandstone on top, the 
whole rising to 900 feet by barometer. In the bad-land tract, I obtained 
satisfactory evidence of the lacustrine character of the formation, a point 
of much importance, inasmuch as the nature of these beds has remained 
very obscure up to the present time. The evidence consists of numerous 
specimens of species of Unio from a number of distinct localities, and frag¬ 
ments of bones and teetli of two or three species of Samians, one of which 
at least was of terrestrial habits, according to our present knowledge. I have 
submitted the Unios to my friend Mr. F. B. Meek, who informs me that they 
belong to five species, which he has described^ three of them under the 
names, U. cristonensis, U. gallkxnsis, and U. terrceruhrcB. He observes, 
“supposing that these shells really come from the horizon of the Trias, 
they are the oldest Unios yet found, so far as I am informed, in this 
country.” 
The Saurian remains above mentioned are those of Dinosamia^ Cro¬ 
codilian and perhaps Sauropterygia. The first named is represented by a 
tooth of the type of that of Lcelaps; the Crocodile is a Belodont, which I 
have described under the name of' Typotliorax coccinarum. 
The evidence derived from the vertebrate fossils is favorable to the 
identification of this horizon with that of the Trias, although it cannot, of 
course, be regarded as conclusive until more perfect specimens are obtained. 
Besides the overlying sandstone bed, the red marls are traversed below 
it by a conglomerate, which is in some localities of a bluish tint. At some 
points, it weathers to gravel, and near this horizon the vertebrate remains 
occur. At other points, it forms a very hard Potomac marble, containing 
pebblesof various colors. Nearthe same level, I obtained specimens of impure 
copper-ore, which simulate petrified wood in form. The sandstones, especially 
those lying obliquely on the mountain-side, I found to contain obscure 
vegetable remains, some of which are replaced by oxide of iron. They 
reminded me of similar remains observed in the same horizon near Taos. 
On passing a mile to the south of the locality which has been described, 
the opposite masses of the Jurassic and Triassic rocks are seen to descend at 
*Anuual Report of Chief of Engiueers, 1875, ii, p, 1003 
