FORMING THE BAD LANDS OF JUDITH RIVER. 
137 
Melania tenuicarinata , Meek and Hayden. Proc. Acad. Nat. Sci., Pa., 1857, 137. 
Cerithium nebrascensis , “ “ “ “ “ viii. p. 125. 
Explanation of the Geological Map. 
I am indebted to the kindness of Lieutenant G. K. Warren, U. S. Topographical En¬ 
gineers, for the beautiful Geographical Map which accompanies this paper. 
A large portion of the map has been coloured inferentially, and therefore can hardly 
be accurate in detail. The formations along the Missouri River to Fort Benton are laid 
down correctly from the result of my own observations. It will be seen that I have re¬ 
presented no rocks of any age between Igneous and Cretaceous. The reason of this is, 
that we have no positive evidence of the existence of any intermediate deposits in that 
region. The discoveries in the Black Hills have rendered it more than probable that 
not only Jurassic, but Carboniferous Silurian, and perhaps rocks of other epochs are ex¬ 
posed by upheaval around the mountain elevations. If they are revealed they occupy 
but a small area, in the form of a narrow belt engirdling the metamorphic rocks which 
constitute the nucleus of elevation. I know, from personal observation, that the broad 
prairie country, very near to the foot of the mountains, is underlaid, for the most part, 
with Formations 1 and 4 of the Cretaceous Period; and it is quite probable that future 
explorations will not make any important changes in the map, excepting in the immedi¬ 
ate vicinity of the mountains. The Cretaceous Formations 1 and 4 are represented by 
one colour, from the fact that we have comparatively little knowledge of their boundaries 
in that region. 
Note.— Through the kindness of my friends, Prof. Baird and Mr. Drexler, I am per¬ 
mitted to refer to an exceedingly interesting group of fossils, recently obtained by the 
latter in the neighbourhood of Fort Bridger, and presented to the Smithsonian Institution. 
In a hasty examination of this collection some weeks since by my associate, Mr. Meek and 
myself, we at once recognised Ilalysites catenulata, (Catenipora escharoides.) In a subse¬ 
quent examination recently, I think I was able to detect three other species of corals, re- 
ferrible to the genera Favosites, Syringopora, and Streptelasma , an association of fossils which 
at once points to the existence of Silurian rocks in this far western locality. The fossils 
are completely silicified, and the matrix is a compact siliceous limestone, corresponding 
very closely in its mineralogical characters to the description given by Prof. Hall of the 
Niagara limestone in New York and Iowa. The locality where these fossils were obtained, 
is about twenty miles east of the South Pass. 
A still more interesting group of fossils, with reference to this paper, forms a portion of 
the collection of Mr. Drexler, discovered near Fort Bridger. The material is composed 
