NATURAL SCIENCES OP PHILADELPHIA. 131 



of the species of Melania appears to be identical with Cerithium tenerum of Hall, 

 (Fremont's Report, pi. 3, fig. 6,) and a small Paludina agrees very closely with 

 Natica? accidentalk, while a third is equally as near Turbo paludinceformis, of the 

 same report. All the other species are new excepting one Paludina, which is 

 identical with P. Conradl of Meek and Hayden, from the estuary beds at the 

 mouth of Judith River, on the Upper Missouri. All the facts point to the con- 

 clusion that this formation holds a low position in the Tertiary System, or, in 

 other words, is probably of Eocene age. 



The succeeding more recent Tertiary beds of this region, are extensively 

 developed along the route traversed, from near the last mentioned locality to 

 Fort Bridger, and thence towards the South Pass. They differ materially in 

 their lithological character from the older deposits just described, and are 

 characterized by an entirely different group of fossils. The upper part of this 

 series consists of greenish sandstones and arenaceous shales, interstratified with 

 sandy and calcareous slates altogether estimated at from two to three hundred 

 feet in thickness, and apparently destitute of fossils. Then comes, (descending,) 

 light colored argillaceous and pure limestones, with at places great numbers of 

 fossils, all of which are strictly fresh water forms, belonging to a few species. 

 Those collected consist of two new species of Melania, two of Limnea, one of 

 Unio and two or three of Planorbls. There is also at the junction of the lower 

 light colored more calcareous deposits with those above, at many places, a 

 band of dark shaly, more or less carbonaceous material, containing many im- 

 pressions of fern and other leaves. 



As all the fossils found in the foregoing series are distinct from those yet 

 discovered in known horizons, in the other Tertiary basins of the North- West, 

 we have no means of drawing parallels, though they are probably miocene. 

 Whether the extensive lignite beds on Bitter Muddy Creeks, east and north of 

 Fort Bridger, belong to this series or to the horizon of the older Sulphur Creek 

 coal is unknown, these localities being too remote from the route to be examined. 



The more modern group described above was never seen in an upheaved or 

 inclined condition, like the estuary beds on Bear River, though it is manifest 

 that the general contour of the country has been considerably modified since its 

 deposition, as this formation was often seen occupying some of the most 

 elevated positions. , 



Beneath this series heavy deposits were observed at several places, consisting 

 of light and whitish fine grained sandstone in thick layers, interstratified with 

 bright red, areno-argillaceous shales. Although these beds appeared to be 

 conformable with the superimposed Tertiary, as no organic remains were found 

 in them, their age must be regarded as doubtful. 



From the foregoing remarks it will be seen that these collections furnish no 

 evidence of the existence of strictly marine Tertiary deposits in the Green River 

 Basin, but like all those yet obtained in Nebraska, point to the conclusion that 

 the Tertiary strata of this central portion of the Continent were deposited in 

 brackish and fresh waters. The oldest of these formations, so far as known, 

 contain a group of mollusca indicating brackish waters, while all the subsequent 

 formations are of strictly fresh water origin. 



Another fact worthy of note is, that all the secondary and Tertiary fossils 

 collected during the survey came from localities east of the Wahsatch range of 

 mountains, while all the specimens collected west of that range of mountains, 

 in the Great Basin, came from Palaeozoic rocks. 



In the ranges of mountains west of the 116th degree of longitude, to the 

 Sierra Nevada, near lat. 39, igneous rocks predominate, and only few traces of 

 stratified rock were found in that district, in none of which any organic remains 

 were observed. 



F. B. Meek and H. Engelmann. 



1890.] 



