GEOLOGICAL RBPOET. 291 



Between the South Pass and Green River a great deal of fossil wood was observed 

 strewn over the surface, all silicified, and some of it changed into transparent agate. 

 It evidently comes out of this formation, probably from No. 3, and Captain Stansbury, 

 who followed a road some miles distant from ours, actually observed some fossil trees 

 imbedded in such sandstones, the trunks of which measured nearly 2 feet in diameter. 

 Near there, we find stated in that report, some imperfect specimens of Nautilus were 

 collected, which would indicate a marine formation, if we may not presume that these 

 fossils either came from a drifted bowlder, or from a limestone corresponding to our 

 No. 2, in which large Planorhis are found, which, when badly preserved, may readily 

 be mistaken for Nautilus. 



THE ESTUARY FORMATION ON BEAR RIVER. 



On Bear River, near the mouth of Sulphur Creek, I observed light-colored shaly 

 slates, gray argillaceous shales, and some strata of sandstone and limestone. The lat- 

 ter is partly light yellowish, coarse-textured, wholly composed of fossils, partly dark- 

 gray slaty, also full of shells, and quite fetid. The outcrop is much covered over by 

 detritus. These strata are considerably tilted: at one point they trend from northeast 

 to southwest, and dip under a high angle to southeast. West of them we find the 

 strata of the lower series of Fort Bridger, with only a slight dip: east of them, a suc- 

 cession of sandstones, to be described hereafter, also strongly disturbed, nearly verti- 

 cal; but the disturbed condition and imperfect exposure of the rocks prevented me 

 from tracing the exact relations between those different formations. 



The fossils collected from these beds belong to the genera Uu'in, Corbula, Jlrlaula^ 

 Paludina, and Melampus. They characterize the formation as a brackish-water or 

 estuary deposit, without any strictly marine forms. Mr. Meek, to whose report I refer 

 for a more detailed enumeration and description of the fossils, among which there are 

 several new ones, considers these strata as decidedly Eocene-Tertiary. The similarity 

 of their organic remains, and their connection with the sandstone serie- east of them, 

 with Ostrea glabra and lignites, indicate that we have here beds formed under similar 

 circumstances with those near the mouth of -Judith River, in Nebraska, of which Dr. 

 Hayden has given an account, under the direction of Lieutenant Warren, Topographi- 

 cal Engineers. 



These estuary beds are undoubtedly older than the Fort Bridger series, because 

 the beds No. 3 overlie, unconformably, the upheaved mountains of which they form 

 part, on the divide on the old road east of Sulphur Creek ; and I hesitate to yield to 

 the paleontological deductions of Mr. Meek in regard to the Tertiary age of this 

 formation. Although, as I have stated, its stratigraphical position is not quite plain at 

 the point where I have observed it, it appears to be closely allied to the sandstone 

 series with Inoceramus, Ostrea ?/!abra, and coal, which is Cretaceous, most probably 

 Lower Cretaceous, and I am inclined to consider it as an estuary local development 

 in that Cretaceous series. In regard to the analogous deposits of the Judith River, 

 the reader will recollect similar doubts were expressed by Dr. Hayden and Professor 

 Leidy in various communications to the Academy of Philadelphia. Estuary deposits 

 are naturally scarce in all formations, but we have no reason to doubt the possibility 



