white.] VICINITY OF BOULDEK CITY. 199 



the formation of those layers in which Mr. Lakes found the Inoceramus 

 here referred to ; and the Atlantosaurian beds seem also to be a part of 

 a continuous deposition of sediment from those of undisputed Jurassic 

 age beneath. This subject will be again referred to on subsequent 

 pages, but our present knowledge is hardly sufficient to warrant any 

 conclusive generalizations upon it. 



From Balston Creek I proceeded northward along the base of the foot- 

 hills, by way of Marshall's coal mines to the valley of the South Boulder. 

 Going thence eastwardly, I found in the slope of the hdls some two 

 or three miles eastward from Marshall's mines at about 50 feet above the 

 horizon of the coal some imperfect specimens of oyster-shells. They 

 were, however, perfect enough to allow their identification with 0. glabra 

 which I had found so abundant at several localities of Laramie strata, 

 as afready recorded. 



Continuing eastward to the village of Erie, on Coal Creek, I found, just 

 south of the village and of the coal mines which are worked there, some 

 considerable exposures of Fox Hills strata that have evidently been 

 brought up by a fault. I could not ascertain the extent of this fault 

 because of the presence of the abundant debris of the plains, but it seems 

 to be a short one and to have its northern end at the south side of the 

 village, and to extend southward only between one and two miles. The 

 throw of the fault is a slight one compared with many of those which we 

 find west of the mountains, as the fossils indicate a horizon near the up- 

 per part of the Fox Hills Group, and the layers containing them are 

 on a level with the coal, which is not far from the base of the Laramie 

 Group. The fault is thus seen to involve the upper strata of the Fox 

 Hills Group and the lower strata of the Laramie, and the throw j>roba- 

 bly does not exceed 500 feet. The fossils referred to are in the form of 

 casts in sandstone, and consist almost wholly of Veniella humilis Meek 

 & Hayden. 



I traversed in different directions a large part of the district between 

 South Platte and Boulder Bivers, hoping to find other exposures of the 

 fossiliferous horizons^ Qf Laramie strata, but without success. These 

 examinations ended my paleontologieal field-work east of the mountains 

 for this season, and I returned to the foot-hills at Boulder City to prepare 

 for crossing the Bocky Mountains by way of Boulder Pass. Many inter- 

 esting and important paleontologieal questions pertaining to this eastern 

 region still remain unsettled, the investigation of which I hope to resume 

 at another time. I have also passed over many important features of 

 structural geology without comment because they have already been so 

 fully reported upon by Dr. Hayden and the late Mr. Marvine. 



The following brief summary of the observations made east of the 

 mountains is presented here for the purpose of facilitating the discussion 

 that on subsequent pages will follow the record of the field-work for the 

 whole season. 



Every practicable opportunity was improved to note the character of 

 the strata at the junction of the well-recognized groups respectively, 

 and also to seek for the precise point or plane where the characteristic 

 fossils of the one cease, and those of the other begin, to appear. Also, 

 as far as possible, the vertical range of each species collected was 

 observed and compared with the vertical range of the same and asso- 

 ciated species elsewhere, especially with those from the typical localities 

 of the Upper Missouri Biver region. 



As to the limits of the formations or groups which I have examined, 

 although each one as a whole is sufficiently distinct and characteristic 

 lithologically, and also paleontologically when fossils occur in them, the 



