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 Avill be again referred to on subsequent 

 jjages, but oiu- x>resent knowledge is hardly suificient to warrant any 

 conclusive generalizations upon it. 



From Ealston Creek I proceeded northward aloug 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 sloi)e of the hills some two 

 or three miles eastward from Marshall's mines at about 50 feet abo^'e the 

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

 were, however, i)erfe(5t enough to allow their identification vn\\i 0. (jJahra 

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

 as already recorded. 



Contiiming eastward to the "\dllage 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 comi)ared mth many of those which we 

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

 jjer part of the Fox Hills Groui), and the layers containing them are 

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

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

 Hills Groui) and the lower strata of the Laramie, and the throw proba- 

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

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

 & Haydeu. 



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

 South Platte and Boulder Eivers, hoping tO'find other exposirres of the 

 fossiliferous horizons of Laramie strata, but without success. These 

 examinations ended my paleontological field-work east of the luountains 

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

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

 esting and important paleontological 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 featiu-es of 

 structural geology without comment because they have already been so 

 fidly reported upon by Dr. Haydeu and the late Mr. Marvine. 



The Ibllowing 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 i)racticable opportunity was improved to note the character of 

 the strata at the junction of the well-recognized groui^s 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 verticnl range of the same and asso- 

 ciated si>ecies elsewhere, especially with those from the t^^ncal localities 

 of the Upper Missouri Eiver 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 

 lithologicaDy, and also paleontologically when fossils occiu- in them, the 



