128 DESCEIPTIVE GEOLOGY. 



with other basins almost impossible, although they present certain features 

 like the Niobrara Pliocene beds east of the Laramie Hills. 



So far as known to us, neither vertebrate nor invertebrate forms have 

 as yet been obtained from these deposits, so that palseontological evidence, 

 so desirable in deteraiining the age of Tertiary basins, is still wanting, for 

 the North Park deposits. It is quite probable that there may be found 

 included within the Park two distinct Tertiary series. Some observa- 

 tions were made at the time of our explorations, which would tend in this 

 direction, showing a lower set of unconformable beds, which, however, only 

 reach the surface in a few localities, the greater part of the area being covered 

 with more recent deposits. From the difficulty of sharply defining the two 

 horizons of these beds, they have been given a local name, the North Park 

 Tertiary, and a distinct designation upon the geological map. Partly from 

 the general appearance of the strata, and in part from- their relation to the 

 basaltic rocks, they have been regarded provisionally as of late Pliocene 

 age. 



Lithologically, as already stated, these deposits within the Park develop, 

 a local character, the uj^permost beds being invariably loose friable sand- 

 stones, formed from the comminuted detritus mixed with the re-arranged 

 sands of the Colorado beds. Where the old crystalline rocks prevail, the 

 sandstone is generally coarse, and of a gray color, while in the other beds 

 yellowish-brown shades prevail. Beneath these overlying sands are finer 

 beds, with inters tratified layers of -impalpable grayish- white and cream- 

 colored marls, which cannot be told from similar beds east of the Laramie 

 Hills, in the neighborhood of Horse Creek and Shelter Blufi"s. Many of 

 these cream-colored beds are exceedingly friable and rich in lime, and, 

 upon being treated with dilute acid, give off a brisk effervescence. Under 

 the microscope, the mass seems made up of exceedingly minute angular 

 crystalline grains : other beds consist largely of trachytic and rhyolitic 

 material. On the south side of Bruin Peak occur beds, which differ some- 

 what from those found in other parts of the Park, being made up of loose 

 coarse crystalline detritus mixed with gravel and fine sand. Beyond the 

 Park, the North Park Tertiary deposits extend down the Platte Valley, and 

 may be traced northward between the two g-reat ranges, along the Medicine 



