34 REPORT UNITED STATES GEOLOGICAL SUEVEY. 



adjacent parts of Colorado and Utah, consists mainly of tbe ordinary 

 indurated sandstones that so largely enter into the composition of the 

 Cretaceous grou{)S of that region, together with somewhat frequent beds 

 of carbonaceous shales, and several beds of coal of greater or less im- 

 portance. This description has also a good general application to the 

 group, as it is developed in this district; but in the eastern part it con- 

 sists very largely of a series of reddish-colored, usually thin-bedded 

 sandstones, together with some shaly and carbonaceous beds. These pecu- 

 liarities, however, gradually merge into the more common characteris- 

 tics of the group to the westward. Besides many more or less distinct 

 carbonaceous horizons, the group contains several beds of coal within 

 this district. One of the best is near its base, and another near its top, 

 with others between, that may, perhaps, be found to be of workable 

 thickness. 



The strata of the Laramie Group occupy a large space between the 

 eastern portion of Axial Basin on the one side, and Agency Park and 

 Hogback Vallej^ on the other. It is brought to view along the whole 

 length of both the Midland and Eaven Eidge flexures, besides occupy- 

 ing considerable spaces to the northeastward and northwestward of 

 Eaven Park. It also occupies the larger part of the space between 

 Williams Fork and Yampa Eiver, within the district, as well as the 

 most of that portion of it which lies north of the Yampa and between 

 the mouth of Williams Fork and Yampa Mountain. For the more pre- 

 cise limits of the surface occupied by the strata of the group within the 

 limits of this district see the geological map accompanying this re[)ort, 

 and also that of the largo atlas of Colorado, published by the Survey. 



So far as known to me, the strata of all the Cretaceous groups of 

 W^estern North America, beneath the horizon of the Laramie Group, 

 are of marine origin, except a few local deposits in different portions of 

 the series, which contain brackish- and fresh-water invertebrate forms. 

 On the contrary, no exclusively marine invertebrate forms are known to 

 have been obtained from the strata of the Laramie Group, as I have 

 defined its limits in this report. The species of Inoccramus that have 

 heretofore been reported from the lower strata of this group, I am now 

 satisfied should be referred to the Fox Hills Group, the error of refer- 

 ence having been made in consequence of the absence of a distinct 

 stratigraphical plane of demarcation between the groups. The com- 

 parative abundance of remains of land-plants in all the strata of the 

 Laramie Group also indicates its sej)aration from the open-sea deposits. 



THE TEETIAEY PEEIOD. 



In the great region that is now drained by the Green Eiver there are 

 three w«ll-marked groups of strata, all conformable with each other, 

 that come in their order above the Laramie Group, and which all agree 

 in referring to the Tertiary period. These are the Wasatch, Green Eiver, 

 and Bridger Groups, named in the ascending order. 



As already mentioned on a previous page, all the groups of the Cre- 

 taceous period, as they are developed in the great Eocky Mountain 

 region, so far as I have been able to observe, or to obtain information of 

 them, are strictly conformable upon each other. I have also shown that 

 the Post-Cretaceous Group is strictly conformable upon the uppermost 

 of the Cretaceous groups, although some unconformity is known to 

 exist among the strata within the limits of the Post-Cretaceous Group 

 (the Laramie Group), to the northward of this district. 1 have been 

 equally unable to discover any unconformity between the strata of the 



