168 GEOGRAPHIC DISTRIBUTION. 



Green River, and the plane of demarkation is obscure, or rather there is no 

 plane of demarkation, but the separation is transitional; but the bad-land 

 beds of the Bridger Group are very distinct from the limestones and sand- 

 stones of the Upper Green River. In the latter, sandstones usually rather 

 massive, and in the case of the Tower Sandstone greatly so, have impure 

 limestones intercalated. In the former, bad-land sandstones prevail, and these 

 are largely green sands, but irregular beds and aggregations of chalcedony 

 are abundant, and high in the series two well marked andr persistent lime- 

 stones are found. These beds of chalcedony afford the moss agates for 

 which the region about Fort Bridger has been noted. I suppose them to 

 have been deposited by chemical precipitation from waters highly charged 

 with silica. The limestones are in many places crypto-crystalline, and break 

 with a conchoidal fracture and often have the ring of phonolite. I consider 

 these also to be chemical precipitates. Fresh water fossils are sometimes 

 found imbedded in the crystalline masses, but can rarely be obtained in a 

 perfect state, but fossils are more abundant in the arenaceous and argilla- 

 ceous partings, and can be obtained in a good state of preservation. In the 

 green sands vertebrate fossils have been found in abundance, and concern- 

 ing them much has already been written by eminent paleontologists. 

 No coal has been discovered in the Bridger Group. 



BROWN'S PARK GROUP. 



These beds are found in the valley known as Brown's Park and a dis- 

 trict of country stretching thence to the southwest beyond the Snake and 

 Yampa Rivers. In Brown's Park they lie in a deep basin of erosion, the 

 bottom and sides of which are composed of Uinta Sandstone. This basin 

 is in the very axis of the Uinta uplift. Eastward, both on the north and 

 south sides of the area of outcrop, the beds are seen to rest unconformably 

 upon all of the Carboniferous, Mesozoic, and Cenozoic formations previously 

 mentioned. Its unconformity with the Upper Green River, Lower Green 

 River, and Bridger beds is well exhibited in the Dry Mountains in many 

 fine exposures. Its structural relations to these beds will be discussed here- 

 after. Its sandstones are bad-land rocks of exceedingly fine texture. In 



