LETTER OF THE GEOLOGIST. XXIII 



between the Park range and the Great Salt Lake, namely, from the 

 Uinta quartzite (which underlies the Carboniferous) to the Brown's Park 

 group, or latest Tertiary, inclusive. Not only has the geographical dis- 

 tribution of these formations been mapped, but all the displacements of 

 the strata have been traced and delineated. The last-named investiga- 

 tions briug out some interesting and important facts in relation to the 

 orographic geology of the region, especially as regards the eastern ter- 

 mination of the great Uinta uplift and the blending of its vanishiog pri- 

 mary and accessory displacements with those of the north and south 

 range above mentioned. Much information was also obtained concern- 

 ing the distribution of the local drift of that region, the extent and geo- 

 logical date of outflow of trap, &c. 



The brackish-water beds at the base of the Tertiary series, containing 

 the characteristic fossils, were discovered in the valley of the Yampah. 

 They are thus shown to be exactly equivalent with those, now so well 

 known, in the valley of Bitter Creek, Wyoming Territory. These last- 

 named localities were also visited at the close of the season's work, and 

 froDi the strata of this horizon at Black Buttes station three new species 

 of Unio were obtained, making six clearly distinct species in all that 

 have been obtained, associated together in one stratum at that locality. 

 They are all of either distinctivtly American types or closely related to 

 species now living in American fresh waters. They represent by their 

 affinities the following living species : Unio clavns, Lamarck ; U. securis, 

 Lea; U. gihhosus, Barnes', U. onetaneoruSjTia&nesque; and IT. complmi- 

 atus, Solander. They are associated in the same stratum with species 

 of the genera Corbulo, CorMcula,' Neritina, Viriparus, &c., and which 

 stratum alternates with layers containing Ostrea and Anoniia. 



The close affinity of these fossil Unios with species now living in the 

 Mississippi Eiver and its tributaries seems plainly suggestive of the 

 fact that they represent the ancestry of the living ones. An interesting 

 series of facts has also been collected, showing that some of the so-called 

 American types of Unio were introduced in what is now the great Eocky 

 Mountain region as early as the Jurassic period, and that their differen- 

 tiation had become great and clearly defined as early as late Cretaceous 

 and early Tertiary times. Other observations suggest the probable lines 

 of geographical distribution during the late geological periods of their 

 evolutional descent, by one or more of which they have probably reached 

 the Mississippi River system and culminated in the numerous and diverse 

 forms that now exist there. 



The work of the season of 1876 shows very clearly the harmonious rela- 

 tions of the various groups of strata over vast areas, that although there 

 may be a thickening or a thinning out of beds at different points, they 

 can all be correlated from the Missouri liiver to the Sierra Nevada ba- 

 sin. The fact, also, that there is no physical or paleontological break in 

 these groups over large areas from the Cretaceous to the Middle Tertiary 

 is fully established. The transition from marine to brackish water forms 



