Geology and Mineralogy. 73 



western Colorado lying between the Yam pah and White Rivers, 

 and between Green River and the subordinate range of mountains 

 that lies west of and parallel with the Park Range. The area is- 

 embraced between parallels 39° 30' and 40° 30', and meridians 107° 

 30' and 109° 30'. 



The rocks of this district embrace all the sedimentary forma- 

 tions yet recognized by the investigators who have studied the 

 region' that lies between the Park "Range and the Great Salt 

 Lake, namely from the Uinta Quartzite (which underlies the 

 Carboniferous) to the latest Tertiary, inclusive. Not only has 

 the geographical distribution of these formations been mapped, 

 hut all the displacements of the strata have been traced and 

 delineated. The last named investigations bring out some 

 interesting and important facts, in relation to the orographic 

 geology of the region, especially as regards the eastern termination 

 of the great Pinta uplift and' the blending of its vanishing pri- 

 mary and accessory displacements with those of the north and 

 south range above mentioned. Much information was also 

 obtained concerning the distribution of the local drift of that 

 region ; the extent and geological date of outflows of trap, etc. 



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 equb.alent 

 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 from the strata of 

 this horizon at Black P.uttes station, three new species of TJnio 

 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 distinctively American types or closely 

 related to species now living in American fresh waters. They 

 represent, by theil j living species ; Unin 



elm-),* Lamarck- TJ. seenris Lea ; TJ. gibbosus Barnes ; TJ. meta- 

 nevrxs Rafinesque, and U. complanatn'* Solander. They are asso- 

 ciated in the same stratum with species of the genera Corbula, 

 < ;„■/, ;,■„!«, Neritina, Vimparus, etc., and which stratum alternates 

 with layers containing OMreu and A„ou,vi. 



The close athnitv of the-, fossil TJnio* with species now living 

 in the Mississippi River and its tributaries -<em- ; lainly sugges- 

 tive of the fa it the ancestry of the hung ones. 

 An interesting -cries of 'farts ha- also been collected, showing that 

 some of th. pes of I'nio were introduced in 

 what is now the great Rocky Mountain region, as early as the 

 Jurassic period, and that their ,\r. e come great 

 and clearly defined as early as late Cretaceous and early ternary 

 times. Other observations suggest the proha I 



eal distribution, dm . >• of their 



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

 reached the Mississip] R - -* ' iated m the 



numerous u now exist there. 



