F. GEOGRAPHY. 255 



their affinities, the following living species : Unio davus, La- 

 marck ; U. securiis, Lea ; U. gibbosus^ Barnes ; U. metaneorus^ 

 Ralinesgue ; and U. complanatus solander. They are associ- 

 ated in the same stratum with species of the genera Corbula^ 

 Corbicida neritina, Viriparus^ etc., and which stratum alter- 

 nates with layers containing Astrea and A?iomia. 



The close affinity of these fossil Unios with species now 

 living in the Mississippi River and its tributaries seems 

 plainly suggestive of the fact that they represent the ances- 

 try 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 

 Rocky Mountain region, as early as the Jurassic period, and 

 that their differentiation had become great and clearly de- 

 fined as early as late cretaceous and early tertiary times. 

 Other observations present the probable lines of geograph- 

 ical distribution, during the late geological periods, of their 

 evohitional descent, by one or more of which they have prob- 

 ably reached the Mississippi system, and culminated in the 

 numerous and diverse forms tliat now exist there. 



The work of the past season shows very clearly the har- 

 monious relations of the various groups of strata over vast 

 areas; that, although there may be a thickening or thinning 

 out of beds at different points, they can all be correlated 

 from the Missouri River to the Sierra Nevada basin. 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 of life commences at the close 

 of the cretaceous epoch, and, without any line of separation 

 that can yet be detected, continues on upward until only 

 purely fresh-water forms are to be found. Dr. White, an 

 eminent paleontologist and geologist, says that the line must 

 be drawn somewhere between the cretaceous and tertiary 

 epochs, but that it will be strictly arbitrary, as there is no 

 physical break to the summit of the Bridger group. 



EXPLORATION OF THE ROCKY MOUNTAIN REGION, BY 



J. W. POWELL. 



As soon as the appropriation for the fiscal year of 1876- 

 77 could be used, the surveying corps left Washington and 



