PEALE.] EESUME VOLCANIC EOCKS. 643 



111 Cache Yallej^ the terraces indicate several periods of comparative 

 permanence below the level indicated by the Provo Beach. The fact 

 that Eed Eock Gap was a pointof ontlet for the lake that filled Cache 

 Yalley was first recognized by Professor Bradley in 1872, although Mr. 

 G. K. Gilbert claimed its discovery in 1876.* It was my intention to 

 enter more fully into this question in this report, but the elevations 

 have not all been determined yet, and even if they were, the time is too 

 limited to take it up at present. Suffice it to say that, 1st ; the terraces 

 point to the existence of a lake having a higher level than Lake Bon- 

 neville 5 2d, the outlet of the lake indicated by the Bonneville Ter- 

 race was considerably north of Eed Eock Gap, probably at the point 

 where the Portneuf Eiver cuts across the axes of the Bannack Eange ; 

 3d, Eed Eock Gap was the outlet when the lake had the level indicated 

 by the Provo Beach.t 



To treat of the subject fully, the valleys of this region should be care- 

 fully studied, which would require an entire season to be devoted to 

 them. 



VOLCANIC EOCKS. 



The volcanic rocks of our district are included under one head, viz. 

 Basalt. The region covered with it is principally the Blackfoot Basin, 

 in which are at x)resent a number of extinct craters. There are indi- 

 cations that there were several flows. In this region the basalt is gen- 

 erally horizontal in position, and fills the valleys and depressed portions 

 of the basin. 



The craters in the Blackfoot basin were the sources of the flow that 

 follows the Portneuf, and below the caiion separates the river from 

 Marsh Creek. This belt of basalt appears to end before the Snake 

 Eiver plains are reached, although below the gap that separates Marsh 

 Valley from the latter there is basalt. The basalt at Station 81 and the 

 surrounding region is evidently older than that of the Blackfoot Basin. 

 It rests on Pliocene rocks, which are tilted, appearing to have been 

 poured out before the end of the Pliocene period; whereas, in the case 

 of the basalt in Marsh Valley, the Quaternary deposits were somewhat 

 eroded before the basalt was poured out. Under the heads Blackfoot 

 Basin &c., in Chapter IV, the basaltic occurrences are described. We 

 have therefore in our district both Pliocene and Quaternary basalt. 

 The former appears to have been a flow at the close of the Tertiary, ac- 

 companying the orographic disturbances which occurred then. There 

 were also two flows during the Quaternary, as indicated by the basalt 

 in Portneuf Caiion and in Gentile Valley. In speaking of the basalt 

 on the Snake Eiver plain and lower portion of the Portneuf Eiver, Pro- 

 fessor Bradley says : | 



Here, again, we encounter basalt, but it seems to belong to a lower layer than that 

 we left at Black Eock. All over the great plain, indeed, we find two or more layers 

 of basalt, separated by greater or less thicknesses of sand and gravel, i^artly loose, 

 partly consolidated by ferruginous, siliceous, or calcareous cement. If two layers 

 should be fovmd superimposed at any point in the upper part of the canon, I should 

 believe that they had resulted from two distinct eruptions from the- volcanic source 

 before mentioned [iw the craters near Soda Springs']. As it is, it is not impossible that 

 these layers in the outer plain' liave been ejected from some central sourcje, have over- 

 flowed the plain, and so have run up into the mouths of the valleys opening uiion it. 

 It seems hardly possible that, after flowing seventy or eighty miles, the lava should 

 still have retained sufficient fluidity to spread out in a solid layer over the plain. 

 Whatever the source, the material had evidently become quite viscid ; for at some 



* See Amer. Jour. Sci. and Arts, Vol. XV, June, 1878, pp. 439-444. 

 tSee Amer. Jour., &c., as above, and Chapter V of this report. 

 tEepoi U. S. (}/-o\ Sm-vey of the Terr, for 1872, 1873, p. 204. 



