266 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



adjacent Portneuf Quadrangle (township 5 south, ranges 39 and 40 east) 

 it also overlies the Salt Lake formation (Pliocene?). The basalt bears a 

 similar relationship to the pre-Quaternary strata, and in the Henry Quad- 

 rangle occupies depressions excavated in the Salt Lake formation. Both 

 the rhyolite and basalt are overspread locally by the earlier Quaternary 

 deposits. 



Age of igneous Eocks 



The age of the hornblende andesite porphyry at Sugarloaf Mountain 

 may be judged only by its structural relations and present state of preser- 

 vation. If contemporaneous with the folding, it dates back to the major 

 deformation of the region, which occurred in post-Cretaceous or early 

 Eocene time. It has already been suggested that the hornblende andesite 

 porphyry may correspond with the early acid breccia of the Yellow^stone 

 National Park. This rock has been shown by its fossil plant remains to 

 be of Eocene age. In the Fort Hall Indian Eeservation^^ andesitic tuffs 

 are overlain by or perhaps in part interbedded with white and yellow 

 conglomerates probably to be correlated with the Salt Lake formation 

 (Pliocene?). On the assumption that the andesites of the region are 

 essentially contemporaneous, they are probably not later than early Plio- 

 cene and they may be as old as early Eocene. 



In the Fort Hall Indian Eeservation the ages of the rhyolite and basalt 

 range apparently from Pliocene into early Quaternary. The similarity 

 of the igneous succession in the Fort Hall Indian Eeservation with that 

 of the quadrangles here discussed has already been pointed out. Hence 

 it seems probable that the igneous activities in the two areas which are 

 neighboring parts of the same general region were essentially contempo- 

 raneous. In the Yellowstone National Park the rhyolite, which is be- 

 lieved to be essentially contemporaneous with that of southeastern Idaho, 

 overlies the Canyon conglomerate, from which were taken bones identified 

 by 0. C. Marsh as part of the skeleton of a Pliocene fossil horse. 



The rhyolitic ash of the Henry Quadrangle, though forming soil at a 

 number of places, does not so clearly overlie the early Quaternary deposits 

 as does' the latitic ash of the Fort Hall Indian Eeservation. However, 

 the recent appearance of the craters in the eastern part of section 7, town- 

 ship 7 south, range 42 east, and of their accompanying debris suggests 

 that their age is probably not greater than that of the latitic ash. The 

 age here assigned to the rhyolite and basalt accords with that of the basalt 

 of the general region as described by Peale^*^ and Eussell.^^ 



1== G. R. Mansfield : Op. cit., p. 60. 



18 A. C. Peale : Op. cit., pp. 643-644. 



" I. C. Russell : Op. cit.. pp. 61 and 105. 



