EPOCHS OF IGNEOUS ACTIVITY 259 



greater age for the andesite than for the other igneous rocks, which were 

 outpoured on the surface and have not been greatly eroded. The ande- 

 sitic tuffs of the Fort Hall Indian Eeservation are much altered and 

 represent the earliest igneous rocks recognized there. 



(3?) The inclusion of basaltic fragments in rhyolite may mean that 

 olivine basalt was erupted before the first outflows of rhyolite. The rela- 

 tive freshness, however, of both the basalt and the rhyolite and the prac- 

 tical identity in mineralogic character of the basaltic inclusions with the 

 broad basaltic flows suggest that the inclusions are parts of the main 

 flows and that the inclosing rhyolite belongs with the later rhyolitic ex- 

 trusions. 



(4) Much of the rhyolite that forms the cones and perhaps the greater 

 part of the flows was extruded during the next epoch. 



(5) Basalt surrounds the rhyolitic cones and floods much of the lower 

 ground in the northwestern part of the region. Some of the basalt in- 

 cludes fragments of rhyolite. Although the basaltic outflows were prob- 

 ably in greater part of the quiet type, the numerous cinder cones and the 

 basaltic ash at some localities indicate that some of the eruptions were 

 explosive. 



(6) A second rhyolitic epoch probably gave rise to the lavas that in- 

 clude fragments of basalt and supplied rhyolitic ash that here and there 

 overlies the basalt. From the relatively smaller volume of the fresher 

 flows recognized, this epoch was probably of shorter duration than the 

 first rhyolitic epoch. The presence of ash beds indicates that the erup- 

 tions were in part of the explosive type. 



(7?) A little basaltic flow on the northwest slope of Middle Cone, in 

 the northwest % of southwest 14? section 7, township 7 south, range 42 

 east, suggests a possible basaltic episode later than the second rhyolitic 

 epoch. The basalt with inclusions of rhyolite in the southeast 1/4 of the 

 same section tends to support such a view. Additional evidence is fur- 

 nished by the presence of a basaltic ash bed between beds of rhyolitic ash 

 in a cut bank of Blackfoot River, in section 16, township 7 south, range 

 42 east. These features may be the result of a single basaltic epoch, but 

 in section 14, township 5 south, range 39 east, in the Portneuf Quad- 

 rangle, rhyolite with gentle easterly dip is both underlain and overlain 

 by basalt, thus indicating two basaltic outflows with an intervening rhyo- 

 litic flow. 



(8 ?) The most recent extrusive activity appears to have been rhyolitic. 

 It is represented by deposits of rhyolitic ash, perhaps the higher bed of 

 the Blackfoot River section above mentioned, and by the explosion craters 

 now occupied by ponds between Middle Cone and North Cone, in town- 



