MODES OF ERUPTION 265 



and 30, township 5 south, range 43 east. Lanes Creek Quadrangle, basalt 

 has emerged along recognized fault lines. Elsewhere, as in parts of the 

 Lanes Creek Quadrangle, the basaltic hills south of Henr}^, in township 7 

 south, range 43 east, and in the basaltic hills and ridges west of Cranes 

 Flat, in township 3 south, range 41 east, the occurrence of the basalt is 

 such as to make doubtful its connection with existing cones or craters. 



Succession of the igneous Rocks 



The succession of the igneous rocks of this region, as previously indi- 

 cated, probably begins with an intermediate type, andesite, as in the suc- 

 cessions of Eichthofen and Iddings, and is followed by alternations of 

 rhyolite and basalt, including at least two and possibly three outbursts 

 of rhyolite and possibly two of basalt. The general conclusion of Lind- 

 gren,^* that eruptions close with outpourings of basalt, does not seem to 

 be sustained in this region. Here rhyolitic ash overlies basalt locally and 

 the craters between the cones in section 7, township 7 south, range 42 

 east, are bordered by j^iles of mingled debris, chiefly rhyolitic. A similar 

 condition was noted in the Fort Hall Indian Reservation, where a dark 

 volcanic sand, composed of latitic lapilli, overlies basalt. It is possible 

 that in each of these regions some flow of basalt later than these siliceous 

 eruptions may exist beyond the district affected by the rhyolite, but no 

 such flow has thus far been distinguished. 



Relations of igneous Rocks to sedimentaey Rocks 



The beds of earlier rhyolitic or latitic ash in the Lanes Creek and 

 Cranes Flat quadrangles, previously described, are not well exposed, but 

 they appear to be interbedded with the Wayan formation and to share in 

 its deformation. 



The relations of the hornblende andesite porphyry to the accompany- 

 ing sediments at Sugarloaf Mountain and vicinity have already been dis- 

 cussed. The folding of the strata is not noticeably greater at Sugarloaf 

 Mountain th^n in many other parts of the field where igneous rocks are 

 not exposed. Thus it would seem that the part played by the intrusion 

 of the porphyry in the upheaval of the mountain was relatively insig- 

 nificant, but the intrusion of the supposed concealed batholith from which 

 the igneous rock was derived may have been an important factor in tlie 

 general deformation of the region. 



The rhyolite in the Cranes Flat Quadrangle overlies inclined strata 

 ranging in age from Carboniferous to Lower Cretaceous (?). In the 



^^ Waldemar Uiidgi-pn : Op. cil., p. 274. 



