CHARACTER AND DISTRIBUTION OF THE ROCKS 253 



one centimeter across, prominent but smaller hornblende prisms, and a 

 little biotite in a ground-mass that has the appearance of a microgranular 

 rock. The thin section shows that the rock contains large phenocrysts of 

 andesine and pale green, zoned hornblende. There are a few crystals of 

 apatite, with smaller crystals of plagioclase, biotite, and iron ore in a fine 

 ground-mass -rthat is chiefly plagioclase, but that probably includes some 

 orthoclase and quartz. 



The second specimen is M. 110-16, section 25, township 2 south, range 

 11 east, summit of Sugarloaf Mountain. In general appearance this rock 

 differs from some of the other types in having no large feldspar or biotite 

 crystals. Phenocrysts of hornblende one-half to one centimeter or more 

 in length and locally in cruciform arrangement are conspicuous against 

 a uniform finely crystalline gray ground-mass. In thin section the rock 

 shows little, if any, biotite ; the ground-mass is coarser than in some of the 

 other types and contains considerable quartz and orthoclase. It might 

 be called a hornblende-quartz latite porphyry, but is not very different 

 from M. 97-16. 



The hornblende andesite porphyry has not in this region been found 

 in contact with any other igneous rocks or in close proximity to such 

 rocks. Its deeply weathered condition is noteworthy and suggests that it 

 may be older than the other types, none of which shows so much altera- 

 tion. In the Fort Hall Indian Eeservation andesitic tuffs of somewhat 

 different composition from the andesites here described are locally over- 

 lain by rhyolite. The andesites of this district show some differences in 

 age, for in section 22, township 2 south, range 42 east, the dikes that 

 contain considerable biotite appear to cut the more hornblendic dikes, 

 though contacts are not well defined because of weathering. The ex- 

 tremely weathered condition of both sets suggests that the difference in 

 age is not great, and that in all probability these andesites all belong to 

 a single epoch of igneous activity. 



Ehyolite 



The rhyolites of the district are confined to the Henry and Cranes Flat 

 quadrangles. In the Henry Quadrangle the exposures include only the 

 three large hills in the northwestern part of township 7 south, range 42 

 east, the largest of which is known as China Hat, and two -islands in the 

 Blackfoot Eiver Eeservoir, in the northeastern part of township 6 south, 

 range 41 east. In the Cranes Flat Quadrangle the rhyolite occupies three 

 areas in the southwestern part of township 4 south, range 42 east, the 

 largest of which includes nearly two square miles. There are other minor 

 occurrences. 



