260 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



ship 7 south, range 42 east. Mounds of debris including both rhyolitic 

 and basaltic fragments with some sedimentary material partly surround 

 the craters. The craters and mounds are so fresh that they might easily 

 have been formed within historic time. 



With the exception of the earliest rhyolitic epoch and the doubtful 

 epochs, the record of igneous activity in this region agrees fairly well 

 with that in the Fort Hall Indian Eeservation, but no nepheline-bearing 

 rocks, such as were found there, have been found here. 



Comparing the record of igneous activity in southeastern Idaho with 

 the more extended record of the Yellowstone National Park,^ some 70 

 miles or more to the northeast, it seems probable that the two major 

 events of the Idaho record, the extrusion of great masses of rhyolite and 

 basalt, were in general contemporaneous with the corresponding events 

 of the Yellowstone Park. It is to be noted that in the latter region some 

 basalt was extruded prior to the outflows of rhyolite. The deeply weath- 

 ered hornblende andesite porphyry of the Idaho record may correspond 

 with the early acid breccia of the YelloAvstone Park, which is described as 

 consisting mainly of hornblende andesite and hornblende' mica andesite. 

 The erosion and weathering which the Idaho andesite has experienced 

 perhaps favor this interpretation rather than that of a correspondence 

 with the late acid breccia of the park, which it doubtless also resembles. 



Origin of the igneous Eocks 



The simplest view of magmatic differentiation as applied to the igneous 

 rocks of this district is that these rocks were formed from an original 

 magma of intermediate composition, from which came first the horn- 

 blende andesite porphyry, and then by continued differentiation the series 

 of rhyolites and basalts. This conception would accord with the rule 

 stated by Iddings,'^ "that in any period of volcanic activity the earliest 

 eruptions are of rocks having an average or intermediate composition, 

 and that subsequent eruptions are of magmas with more and more diverse 

 compositions, the last eruptions producing the most diverse kinds." 



Lindgren,^ summarizing the extrusive activity of the Cordilleras, pre- 

 sents a different view of the problem. He divides the extrusives of the 

 Cordilleras into two groups, one of which "embraces the volcanoes of the 

 Sierra Kevada, the Cascades, innumerable vents in Nevada, the Yellow- 

 stone Park region, and the San Juan country, in southwestern Colorado. 



» U. S. Geological Survey atlas, Folio 30, 1896. 



■^ J. P. Iddings : Igneous rocks, vol. 1, p. 257. New York. 1000. 



8 Waldemar Lindgren : The igneous geology of the Cordilleras and its problems. Yale 

 University, Silliman Foundation, lOl.S. Problems of American geology, pp. 234-286, p. 

 285. New Haven, 1915. 



