ORIGIN OF THE IGNEOUS ROCKS 261 



All these yield predominant andesite with considerable rhyolite and minor 

 masses of basalt, and it seems fair to advance the hypothesis that they 

 are caused by explosive action from the magmas of older granodioritic or 

 quartz-monzonitic batholiths, which have had time to differentiate in their 

 upper gas-charged ''cupolas/ or from satellitic intrusions of such batho- 

 liths. Wherever we find local intrusions in such volcanoes they appear to 

 be of magma of intermediate composition." The other group includes 

 "the Columbia Eiver lavas, many fields in Nevada, and those of central 

 and eastern Arizona. These eruptions go over into the type of latest 

 Pliocene and of Quaternary age, in which only basalts were poured out. 

 It seems probable that these eruptions are not connected with the grano- 

 dioritic magmas, but are of more deep-seated origin." 



In the region here described the hornblende andesite porphyry is ap- 

 parently the oldest of the igneous rocks in place, and thus represents the 

 first products of the magmatic intrusion of the region, perhaps before any 

 significant difi'erentiation had taken place, if the simpler view implied by 

 the citation from Iddings is assumed, or it may represent a differentiation 

 product from a granodioritic magma such as those postulated by Lind- 

 gren. No effusion appears to have occurred in this immediate region, but 

 in the Fort Plall Indian Reservation considerable areas of andesitic tuff 

 show that there were actual volcanic outbursts not far away. 



The rhyolites and basalts may represent the products of further differ- 

 entiation of a magma of intermediate composition or a granodioritic 

 magma, but the basalts, according to Lindgren's view, more probably 

 come from a different and more deeply seated magmatic source. The 

 region has thus been underlain by at least one great body of rock magma 

 or possibly by an earlier and a later magma at different depths. From 

 these the igneous rocks here described have been derived. It is possible 

 that further intrusions or effusions may develop from the same source, 

 although present evidence points to the dying away of volcanism. 



Modes of Eruption 



generally effusive in character 



Most of the igneous rocks of the region are effusive, though a few dikes 

 and sills and an incipient laccolith have been noted. The hornblende 

 andesite porphyry in the Cranes Flat Quadrangle has been found only in 

 intrusive form. 



INTRUSION AT SUGARLOAF MOUNTAIN 



St. John" regards the intrusion of the andesite at Sugarloaf Mountain 



9 Orestes St. John : Report of the geological field-work in the Teton division. U. S. 

 Geol. and Geog. Survey Terr., 11th Ann. Rept., 1879, p. 356. 



XVIII— Bull. Geol. Soc. Am., Vol. 32, 1920 



