Vol. 52.] AND INTRUSIVE IGNEOUS ROCKS. 615 



tions, whose volumes may still be estimated in cubic miles. There 

 are besides many bodies of comparatively small size, both extrusive 

 and intrusive. To the first category belong the andesites of various 

 kinds, rhyolites and basalts ; to the second, the medium-grained 

 and coarse-grained granular rocks and porphyries ; while to the 

 third belong any of these, and the exceptional varieties such as 

 absarokites, shoshonites, and banakites. All are inseparable parts 

 of the volcanic history of the region in Tertiary times, and their 

 testimony as to the sequence of events is necessarily concordant. 

 From it we learn that the earliest eruptions were of magmas which, 

 as laccolites, became hornblende-mica-andesite-porphyry, and as ex- 

 trusive rocks became hornblende-mica-andesite ; with these magmas 

 were associated hornblende-andesites and porphyries, and some 

 that might be classed as dacite. Later there were great eruptions 

 of magmas solidifying as pyroxene-andesites of several kinds, or as 

 diorite, followed by those yielding andesitic basalt, and basalt or 

 gabbro. 



"We find at one volcano, that of Electric Peak and Sepulchre 

 Mountain, a succession of more and more siliceous magmas follow- 

 ing those of diorite and pyroxene-andesite, reaching dacite. In the 

 region of Emigrant Peak and Mill Creek numerous and large bodies 

 of dacitic porphyry, or acid andesitic porphyry, traverse the basic 

 andesitic breccia. At Crandall volcano there was first acid andesite, 

 then basic andesite and andesitic basalt. Then in the core of the 

 volcano we have a reversal of the series, after the basaltic magma had 

 crystallized as gabbro, yielding diorite and granite in small volumes. 

 But away from the core occur complementary basic dykes. In 

 these volcanoes we find evidence of a quite local differentiation, 

 yielding smaller volumes of magmas, becoming more and more 

 extreme, and furnishing an aplite in the core, and at Crandall 

 volcano the basic complements at a distance. 



There is a repetition of this history in the extrusive breccias 

 forming the southern half of the range. Then a long period of 

 rest, followed by sudden and violent eruptions on a scale almost 

 without parallel, producing two distinctly marked and contrasted 

 types of magma, rhyolite and basalt, whose combination would 

 reproduce a magma like that with which the first eruptions began. 

 Furthermore, while they closed the period of activity, the com- 

 positions of these magmas are not so extreme as are those of 

 some of the small bodies of magma that closed the eruptive activity 

 centred about some of the great volcanoes just mentioned. If they 

 represent the products of a differentiation, as I believe they do, 

 the differentiation was not so extreme, or possibly not of the same 

 kind, as that which took place at the base of one of the volcanoes. 



In the assemblage of activities that occurred in Tertiary time in 

 this region, there must have been processes of differentiation of 

 different orders ; some taking place in large reservoirs of magma, 

 others affecting smaller ones, some in deeply-seated magmas, others 

 in magmas nearer the earth's surface. The differentiation was 

 undoubtedly due to a disturbance of the physical equilibrium pre- 

 viously existing between the molecular constituents of the magma, 



