Geology and Mineralogy. 389 



Porphyritic cliorite and Earlier-Hornblende-andesite. All shades 

 of gradations between these two rocks occur. Transitions from 

 glassy varieties to the most crystalline are well seen about Mt. 

 Davidson and elsewhere. A comparison of sixty thin sections 

 failed to make out any distinguishing features. Some of these 

 Earlier Hornblende-andesites are glass-bearing. 



Mica-diorite and Later Hornblende-andesite. For the latter 

 rock the name hornblende-mica-andesite is stated to be better, as 

 the mica is an essential ingredient, and distinguishes it from the 

 earlier-hornblende-andesite. But from the varieties of mica- 

 diorite it is indistinguishable ; there is great variation in macro- 

 scopic aspect and this makes more forcible "the absolute identity 

 of the two rocks." The coarsest variety of the mica-bearing 

 diorite is " thoroughly granitedike in all its mineral constituents" 

 while at the other end of the scale of gradations there are kinds 

 having a glassy ground-mass crowded with microlites, and these 

 are the surface kinds. 



Quartz-porphyry, Dacite and Mhyolite. These rocks were 

 made pre-Tertiary by von Richtofen and Becker, and Tertiary by 

 Mr. Clarence King ; and the latter conclusion is that of Messrs. 

 Hague and Iddings. They are for the most part rich in crystals 

 of quartz, feldspar and mica, with some hornblende, but the 

 dacite contains less quartz and less hornblende. The examination 

 leads to the conclusion, that while there are great variations in 

 texture, color and density, there exist no petrographic distinctions 

 between them. 



Younger-Diabase, Black Dike, Basalt. The younger- diabase, 

 a pre-Tertiary rock, occurs as a narrow dike for more than a mile 

 along the lower levels of the Washoe mines ; its upper part makes 

 the Black Dike. A comparison of thin sections of the surface 

 basalt and the diabase shows that there are no petrographic dis- 

 tinctions between these olivine-bearing rocks. 



The comparisons of these various rocks as regards chemical 

 composition — using analyses in Becker's work, with three new 

 ones — sustains the close similarity of the augite-andesites, horn- 

 blende-andesites and hornblende-mica-andesites. The silica in 

 augite-andesites and hornblende-andesites varies most in per- 

 centage, but only from 56*40 to 60*82. The hornblende-mica- 

 andesites and mica-andesites have 63*30 to 65-68 per cent of silica. 

 Silica determinations of five pyroxene-andesites and hornblende- 

 andesites from other localities (near the base of Mt. Davidson), 

 give for the silica 55*66 to 57*06 per cent, showing remarkable 

 uniformity. 



The authors conclude : 



That the degree of crystallization developed in igneous rocks 

 is mainly dependent on the conditions of heat and pressure under 

 which the mass has cooled, and is independent of geological time. 

 The same magma has given origin to the most glassy and the 

 most granite-like kinds, the most modern-looking and the most 

 ancient-looking, the glassy kinds occurring at or near the surface, 



