ROCKS RELATED TO SHOSHONITES. 345 



chemically (analyses 3, 4, and 6), are quite unlike mineralogically. The 

 first has abundant phenocrysts of labradorite, olivine, and augite, Avhile 

 the second contains no olivine. The groundmass of the first contains 

 some brown mica which may be secondary, while that of the second con- 

 tains much mica that is primary. Other differences will appear when sev- 

 eral rocks which are closely related to shoshonites have been described. 

 The chemical analyses of three of these are appended to the table on 

 p. 340 — Nos. 9, 10, and 11. These rocks are not so different chemically 

 from normal basalts, except that alkalies are rather higher, and potash is 

 specially so. The alkalies are like those in analyses 1 to 5; magnesia is 

 higher than in shoshonites, and more nearly that of normal basalts. 



Analysis 9 is of a dike rock from near the head of Stinkingwater 

 River (1462). The rock is very dark gray, with long megascopic crystals 

 of hornblende, and some of augite, but none of feldspar. In thin section 

 it is holocrystalline. The groundmass is composed of lath-shaped plagio- 

 clase with low extinction angles and of cryptocrystalline material, probably 

 an alteration product, besides augite and magnetite, and considerable serpen- 

 tine, which is evidently the alteration product of small olivines whose origi- 

 nal form is still preserved. The phenocrysts are lai'ge hornblendes with 

 irregular outlines and a border of magnetite grains, and smaller augites. 

 There are none of feldspar. The rock might be called a hornblende-basalt, 

 with feldspar averaging about the composition of oligoclase. It is possible 

 that some orthoclase may exist in the groundmass, but it was not recog- 

 nized in the rock section. The shoshonite nearest to this rock in chemi- 

 cal composition is the lower sheet from the southeast fork of Beaverdam 

 Creek (1647), analyses 3. It differs from it mineralogically in not having 

 hornblende, in having large phenocrysts of labradorite, and in the presence 

 of orthoclase in the groundmass. 



The rock whose composition is given in analysis 10 is a basalt-like 

 lava, forming large blocks in the basic breccia on the ridge south of the 

 core of Crandall volcano (1325). It carries large phenocrysts of labradorite- 

 bytownite, and abundant ones of olivine and augite. The groundmass is 

 holocrystalline, and consists of irregularly lath-shaped feldspars that are 

 labradorite at the center and orthoclase in the margin. Orthoclase is in 

 subordinate amount. Augite, magnetite, and some serpentine, with delicate 

 needles of apatite, are the other constituents. . It is like the shoshonites 

 already described, but is richer in labradorite. 



