456 II. W. Turner — Lavas of Mt. Ingalls, California. 



rock, but a small amount of basalt-breccia and scoriaceous 

 basalt forms two points on the summit of Mount Ingalls, per- 

 haps remains of a former crater. In the ravine that heads just 

 east of the summit of Mount Ingalls the basalt is glaciated. 

 Its age, therefore, antedates that of the glaciers of the Sierra 

 ISTevacja. 



II. Andesite. — This is chiefly a breccia. The angular frag- 

 ments of andesite from this breccia are usually coarsely crys- 

 talline and frequently vesicular. The color is dark gray, the 

 feldspars showing as white spots. Frequently hornblende 

 needles are macroscopically visible. 



Under the microscope the andesite is composed of pheno- 

 crysts of plagioclase and pyroxene and of occasional horn- 

 blendes in a groundmass sometimes very glassy, sometimes 

 hypocrystalline with abundant plagioclase microlites and 

 usually considerable magnetite. The pyroxene in some cases 

 is largely. augite ; but there is frequently a good deal of hyper- 

 sthene. Sometimes the rock is nearly holocrystalline. In some 

 dark compact specimens of the andesite hypersthene is the 

 most abundant bisilicate, but the prevailing type is the coarser 

 one above described, and usually the predominant bisilicates 

 are augite and hornblende. 



Andesitic breccia forms most of the ridge that extends 

 southeasterly from Mount Ingalls. The same area continues 

 to the north of Red Clover Yalley. 



III. Older basalt. — This is a dense black rock usually show- 

 ing macroscopically no porphyritic constituents except occa- 

 sional olivines. It is frequently roughly columnar, the prisms 

 being small, seldom more than three or four inches in diame- 

 ter. The rock has been nowhere seen by the writer except in 

 the massive form. 



Microscopically the rock is composed of lath-like plagioclase 

 with more or less olivine and some augite in a groundmass 

 which is rendered dark by the abundant magnetite. The 

 groundmass sometimes contains considerable glass and fre- 

 quently specks and minute prisms of augite. Much of the 

 olivine is altered to serpentine. The plagioclases are usually 

 small and of nearly even size. One slide, however, shows 

 porphyritic plagioclases and olivines in a groundmass of plagi- 

 oclase microlites, magnetite and glass. 



The older basalt forms an area of several square miles to 

 the east of Mount Ingalls and west of Red Clover Yalley. 

 There is a smaller area at the southwest base of Mount Ingalls 

 east of Little Grizzly Greek, and also another just southwest 

 of the last and on the west side of Little Grizzly Creek. It is 

 the lava of Walker Plains, in Plumas county, and of Moore- 

 ville Ridge and Oroville Table Mountain, and the Iron Caiion 



