H. W. Turner — Lavas of Mt. Lngalls, California. 455 



Art. LX. — The Lavas of Mount Ingalls, California; by 

 H. W. Turner, Washington, D. C. 



[By permission of the Director of the IT. S. Geological Survey.] 



Mount Ingalls is situated just south of the fortieth parallel 

 in Plumas county, California. It forms a portion of the 

 eastern crest of the Sierra Nevada. Its elevation is 8,484 feet 

 above sea level, according to the determinations of the topo- 

 graphers of the U. S. Geological Survey. 



Mount Ingalls was a center of volcanic activity during much 

 of later Tertiary time. The lavas which originated in its 

 vicinity form an interesting series. The following rocks are 

 represented : 



I. Late basalt, a rather coarse gray rock with much pyrox- 

 ene and some olivine. It is usually massive. 



II. Andesite, containing both pyroxene and hornblende, 

 and occurring chiefly as a breccia. 



III. Older basalt, a dense black rock with little olivine, and 

 a good deal of magnetite. It seems always massive. 



IV. Rhyolite, in very small amount. 



The order of succession is that indicated by the Roman 

 numerals with the exception of rhyolite (IV.) As has been 

 previously noted* the rhyolite of the Sierra Nevada underlies 

 the andesites, but its relation to the older basalt (III) is un- 

 determined. The rocks may be roughly characterized as fol- 

 lows : 



I. Late basalt. — This is macroscopically a coarse to medium 

 grained light gray rock, sometimes pinkish, with large scattered 

 olivines. Under the microscope it is seen to be nearly holocrys- 

 talline and to be composed of lath-like plagioclase, augite and 

 magnetite, with occasional large olivines. One of the slides 

 shows a hypocrystalline glassy base containing abundant grains 

 of magnetite, with numerous phenocrysts of plagioclase and aug- 

 ite, and a few hypersthenes and olivines. In general, however, 

 very little glass is to be seen. 



The late basalt is the most recent of the lavas of this 

 portion of the Sierra Nevada. It forms the summit and nearly 

 all of the sides of Mount Ingalls down to and in places below 

 the 7,000 foot contour. It rests, to a large extent, on andesitic 

 breccia. Similar basalt caps a number of the higher points to 

 the south and southwest of Mount Ingalls. Two of these are 

 Mount Jackson and Penman Peak. There also the basalt rests 

 on andesite The later basalt is almost everywhere a massive 



* Mohawk Lake Beds, Bull. Wash. Phil. Soc, vol. xi, p. 389. 



