H. W. Turner — Lavas of Mt. Ingalls, California. 457 



of Chico Creek in Butte county. So far as I am aware, this 

 dense black older basalt is found only in Plumas and Butte 

 counties. At numerous points it rests on Tertiary sediments, 

 chiefly river gravels. It has nowhere been seen by the writer 

 overlying or cutting through other Tertiary lavas. Its rela- 

 tion to the andesite (II) is very evident about four miles east- 

 southeast of Mount Ingalls on the ridge to the east of the 

 road from Red Clover Valley to Genesee Valley. Here a 

 nearly horizontal bluff of andesite above five hundred feet 

 vertically above the road may be seen resting on the older 

 basalt (III). A vertical section here would be approximately 

 as follows : 



1. Andesitic breccia, thirty feet in thickness. This is made 

 up of angular fragments of andesite cemented by volcanic ash. 

 It shows no evidence of stratification. A little to the east of 

 the bluff, lying on this breccia and presumably weathered 

 from it, were found fragments of silicified wood. 



2. Fine even-grained andesitic tuff, one to four feet thick. 

 The tuff layer is approximately horizontal. It is made up of 

 minute fragments of plagioclase, augite, hypersthene, green 

 hornblende, and apparently some quartz, with opaque and dis- 

 colored particles. It is very plainly fragmental. It contains 

 also fragments of plant stems. 



3. Andesitic conglomerate, fifteen feet thick. Immediately 

 under the tuff and plainly resting on the older basalt is a vol- 

 canic conglomerate containing some fragments of the older 

 basalt as well as granite pebbles but chiefly made up of peb- 

 bles and fragments of andesite. 



4. Older basalt, five hundred feet thick. This reaches from 

 the bluff of the above fragmental materials 1, 2, and 3, down 

 to Red Clover Creek below, a vertical distance of about five 

 hundred feet. 



To the observer it is very manifest at this locality that the 

 above series of volcanic fragmental rocks are of later date 

 than the underlying basalt. The great thickness and extent 

 of the basalt and the thinness of the overlying andesite ren- 

 ders it extremely unlikely that it is an intruded sheet. More- 

 over, the same superposition of the andesite on the older 

 basalt may be noted at other points, as for example, at the 

 southwest base of Mount Ingalls, at Iron Canon in Butte 

 county, and on the ridges to the north and south of Onion 

 Valley Creek, in Plumas county. It is most probable that a 

 period of erosion occurred between the time of the older basalt 

 flows and that of the andesitic eruptions. 



IV. Rhyolite. — A small area of this rock was noted four 

 miles to the southeast of Mount Ingalls, to the west of Red 

 Clover Valley. The exposure is surrounded by andesitic 



