Geological Notes on the sierra X inula. — Turner. 315 
and Mr. W. A. Goodyear* reports that the white lava (rhyo- 
lite) always underlies the gray lava (andesite). 
Mr. Dillerf has described some fragmental volcanic beds 
which cover large areas in Butte county as the Tuscan form- 
ation. This tuffaceous formation consists in part of breccia 
and resembles, in a general way. the andesite breccia area- 
further south, but it appears t<» consist in part of basaltic ma- 
terial of the type of the later basalt or dolerite. The exact 
relation of the Tuscan formation to the andesite-breccia areas 
of the Gold belt therefore remains t<> be determined. 
The relations of the older basalt to this tuff is shown in the 
Iron canyon of Chieo creek, where the older basalt (No. 1. 
Butte county ) rests directly on Chico sandstone, and underlies 
the Tuscan formation. 
1 also obtained confirmation of this relation by finding peb- 
ples of the older basalt under the tuff at two points, one (No. 
15, Butte county) from a tunnel about three miles southwest 
of Ximshew, and another (No. 52. Butte county) from a little 
butte about one mile west of Pentz post office. 
The relation of the later basalt or dolerite to the andesite is 
evident on Mount Ingalls. It is plainly later than the an- 
desite. It forms the summits and slopes of Mount Ingalls 
down to, and in places below, the 7,000 foot contour, and evi- 
dently poured forth from the volcano as a molten flood of lava 
covering the andesite breccia and other rocks. The dolerite 
may be seen capping the andesite on Penman peak. Mount 
Jackson, Big hill about three miles northwest of Mohawk val- 
ley, on the- ridge one-half mile northwest of Hell's bar, and at 
other points. 
The flow of dolerite one mile southeast of Penman peak ap- 
pears to have taken place after the andesite areas had been 
much eroded, and the slopes of the mountains were approxi- 
mately as they arc to-day. suggesting that the age of the dol- 
erite flows is post-Tertiary. 
Since many of the mortars found among the gravels of Cal- 
ifornia, which have led to so much discussion as to the 
age of man in that part of the world, are of volcanic rocks, it 
* Auriferous Gravels, p. 522. 
fDescriptive text, Lassen peak sheet. 
