314 The American Geologist. May, 1894 
The dolerite of Grizzly peak in the slide examined contains no olivine 
and comparatively little pyroxene. In the slides examined of the dole- 
cite of Penman peak and other areas near thai poinl there appears to be 
lid hypersthene, but olivine in more abundanl than at Mount [ngalls. 
Other late Basalts. — Occurring usually in small areas are dikes and 
Hows ofbasalts, some of them probably Pleistocene in aye. as lor exam- 
ple the columnar dike-like occurrence at Lava peak on the Downieville 
sheet where the basalt appears to have cut through the Neocene gravel 
on the ridge about the peak. The aye of the little buttes of columnar 
basalt, and the isolated flows, relative to the other Tertian lavas, cannot 
usually be determined, but thej are doubtless all of them late Neocene 
or Pleistocene. The basalts here referred to always carry an abuu- 
da rice of oli\ ine. 
Analyses ok Basalts. 
Older basalt. Tuolumne Dolerite. 
Table mountain. 
No. 276, N<>. 36, No. 311, 
Plumas county. Sierra Nevada. Plumas county. 
Silica 50.5G 56.19 53.91 
Id me 7.58 <'>..->:; 10.40 
Iron oxides 12.44 I.".':; 7.01 
Magnesia 1.07 :>.:u 5-. 52 
Potassa 2.10 1. 16 1.34 
Soda 2.94 2.53 2.90 
The Succession of the Tertiary Volcanic Hocks. 
There is a considerable variety of Tertiary volcanic rocks in 
the Sierra Nevada, but there are only a few general types 
that have an extensive distribution. The oldest Of the series 
are perhaps the hornblende-mica-andesites, which occur in 
small areas on the Downieville sheet and elsewhere, but the 
earliest flows of which the relative age is definitely known are 
of rhyolite and the older basalt. 
Of these two older rocks the rhyolite has much the wider 
distribution, it being Cound throughout the Gold belt. The 
basalt referred to above as the older basalt is, so far as known 
to the writer, confined to Butte and Plumas counties. That 
the rhyolite and older basalt of the Sierra Nevada in general 
underlie the hornblende-pyroxene-andesite, which, in the form 
of a breccia, covers so much of the range, has been noted in 
two previous papers.* It appears, however, that the writer 
was not the first to have called attention to these relations. 
In a report made to Prof. J. D. Whitney in 1879, Prof. W. II. 
Petteef notes that the older basalt was overlain by andesite, 
*Bull. Phil. Soc. vol. «.). p. 389; and Am. .lour. Sci.. vol. 44. pp. 455- 
159. 
f Auriferous Gravels, p. 468. 
