IGNEOUS ROCKS OF THE SIERRA NEVADA. 411 
of analyses of Tertiary rocks is not a dacite. The quartzes are 
much corroded and surrounded with reaction rims of pyroxene 
microlites. They may perhaps be regarded as inclusions. The 
rock has a pilotaxitic structure, and seems related to the. fine- 
grained pyroxene-andesites. It forms a flat-topped hill on the 
ridge two miles northwest of Downieville. _ 
Following the andesite eruptions we have at many points 
flows of basalts. Some of these, as, for example, the coarse 
doleritic basalts of Mt. Ingalls in Plumas county, are believed 
to belong to early Pleistocene time, for the eruptions seem to 
have occurred after the present drainage system was partly 
formed. However, on the north side of the mountain glacial 
striae are to be seen on the lava, showing the flow to have taken 
place before the close of the Sierra Nevada glacial epoch. The 
doleritic basalt is represented in the table of analyses by Nos. 
311 and 314 Plumas. 
There are at many points isolated buttes and dikes of fine- 
grained basalts that do not resemble closely either the older 
basalt above described or the Pleistocene doleritic basalt. There 
is usually abundant olivine in these rocks, but their groundmass 
is much finer grained than that of the doleritic basalt. Their age 
is presumed to be Pleistocene. A dike of one of these rocks cuts 
the coarse andesite, and the underlying gravel of the point some- 
times, called Mt. Bina, one mile northeast of Mt; Fillmore 
(Downieville sheet) by the road to Johnsville, and similar dikes 
cut the Neocene fragmental material exposed on the southeast 
slope of Mt. Fillmore. 
To the north of the Sierra Nevada at Cinder Cone,’ near Las- 
sen Peak, and about Mono Lake? just to the east, as well as in the 
Coast Range to the west near Clear Lake,? craters exist that 
retain nearly or quite their original forms. These volcanoes are 
without doubt of late Pleistocene age. Within the Sierra Nevada, 
however, if the range be limited as suggested in the beginning of 
t Bull. No. 79, U. S. Geol. Survey, by J. S. Diller. 
2 Kighth An. Rep. Director U. S. Geol. Survey, pp. 378-389. 
3 Monograph XIII., U. S. Geol. Survey, p. 246. 
