IGNEOUS ROCKS OF THE SIERRA NEVADA. 387 
Basin depressed (or perhaps movements in both directions have 
occurred) in Tertiary time. 
The Sierra Nevada, as thus outlined, appears-to constitute a 
block of the earth’s crust that has been practically rigid since 
middle Cretaceous time, although it has since, in common with 
most of California, experienced a considerable elevation, and 
there has been displacement by normal faults within the mass, 
particularly in Plumas county, at the north end of the range. } 
In another respect also the Sierra may be considered a geo- 
logic unit. The Tertiary lavas throughout the mass that have 
the widest distribution are very similar at distant points, and 
unlike Tertiary lavas in other adjacent areas, especially as to 
their form of occurrence and their relation in time. Thus the 
oldest flows of large extent were of rhyolite, succeeded by horn- 
blende-pyroxene-andesite, chiefly in the form of tuff and breccia. 
This relation does not appear to hold good in the Lassen Peak 
region or in the Great Basin. The sediments of which the Sierra 
Nevada are in part composed are presumed to have been derived 
chiefly from an Archean land mass lying west of the central part 
of the state of Nevada, although it is by no means impossible 
that an Archean land area once existed on the site of the present 
Coast Range, and in that case part of the sediments may have 
been derived from the West. It is probable that there are within 
the Sierra Nevada formations ranging in age from Archean or 
Algonkian to Recent. Excepting, however, some Silurian fos- 
sils at the north end of the range, no evidence has thus far been 
found of rocks older than the Carboniferous, although in the 
northwest extension of the Auriferous Slate series of the Sierra 
Nevada in Siskiyou and Shasta counties, Diller and Fairbanks 
have collected Devonian fossils. On the geologic map (Plate 
VII.) only the older formations, constituting the Auriferous Slate 
series with the associated igneous rocks, are represented. On 
this map the Paleozoic corresponds with the Calaveras formation 
of the Gold Belt maps; the two narrow belts of Jura-Trias, on 
the west slope of the range, are the Mariposa beds; the Jura- 
Trias beds northwest of Lake Tahoe are the Sailor-Canyon and 
