318 The American (jeol agist. Nov.mber, i89?. 
when they are all made, can be bound together separately in ronn of aa 
atlas with briefer accomi)anying descriptions, if desired. In other 
words we should make tlu i-ouniy the unit of mapping, just as it is the 
civil and geographic unit, and if the scales differ it is of no moment, 
since generally, especially in all western states, the townships are cut 
by section lines which on the face of the map constitutes a Muih- of miles 
for every map. Thus the map is always handy and will last as long as 
the book itself. 
Stratigraphy ((nil Succession of the Hocks of the Sierra Necada of Cali- 
fornia. By James E. Mills. Bulletin of the Geological Society of 
America, Vol. iii, pp. 418-444, with map; Aug. 8, 1893. This description 
of the northern portion of the Sierra Nevada between the North and 
Middle forks of Feather river is a summary of the author's observations 
during many years as a mining engineer. The accompanying map 
shows the rock formations on the greater part of an area of six town- 
ships from east to west and four from north to south, lying in Plumas 
county, immediately southwest of the Taylorville district and Mt. Jura, 
which have been described in the same volume by Diller and Hyatt, as 
noticed in the September Geologist (p. 183). In this northern part of 
its extent the Sierra is double, consisting of eastern and wes'ern divis- 
ions, the latter being in the course of continuation of the axis of the 
Cascade range, to which it is nearly related by its geologic features. 
Barometric measurements in the district mapped give the hight of 
Grizzly ridge, in the eastern division, as 7,!»52 feet above the sea; of 
Spanish peak, the culminating point of the western division, 6,990 feet; 
and of an intermediate mountain called C'laremont, 0,962 feet. Con- 
tiguous valleys are between 3,000 and 4,000 feet. The northern end of 
the Sierra Nevada rises thus only about half as high asMt. Whitney and 
other peaks near its southern end. 
The present altitude of this mountain range is ascribed principally to 
Tertiary and Quaternary uplifts by faulting; but the axes of greatest 
uplifting coincide approximately with those of previous ranges within 
the same area. "Repeated orographic movements have taken place 
along the same axes, and recurring uplifts along these axes have fol- 
lowed recurring erosion. In this way a pre-Meso/oic range arose, carry- 
ing up both crystalline and metamorphosed sedimentary rocks, and par- 
tially disappeared through erosion and subsidence; then a Mesozoic 
range arose and its strata became uptilted, and it in turn was reduced 
by erosion and subsidence to very small proportions (in its northern 
half, at least, nearly or ([uite to base-level of erosion); and then in Ter- 
tiary and (iuaternaiy time has arisen the present range, which is now 
undergoing its erosion, but whether it is now rising or subsiding is not 
determined." 
Granites form the greater part of the pre-Mesozoic rocks of the Sierra 
Nevada, making up the core of the range and of each of its two north- 
ern divisions; and they are overlain by metamorphic slates and (|uart/.- 
ites, which may represent several Paleozoic periods. 
