16 
GEOLOGY. 
From the hill at Bear creek to the Mariposa liver the road passes over the border of the 
metamorpliic rocks, and an occasional quartz vein is seen. The earth is also more or less 
colored by oxide of iron, and fragments of quartz are numerous. These indicate the presence of 
gold. Bounded hills, with flat caps of sandstones, continue in view. They are of various altitudes, 
hut below 200 feet. 
We arrived at camp on the Little Mariposa about six in the afternoon of the 22d ; the ther¬ 
mometer standing at 96 degrees in the shade, and a breeze blowing. The heat of the sun 
during the day on the broad, open plain was intense and oppressive. The bed of the river is 
now dry, except in the deep and shaded places. 
Mariposa river to the Fresno river , July 23, 22.5 miles .—We left camp at 2.30 a. m., and 
about five miles south of it passed upon granitic or metamorphic rocks, both compact and 
gneissose, containing beds of mica and hornblende slates. These have a trend of N. 60° W., 
dip 75° N.E. The granite, at a short distance, resembles syenite, but it is composed of feldspar 
and mica, with some quartz. It decomposes rapidly into a granular mass. The beds of a slaty 
character were very thin, being not over one foot in width; narrow and irregular quartz veins 
were also observed. 
Choivchillas river .—The denuding action of rivers, and their power to cut a bed downwards 
through the sedimentary rocks, is well shown on the Chowchillas on the left side of the ford. At 
that place, hills with broad, flat tops rise abruptly to an equal elevation on each side of the stream. 
SECTION AT THE CROSSING OF THE CHOWCHILLAS. 
They are peculiarly interesting, as they contain a bed of conglomerate fifteen feet thick; of 
pebbles and round masses of white quartz , some of them twelve inches in diameter. They are 
all rounded and smooth, showing that they have been much rolled and waterworn. This 
stratum forms the protecting layer on the tops of the hills, the pebbles being firmly cemented 
together, and offering resistance to decomposition. The softer sandstones underlying it are 
more rapidly worn away, and allow large masses of the conglomerate to fall off and roll down 
the slope. 
Fragments of slate-rock, enclosing large numbers of crystals of andalusite, were intermingled 
with the quartz ; and detached crystals of this interesting mineral, of unusual size, were abund¬ 
ant. 1 The cementing material of the mass was sand and sesqui-oxide of iron, the latter being 
in quantity sufficient to give a dark color to the matrix. 
This peculiar conglomerate, together with the contiguous beds of sandstone, project from the 
tops of the hills at the same height on both sides of the river; rendering it strikingly evident, 
even to the most ordinary observer, that the strata were once continuous. Similar phenomena 
are presented all along the slope of the Sierra Nevada, wherever the streams have cut through 
the horizontal sedimentary sandstones. 
At the point where we crossed the Chowchillas, below the hills just described, its banks are 
low and sandy, and a broad but shallow current was running over a bed of fine sand filled with 
small glittering crystals of brown mica. There was very little river-drift of rounded stones or 
1 These crystals of andalusite are described in Chapter XX. 
