GRANITE—-TERTIARY STRATA. 
31 
was evidently not far below us, as we several times crossed limited exposures of its surface rising 
up in the valley among the sedimentary hills. 
All these sediments rest immediately upon the granite, and are generally horizontal, or dip¬ 
ping gently westward. 
The preceding sketch was taken from the top of a hill of granite at the head of a small valley 
hounded by the rounded hills. At its lower end the nearly horizontal strata were exposed in a 
series of beds of sandstone. 
The streams that traverse this region have excavated deep channels in these nearly horizontal 
formations, producing good natural sections of the stratification. 
One of these vertical exposures of the sedimentary beds consists of white argillaceous sand¬ 
stone in a soft, earthy state, filled with small grains of white quartz, apparently derived from 
the decomposition of feldspatliic granite. The mass emits an argillaceous odor when breathed 
upon ; and some portions of the beds resemble the crude Chinese kaolin. These beds are near 
the underlying granite. Other exposures in the hill-sides displayed strata of similar materials, 
and an occasional admixture with layers of rounded pebbles. One of the outcrops presented a 
section in which the underlying granite was visible, as is shown in the little figure at the end 
of the chapter, which will also serve to give a general idea of the character of the beds of the 
small ravines along the borders of this tertiary, sedimentary formation, where the granite is 
near the surface. 
Sedimentary hills, of the character previously described, bounded our trail for several miles 
before we reached camp on Pose, or Ocoya creek. Many of these hills are so steep that it was 
difficult to get the wagons of the train over them ; but the absence of any vegetation, or of out¬ 
cropping rocks, permitted the ascent to be made, in many cases, by winding around the slopes 
up the side ravines. The great number of these narrow valleys or ravines that we were obliged 
to cross rendered the journey exceedingly irksome and fatiguing. Occasionally in the hill-sides 
the edges of some of the harder strata were visible, but the degradation and wearing away of 
the formation has generally left a uniform coating of debris or soil, which gives a rounded con¬ 
tour to the hills, and covers the strata from view. 
