di SESS E ee 
PLEST M 
WHITE DYKES Qa NDSTONE, 21 1 
tional series of ridges coming to the surface opposite station 74, at the extreme eastern end of 
the Pass. 
This volcanic rock presents various shades of red and dark-green, and some portions of the 
dykes are distinctly laminated. Other parts are vesicular, and contain small masses of chal- 
cedony, often forming only a thin lining on the border of the cavity ; the inner surfaces being 
drusy with small and brilliant quartz crystals. Seams of white and translucent silica are also 
present. These quartzose nodules are the most abundant in the red rock. The dark brown 
and greenish portions of the dykes contain earthy nodules of a brilliant, dark, chrome-green 
color, and the same substance forms thin coatings upon the surfaces of small fissures. I regret 
that all the specimens from this vicinity were lost, as some of this peculiar green mineral was 
collected for examination. 
The direction or trend of these intruded rocks appeared to be nearly north 20? east, south 
20° west. An adjoining outcrop of sandstone strata trends north 45? east, and south 45° west; 
dip 54? to 55? towards the southeast. 
By reference to the map, it will be observed that the eastern part of the Pass, from the sum- 
mit to station 74, extends nearly parallel with, and skirts the foot hills of, the east and west 
range of mountains that divides the Great Basin from the valleys and slopes of San Fernando 
and Los Angeles. The hills on the right or south of the trail thus gradually rise to the 
elevated ridges of that range; while on the north the observer, after ascending about one hun- 
dred feet, stands on a wide and gentle slope, and has an unobstructed view of the Great Basin 
and the numerous barren ridges that rise from its broad surface like islands in the ocean. 
The outcrops of igneous rock and the uplifted sandstone strata may be regarded as pertaining 
to this transverse chain of heights—the Bernardino Sierra—rather than to the Sierra Nevada. 
The central axis of this chain is granitic and metamorphic, as shown by the examination of the 
Pass of San Francisquito and Williamson's Pass, both of which are represented on the little 
Geological Map. The sedimentary formations of the Cafiada, the Tertiary formations, and the 
more modern drift accumulations, are described in Chapter XIII and in the Itinerary. 
