70 
GEOLOGY. 
lower parts of the Pass of San Francisquito. The colors presented, were varying shades of gray, 
red, and brown ; the materials were coarse, and, so far as I observed, were not accompanied by 
beds of shale. 
The series attains a great thickness, hut it could not he determined with any accuracy. The 
uplifts and disturbances appear to he produced by the dyke or ridge of intruded porphyritic 
rock, which extends from the summit nearly half way down to the plains, hut appears to be 
broken in several places. For a part of the way it was hidden from the trail by intervening 
hills of granite ; but there were good evidences of the presence of the rock. The representation 
of the outcrops on the map is presented as an approximation to their position and relative mag¬ 
nitude. The true direction and extent of the ridges can only he ascertained by a survey for the 
purpose. It is very probable that several intrusions would be found. A short distance below 
the end of the ridge of erupted rock, a long valley was seen extending off towards the east, and 
the dry bed of a stream, very broad, and lined with large boulders, indicated the existence of 
a powerful current there at some seasons of the year. The rocks thus transported along this 
creek were mostly granitic and metamorphic, much white granite being found. I picked up 
several masses, which had a delicate purple or lilac tint, produced by the feldspar. This, how¬ 
ever, was a syenite, no mica being visible, hut an abundance of hornblende of an olive-green 
color. The crystals were so disposed throughout the rock that the surface looked as if it had 
been written upon. The rock in fact is a beautiful graphic syenite. The appearance of the 
surface of one of the pebbles is shown in the annexed figure, the black portions representing 
the hornblende, and the light the feldspar. The latter is also crystalline, and apparently the 
cause of the peculiar form of the masses of hornblende. Neither mica nor quartz were observed 
