202 GEOLOGY. 
the adjoining rocks. These lens-shaped and elongated masses appear to differ from the enclosing 
rock merely by the greater amount of either hornblende or mica that they contain. They are, 
doubtless, the result of the action of the same forces that produced a more distinct and laminated 
Structure in other parts ofthe same range. They are, also, harder, and offer greater resistance 
to the disintegrating action of the atmosphere; they, consequently, stand out in relief from 
surfaces that have been exposed to weathering. This is distinctly shown on the great erratic 
masses that lie on the surface near the entrance to the Tejon. 
Some portions of the compact rock disintegrate rapidly, cracking up into soft and friable 
masses, that crumble readily when struck by the hammer, and fall into grains about the size of 
peas. This is more especially the case with the granite in which the crystals of mica and feld- 
spar predominate, and appears to be a separation between the constituent minerals rather than 
a division-into their particles. 
Tejon Pass.—The Tejon Pass is divided from the Tejon ravine by one ridge only, and the 
granitic exposures in each have a general similarity. The Tejon Creek, which rises at the 
summit of the Sierra, flows through the Pass at the base of the ridges on the south side. It 
here makes good exposures of the rock, while on its left bank, or north side, it is bounded b 
high banks of drift accumulations, in which it appears to have excavated its channel. This 
drift occupies a large area in the valley of the Pass, and is deeply furrowed and cut into ravines 
by the side streams that have descended from the high ridge on the north. As this drift 
completely obscured the rocks on the north side, they were not examined, and the following 
descriptions, as far as the summit, apply to the exposures on the south side of the Pass only. 
The trend of the planes of structure in the granite, at the point of rocks at the entrance to the 
Pass, (station 11, see Section 5,) is N. 48° W. by the needle, and their dip is nearly vertical. 
The rock is laminated and slaty, but quite hard, and the surface is rough and angular. Directly 
over the summit of this low projecting point the rocks are well exposed, and a narrow belt of a 
very finely laminated syenitic granite appears, which contains a large amount of small garnets, 
epidote, and granular quartz. The rock also contains black hornblende in such quantity as to 
give it a dark shade, which distinguishes it from that adjoining it on each side. All these 
minerals are arranged in numerous narrow seams, forming an exceedingly hard and compact 
rock, cleaving most readily in the direction of the structural planes, wnich are sufficiently 
numerous and regular to give a slaty appearance to the mass. 
The mass also breaks up readily in two directions nearly transverseto the planes of structure, 
producing triangular and rhombic masses, similar in their forms, and in the value of their 
angles. These were measured as accurately as possible, and found to be 110° and 70°. The 
trend of this rock is N. 25? W., (magnetic ;) its dip nearly vertical, inclining slightly eastward. 
Although the outcrop has a width of only twenty feet, I have deemed its interesting structure 
worthy of a minute description, especially as it is intercalated with rocks of a more micaceous 
and granular structure. Á 
A few rods further to the east the structural planes of the granite have a well-defined dip to 
the northeast of 70°, and the rock is traversed by nearly horizontal veins of feldspar, composed 
of an intermixture of beautiful white and reddish feldspar, with a thin central seam of epidote, 
and an occasional grain or crystal of quartz, or black mica. This vein was traced along 4 
vertical wall of granite for about one hundred and fifty feet. At several points it was disjointed, 
the ends being thinned down to a mere line, and overlapping each other. They were separated 
