210 GEOLOGY. 
drift, that the character of the rock is obscured, and a rounded and smooth outline is given to it. 
It forms a projecting point, around which the valley of the Pass bends, as will be seen on the 
map. The granitic character of the underlying rock is shown by several limited outcrops along 
the base of the ridge. At a point on the southern side, the rock appears to be composed of a 
simple mixture of quartz and feldspar, or albite, in fine grains, without mica. The mass much 
resembles some compact or metamorphic sandstones; it appears to decompose rapidly, crumbling 
to a white powder, which mingles with the debris of the overlying sediments. A short distance 
further west, and near the summit, but at the southern foot of this granitic ridge, (between 38 
and 40 on the section,) we find a third series of narrow ridges of white limestone. They are 
accompanied by trappean dykes, with a nearly east and west trend. These give rise to a series 
of small, local valleys, parallel with the outcrops. A vein of iron-ore is intercalated in one of 
these limestone ridges. 
SECTION NORTH AND SOUTH NEAR THE SUMMIT. 
d o 1 t g ۱ 
g. Granitic. t. Trap, or igneous rock. l. White limestone. o. Bed ofiron ore. d. Drift deposits and Tertiary. 
These outerops and the dykes of trap, with the exposed surface of the adjoining granite, all 
trend nearly east and west, and thus are transverse to the line of the section. They are, 
therefore, represented upon it near station 38, or at the extreme southern part of the ridge, 
this being the point at which they would appear if prolonged in their line of trend. It is 
probable that another series of beds of limestone occurs on the north or opposite side of the 
ridge fronting on Casteca Lake. If so, there would be scarcely a doubt of the identity of these 
formations, in respect to age and metamorphism, with the limestones of the eastern slope of the 
mountains at the Tejon Pass. "The three outcrops described, trending parallel with each other, 
and separated by granitic or metamorphic rocks, taken in connexion with the similarity of the 
adjoining granite to that of the Tejon, are, however, sufficient to justify the conclusion, that 
the formations are synchronous and similarly plicated. They are, in all probability, continuous 
along the eastern slope of the mountains, from one pass to the other. 
The intrusive rocks which appear in connexion with the outcrops of limestone on the south 
side, or Great Basin slope of the ridge, are continuous towards the east on the north side of the 
valley of the Pass, among the sandstone formations ; being best defined between stations 40 and 
48, near the little ponds or lagoons.—(See 40 on the Section.) These dykes are of a dark color, 
very compact, and slightly porphyritic. Some peculiar rocks were observed in connexion with 
them, which appear to be the granite in a metamorphic condition. . It appeared to have n 
fused, so as to produce an intimate mixture of the minerals, and to have cooled without taking 
a crystalline structure. 
This part of the Pass may be considered as the boundary or foot of the Sierra on the side of 
the Basin; the remaining extension of the valley, until it finally opens out upon the broad 
slope of the Great Basin, (see Geological Map,) may be regarded as a hollow, or valley of denu- 
dation, in the sandstones and hills of drift that form the upper and higher parts of the great 
slope. The hills on each side are low; but the strata of the sediments are not horizontal, 
having been uplifted by the intrusion of the volcanic rocks already mentioned, and by an addi- 
