INLAND VALLEYS—SAN LUIS PASS. 
59 
the valley of San Luis Obispo ; the lowest of them, the first named, being nearly 800 feet above 
its neighbor to the west, the valley of San Luis. Hence this range, viewed from the west, appears 
a much more imposing mountain mass than when seen from the interior valleys. 
This increased altitude of the inner valleys named is not to be attributed to this volcanic range; 
its influence upon the valleys has been but slight, and perhaps only resulted in defining its limits 
on the west; the elevation must be attributed to the San Jose granite range, which has elevated 
the whole district east and west of itself; in the Santa Margarita, San Jose and Santa Maria, 
the dip of the strata under the level of the plain, wherever observed, has always been noticed 
as westward or toward the Santa Lucia range ; this direction being observed up to the very 
base of the latter chain, and the dip less abrupt than that of the volcanic range ; from which 
it would appear that the district was first upheaved by the San Jose range, and that subse¬ 
quently the trappean and trachytic eruption along the line of Santa Lucia elevated the sand¬ 
stones to a high angle, carrying up those in proximity to it, but not influencing the strata to 
any distance. The range is thus looked upon as produced by a volcanic outburst upon the 
western edge of the granitic slope, which view explains the sudden dropping down of the valley 
on the west, and the non-existence of granitic rock west of this chain. 
The lowest pass, indeed the only one known over this range, is that from Santa Margarita 
valley into San Luis Obispo valley ; the height of this pass is 1,350 feet, being 400 feet above 
the level of Santa Margarita and 1,200 feet above San Luis valley. In going through this pass, 
which is about six miles across, upon the summit the trappean rock is met with—an amygdaloid 
augitic variety—the cavities filled with carbonate of lime. A short distance east of this a brown¬ 
ish trachytic vein appears running westnorthwest. The serpentine conglomerate is found lying 
next to this and dipping southeast or toward the valley ; lying upon these conformably are \ he 
brown and yellow sandstone and fine grits and conglomerates similar to those upon the eastern 
edge of the valley Santa Margarita ; these rocks varied in dip from 15° in the lower part of the 
valley to 55° near the summit. On the west side of the trap occurs an immense mass of serpen¬ 
tine, compact, in places foliated and rising in a crested hill 1,000 to 1,200 feet above the level 
of the pass ; the descent is very abrupt, over masses of serpentine and sandstone, sloping 
steeply to west; the brown grits were observed on the west slope ; further south and high up 
on the top of a crest were observed the agatic layers and the beds of ostrea similar to those in 
the Santa Margarita valley. 
Near the summit on the east side a bed of calcareous tufa, of small local extent and not more 
than 20 feet thick, was observed ; on fracturing it some of the exposed portions exhibited the 
impressions of dicotyledonous leaves ; it was looked upon as of recent origin arising from the 
drainage from the calcareous traps. Observing that the cavities of the trap were found filled with 
crystals of calcite, search was made for the rock affording it; crystals of cromic iron abounded 
also in the trap. Between the amygdaloid and the brown trachyte a dark blue rock, which 
effervesced strongly, was met with—tinged greenish by intrusions of serpentine, veined with 
quartz and intersected by rhomboidal crystals of calcite—it was evidently a metamorphic sedi¬ 
mentary limestone; no traces of stratification were observable; only 20 feet of it was exposed, 
dipping at a high angle apparently to the west. No fossils were observable in the fractured 
surfaces. 
The party of survey crossed this range again, where the Santa Maria canons through it, to 
reach the Guadalupe Largo plain ; not being present, a geological section was not accurately 
made, but, from the observations kindly made by Mr. Campbell, the same volcanic rocks were 
