76 
STRUCTURE OF THE SIERRA MONICA. 
Sierra Monica .—These hills lying parallel with the road were better observed ; like the fore¬ 
going, they are made up of parallel ranges running easterly, and rising to a considerable height 
in the centre of the range. The volcanic rocks have there elevated the whole mass of the sur¬ 
face considerably, so that even the valleys are several hundred feet above the Santa Clara river. 
Semee plain, which lies at one of the most elevated points of this region, having an altitude 
of 1,400 feet, is a small valley, surrrounded by igneous rocks, which rise a few hundred feet, 
forming bounding walls. The buttes, which lie east and west, are a reddish felspathic rock 
full of amygdaloidal cavities, some of which are encrusted with a yellow zeolitic powder. It is 
not unusually rough to the touch, but is slow of decomposition, forming a thin soil, which does 
not remain attached to the hill, but is washed down, leaving the sides of the butte naked, and 
incapable of supporting any growth of timber. The absence of trees of any kind upon these 
hills strikes the observer at once as a phenomenon very remarkable, where a few miles travel 
could present him with luxuriant growths ; as the hills are unclad, so of course are the vallies, 
the latitude being too low for arborescent growths, and thus an air of perfect sterility crowns 
the whole; while, a few miles south, lying upon the calcareous sandstones, the small valleys of 
the ranches Encima and Triumpho are covered with evergreen oak, loaded with the hanging 
ramalina, and growing on rounded knolls well covered with grass. South of these valleys a 
high range runs east and west, cutting them off from the coast, which may be reached by deep 
cuts or drops in the range, the centre or axis of which is the red felspathic amygdaloid alluded 
to—a trachytic rock ; this upheave tilts the strata to the northwest, (between Camps 34 and 35,) 
which is the general dip of all the sedimentary rocks of the vicinity, save where they are again 
broken by dykes of augitic rock. There appear to be two volcanic forces at work in these hills: 
1st. The trachytic rocks, including the amygdaloid and the compact red felspathic variety, 
which encloses small angular crystals of quartz and glassy felspar. These occupy both the 
small isolated buttes on the plain as well as the centres of the lofty ridges; and 2d, augitic trap, 
which does not form an axial rock, but partially uplifts, and cutting through the strata forms 
dykes of more or less thickness ; the former of these volcanic rocks runs from east to west, (N. 
80° W., S. 80° E.,) the other more north and south; thus they cut across and intersect each 
other, and contort the strata considerably. 
The sedimentary rocks are brownish, white, and greenish sandstones, including calcareous 
layers, which are highly fossiliferous. The arenaceous beds of the sandstones are poor in fos¬ 
sils where examined, containing mytilus Inezensis, and are either a felspathic sandstone, with 
angular quartz particles, or they are a soft greenish sand rock, similar to that which is every¬ 
where along shore associated with the bitumen. 
The order in which these beds are exposed are north of Semee plain, reckoning from above 
downward: 
1st. Reddish sand rock, 100 feet exposed; 2d. Whitish grit, 80 feet exposed; 3d. Yellow 
sandstone, with a calcareous layer 14 feet thick,150 feet exposed. This last rock is in contact 
with an augitic dyke; in the calcareous layer are found well preserved natica, cardium, 
and turritella. 
The foregoing strata have a general dip to the north, which, in the majority of observations, 
did not exceed 15°. 
In passing from Camp 34 to 35 the road lay over some elevated strata which formed the 
divort between the plain Semee and the ranch Triumpho. The capping rock of this locality 
is a soft brown sandstone grit, which readily decays, forming a quartzose sand, which accumu- 
