CHAPTER XI. 
GEOLOGY OP THE SIERRAS SANTA SUSANNA AND MONICA. 
Geographical position of these ranges.—External form of the hills.—The two chains separated by conejo pass and 
SEMEE CREEK. — ANALOGY OF THE STRATA OF THE SIERRA SUSANNA WITH THOSE ON THE SAN BUENAVENTURA AT MATILIHAII.-CON¬ 
NEXION OF THE RANGE WITH THE CORDILLERAS.—‘SlERRA MONICA.- TrAPPEAN ROCKS.-SLOW DISINTEGRATION.—EFFECTS OF.- 
Absence of timber.— Contrast of semee plain with the valleys of encima and triumpiio.—Trachytic upheaves.— 
Traces of two different volcanic forces.—Trachytic rocks and augitic trap.—Enumeration and order of the sedi¬ 
mentary BEDS.—FOSSILIFEROUS SANDSTONES AND LIMESTONES.-LlST OF FOSSILS. — SIMILARITY OF THE SUSANNA AND MONICA 
ranges.—Occurrence of the asphaltic group in the latter range.—Extension into los angeles valley.—Thickness 
of the beds of that group.—Resume of the structure of these beds.—Probable thickness of the sedimentary strata. 
These hills occupy the triangular space comprised between the Santa Clara river, with its 
valley on the north, the ocean on the south, and the plains of Los Angeles and San Fernando 
to the east, embracing a district 40 miles long from west to east, and 26 miles from north to 
south. They constitute distinct ranges of hills, running in a direction nearly east and west, 
having a slight northeast trend. 
They present in outline a series of rugged angular crested hills, which run parallel and rise 
to an altitude of two to three thousand feet. As many as four of these ridges may be counted 
in the Sierra Susanna, which lie most northerly, and are first met in passing up the Santa Clara, 
against which river valley they abut their western edges. Each chain to the north lies a little 
east of its neighbor, and thus the Santa Clara valley runs northeast until the river finally winds 
behind the most northern ridge, and separates it from the Sierra Madre or Cordilleras, leaving 
a communication by a pass between the Santa Clara valley and the plains of San Fernando. 
The two sierras are separated from each other by the Conejo pass, a considerably elevated 
plain rising from the Santa Clara valley and passing in a line N. 30° E., enters the San Fernando 
valley at its northwest corner ; as the pass narrows it loses its plane character, and is cut 
across in places by ridges of plutonic rock. Down the slope of this pass rolls the Semee creek, 
a small stream which fertilizes the pass, and finds its way into the ocean at the eastern end of 
the Santa Clara valley. The ranch Las Posas lies upon this creek, at the point where it leaves 
the pass to flow through the valley below. To the north of the pass lie the Sierra Susanna; 
to the south, the Sierra Monica. 
The Sierra Susanna, where observed, was made up of the red and yellow sandstones, similar 
to those observed upon the Buenaventura river, crossing it 12 miles above the Mission. In the 
most southerly of the ranges the strata dip N.E. about 18° to 20°. No primary rock is observed 
in this sierra, the intrusive masses being amygdaloidal trachyte and felspathic porphyry rock. 
Only the southern slopes of these ranges were observed, those upon the north side not having 
been traversed. This range is connected at its southeast extremity with the Cordilleras by an 
upheaval of trappean rock, which, starting from these hills, runs northwest toward the shore, 
carrying upon its flanks the same sandstones, which may be traced from the Cordilleras to San 
Buenaventura. 
