98 
STRUCTURE OF CAJON PASS. 
extremity, the greatest elevation lying north is occupied by the sedimentary beds on the north end 
upraised by the axis. 
In ascending the pass from the Pacific slope primitive rocks only occur along the first three 
miles. The depressed axis of the Kikal Mungo hills is there met with at an elevation not 
exceeding 2,000 feet, consisting of a nucleus of granitic porphyry, hornblende, and mica slate, 
intercalated with beds of crystalline felspathic rock, gneiss, and talcose slate. 
The stream which flows down the pass (Cajon creek) winds its way through and between 
these metamorphic and primary rocks, and enabled a thorough section to he obtained. 
In ascending the pass, as soon as the upper drift beds, which are continued into the valley of 
the Santa Anna river, are passed over, and which are here cut through by the Cajon creek, the 
first rock in situ exposed is mica slate, forming a hill about 900 feet high above the pass, with 
a dip of 30° northeast. A couple of hundred yards higher up granitic porphyry and horn¬ 
blende slate appeared on both sides of the stream ; one mile further up, the creek bed traversed 
a small flat, with well marked terrace hanks, composed of the detritus of the primary rocks 
around, varying in depth from three to 10 feet, and lying upon mica slate, dipping northeast 
50°, accompanied by gneiss, talcose slate, and felspathic rock. These erupted and metamor¬ 
phic beds were much contorted, the gneiss and slates lying at a high angle. 
Ascending the creek, which here flows almost south, a series of pink colored strata occur, 
dipping 25° south 10° west, lying on the right hank of the stream; three hundred yards higher 
up the same beds dipped in the opposite direction, or to the northeast; both series presenting a 
prominent feature in the landscape by the peculiar tint and conical shape of the edges of the 
strata ; being very friable they weather readily, and have their caps rounded, the degraded ma¬ 
terial being either washed down by the creek, which carries in its current vast quantities of fine 
and course debris, or accumulates in the little valleys between the upraised strata. These beds 
might he at first sight mistaken for a granitic rock, were not its sedimentary character and the 
lines of stratification so well marked, for it is wholly felspathic in its constitution, presenting 
small rhomboidal crystals of pink colored felspar, imbedded in a felspar paste of the same 
color loosely cemented, so that the mass can be readily cut with the penknife, or even removed 
by impressing it with the shoe ; each crystal of felspar being complete in its form, and showing 
no sign of having been transported any distance before it was consolidated. Ho trace of fossils 
was observed in these strata. 
These strata are repeated several miles up the pass, almost to the head of the creek, which 
rises in a depressed area, surrounded on all sides (save at its southern outlet) by bluff walls of 
steep ascent, varying from 150 to 500 feet, which, when gained, are not hills, as they appear 
from below, hut merely the summit of the sandstone sloping eastward to the Great Basin. 
This peculiar local configuration has given rise to the name of the pass, (Cajon,) the origin of 
the creek being enclosed, as it were, in a box. 
The rounded drift stones, some of which are of large size, carried down by the creek during 
freshets are not to any extent derived from the immediate wearing away of the primary rocks of 
the axis alluded to, hut are mostly derived from the drift covering of the little valleys of the 
pass, and from the terraces alluded to. They are gneiss, hornblende, porphyry, felspar rock, 
a few mica slate specimens, and white crystalline limestone. This latter rock, though an 
abundant constituent of the drift, has nowhere in this pass been observed in situ. That it exists 
in the pass in place there can he little doubt, near its summit. It is a constituent rock of the 
chain, being found extensively in place at Tejon, and having been observed in situ by Mr. 
