i)S 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. Tiie depressed axis of the Kikal Miingo hills is there met with at an elevation not 

 excecMng 2,000 feet, consisting of a nucleus of granitic porphyry, hornblende, and mica slate, 

 intercalated with beds of crystalline felspatliic 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 be 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 teirace banks, 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 bank 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 be at first sight mistaken for a granitic rock, were not its sedimentary character and the 

 lines of stralification 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. No 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, but 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, but 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 be little doubt, near its summit. It is a constituent rock of the 

 chain, being ibund extensively in place at Tejon, and having been observed in situ by Mr. 



