GEOLOGICAL STEUCTURE. 43 



tine and trappean rock, spurs of which are given off and enter the plain at various points. One 

 of these protrusions occurs in the centre of the valley, within one and a half mile of the entrance 

 of the pass to San Luis valley ; it is a mass of augitic trap, which rises up from the plain in 

 knobs ten to fifteen feet high, increasing in height to the south, where it is found elevating the 

 brown sandstones from one hundred to five hundred feet high, and forming central bosses from 

 fifty to one hundred feet on the summit. These hills are covered with wild oats and oak, and 

 produce the finest pasture in the valley ; as they increase in height they merge into the main 

 mass of the Santa Lucia. To the northwest the trap vein can be traced along the valley several 

 miles, until it passes into the same range west of the Old Mission, Santa Margarita, (Don Joa- 

 chin de Estrada's residence.) 



The Old Mission establishment stands on a terrace raised about sixteen feet above the plain, 

 on its western side ; at the base of the terrace lies one of the forks of the Salinas, which heads 

 up in the Santa Lucia range a few miles southwest ; the terrace is not more than one-fourth of 

 a mile wide, and stretches northwest for one and a half miles ; where the river cuts its margin 

 it exposes here and there sandstone and argillite beds similar to those of the San Antonio river, 

 cut up and altered by intrusion of trap rock, steatitic and talcose clays, and shales, which have 

 flexed and contorted the strata in various directions. This intruded rock can be traced several 

 miles, both north and south, preserving a direction of N. 60° W., S. 60° E. 



The valley is closed at its south extremity by an elevation of the sandstones, caused by the 

 intrusion of a mass of serpentine with augite, which runs in an easterly direction towards the 

 Toro hills, small masses of granite, outliers of the San Jose mountains, and forms a natural 

 division between the Santa Margarita and San Jose valleys. 



On account of the frequent and extensive intrusions of augite and serpentine in this valley, 

 it is difficult, in many places, to say what is the original dip of the strata ; thus, in the north, 

 the white fossiliferous beds dip to the southwest, conforming to the dip of the strata of the San 

 Antonio river, and there on the west flank of the Point Pinos range, while in the south of the 

 valley the same strata dijj eastward. Observation had however shown, that as an elevating 

 agent, the Point Pinos (or San Jose) mountain range extended its influence to a much greater 

 extent laterally than the Santa Lucia mountains, and that ,where the strata repose without any 

 subsequent alteration or flexion they are found to be conformable to the Point Pinos granitic axis. 



The sedimentary bed lowest in position in this valley is the same as that observed at San 

 Antonio, a breccia conglomerate of quartzose and jaspery pebbles in an aluminous paste, the 

 ■whole having a light brownish green color. This was not occupying a prominent position, but 

 was found cropping out near the river beyond the low hills on the east side of the plain. 

 Above this is a fine-grained sandstone, greyish white in color, friable, and weathering readily 

 into holes, intersected with threads of sulphate of lime, which traverse the rock in horizontal 

 lines, having a direction north 20° west ; the gypsum was in places granular and compact ; in 

 others, crystalline ; the seams one-half to an inch thick, and stained green with carbonate of 

 copper, (malachite,) this appearance was presented wherever augitic rock cut through the 

 elevated sandstone, and was well displayed at the Rinconada hills. The gypsum, being readily 

 removed by the weathering of the rock, is dissolved by the waters, and finds its way into the 

 Salinas, to which it imparts its flavor and unhealthy action, and from which being present the 

 river has derived its name. Besides the gypsum Veins, another set of threads cross the fore- 

 going at an oblique angle ; these are filled with limonite, (peroxide of iron.) These two 

 classes of veins render the sandstone readily recognizable wherever found. The thickness of 



