GEOLOGICAL STRUCTURE. 
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 he 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 dip 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 Kinconada 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 
