VALLEY OF SANTA MARGARITA. 
45 
brittle so as to render it difficult to preserve the specimens from falling to pieces ; while the 
ostrea layers were sometimes imbedded in so hard a matrix that it was difficult to remove them 
without fracture. Measured by the foot, several of the oyster shells were fourteen inches long 
by seven inches wide, and few of them were less than six inches long. The thickness of the 
under shell is remarkable, some of them being five inches in depth. 
It is worthy of remark that the beds, although within few yards of each other, had not 
their fossils intermingled, as it were; thus the asterodapsis, though the most abundant shell 
of the upper bed, was not found in the second lowest ostrea bed—this might more properly 
be called the hinnites hed—which latter was also unattended by the Janira estrellana, which, 
however, was freely intermingled with the laganum ; and, lastly, that the layers perforated 
by the pholadines were only found above the upper bed, and formed loose stones upon the 
surface, accompanied by rounded pebbles of jasper and quartz. 
The height of these hills rarely exceeded 100 feet, more frequently about 80 feet, with 
rounded sides and summit, and presenting bluff edges to the west. As this was the direction 
of the dip, which did not anywhere in these beds exceed 40°, the strata were uncovered by 
denudation ; indeed-, everywhere these hills were examined they were found to have suffered 
extensively by the denuding effects of currents, which, sweeping in a meridional direction, 
removed large masses of this stratum, and converted what was an inclined .slope into a series of 
rounded hills. 
The flattened summits of these hillocks with the bored slates tend to show that, at the time 
when these were last covered by water, they must have nearly reached the level of the sea of 
that period. It may be remarked of the sandstone hills of the west side of the valley, that 
those which are first met with are also flat on their summit and under 150 feet in altitude. 
Below these heds, interesting from their fossil contents, and separating them from the 
gypseous sandstones, lies the white argillaceous rock with the layers of chalcedonic and opales¬ 
cent quartz, already spoken of as met with on the San Antonio hills, beds containing dosinia, 
venus , and natica. In a geological sense, as well as topographical, the argillite is inferior ; for 
it here occupies the middle of the valley beside the stream ; it dipped N.W. from 60° to 70°, 
and was so hidden up by alluvium as to prevent any exploration of its beds. It has a light 
yellow tint, becoming in places almost white; granular in texture, and breaking with a splintry 
fracture. In places the strata were almost vertical, not more than 50 feet was exposed. 
These white ostrea beds were the most modern rocks observed in the valley. Passing west¬ 
ward for a couple of miles, no rock was exposed ; and when the Pacific side of the valley was 
reached, protrusions of trap and serpentine presented themselves to view, with the brown sand¬ 
stones dipping toward the centre of the plain. 
These are the foot hills of the Santa Lucia range, a mountain chain which is described else¬ 
where in the report. 
These sandstones underlie the valley, dipping beneath the strata already described, and 
having a position below the breccia conglomerate of the east of the plain, and lying between it 
and the granitoid axis. I look upon them as repetitions of the gypseous and saline sandstones 
found near the base of Panza. 
The relative positions of the strata of this valley are shown in plate 1, fig. 4; and a view of 
the characteristic beds in fig. 1, plate 2. 
The valley is thus a true basin : the conglomerate of the east rising up near the summit of 
Santa Lucia, range; above it the brown and yellow gypseous sandstones ; then the San Antonio 
