72 
RINCON-MATILIHAH-BUENAVENTURA RIVER. 
and then seams of gypsum, the stratum fractured and re-cemented; while veins of pyrites 
accompany the white clay rock, which is there hard and flinty. The bright red tint of the 
stratum near the trachyte is very noticeable, and, with the hardening of the strata, is the result 
of plutonic action decomposing the pyrites. A similar appearance of the strata was observed 
by Mr Campbell on the San Marcus pass, where he also found a serpentine axis accompanying 
the trachytic lava ; the decomposition of the pyrites was there effected on a much larger scale, 
considerable deposits of sulphur being found in clefts and chinks of the sandstone strata 
impinging on the axial rock. Limonite occurred in partial deposits near that locality. 
Eincon is the most easterly locality where the axial rock of the Sierra was observed along 
shore. In passing up the river San Buenaventura, about two miles above its mouth, an anti¬ 
clinal axis occurs on the right hank of the stream ; the sandstone strata form low hills there, 
and the igneous central rock is not upraised ; this low range, which the river cuts through, 
and has worn into it a wide valley bed, is two and a half miles across in width, from the shore 
northward ; still higher up the river, another range is met six miles north of the first, there 
reddish amygdaloidal trachyte, having a dull earthy fracture, crosses the river in a line northwest 
and southeast. On the east side of the river, and contiguous to the igneous rock, a spring, 
which deposits sulphur, is met, and beside it a large outflow of asphalt, solid along the margin, 
hut toward the centre readily yielding to the pressure of the mule’s foot, and in places oozing 
up in the condition of a syrupy liquid. The rock through which the bitumen is effused is a 
yellow sand rock. 
Above this point the river bed widens and opens into a valley hounded by mesas or terraces 
given off from the hills, and having an elevation of fifty feet above the river valley; these mesas 
are of sandstone covered over with its own detritus, the wear of the foot hills. A lofty range 
hounds this valley on the north and northwest, which may he considered as the eastern continu¬ 
ation of the San Luis Obispo hills, while those to the south may he looked upon as the Santa 
Inez and San Eafael mountains, the two ranges being here blended thus closely together so as 
to become intimately one. The elevation of this valley is about 700 feet above tide level, 
and is called by the Indian name Mat-ili-hah. 
The Buenaventura river has its source in this northern range. In the glens between the 
ridges it courses its way downwards, crossing augitic dykes and metamorphic rocks, over which 
it forms cascades, until it finds its way into the valley of Matilihah; in this upper course it 
heads considerably in the west, and finds its source not many miles apart from the sources of 
the Santa Inez river. The exploration of the upper waters of this river was a fatiguing and 
difficult performance. The strata met with were hard sandstones, conglomerates, chloritic 
slates, and augitic trap, with amygdaloidal trachyte. Beds of black shale, accompanied with 
sulphuret of iron, occur, intercalated with the hard gray sandstone. Sulphur springs are 
frequent in these strata, which are sometimes elevated, and sometimes only cut through by the 
trachytic rocks. Ho developments of serpentine were met in these hills. These strata may 
be looked upon as very much altered in character, and have little resemblance to those lower 
down and nearer the coast; they resemble more the beds found flanking San Emilio, and 
the whole region may be considered as the commencement of that mountainous district which 
extends north to Tejon and the Sierra Nevada. 
From this region the Santa Inez mountains are separated by the little valley Matilihah, and 
from the sierras Susanna and Monica by the valley of the Santa Clara river. Their eastern 
and northern termination has been now traced. The whole sierra is made up of sandstones of 
various degrees of fineness, highly calcareous in a few beds, and fossiliferous in almost all; the 
