northeasterly from Point Loma and south from the Soledad Hills. A
violent disturbance, forming a great uplifted fold or perhaps a fault,
has taken place along a line extending southeast from La Jolla through
the Soledad Hills. At the eastern end of False Bay there is a small
exposure of Eocene strata, dipping west. Near the mouth of Rose Cañon
the strata dip southwest, and at the mouth of the cañon they dip 40°
northeast. Along the road which leads over the hills to La Jolla the
rocks are tilted at a very high angle to the southwest. The highest
point of the Soledad Hills, rising 700 feet, lies over this disturbed
region. Unconsolidated bowlder deposits lie on the top of the hills. The
strata on the east side of Rose Cañon are well exposed, but do not seem
to partake of the disturbance shown on the west. This is probably no
unconformity, as they contain Eocene fossils, and the Eocene in other
spots appears to be conformable with the Chico. Along the coast between
False Bay and La Jolla the strata dip south at a small angle. At La
Jolla, near the caves, they have been folded so as to dip very steeply
to the southwest for nearly a quarter of a mile across the strike. Near
the eastern end of the cliffs a reversal takes place, and they dip
northeast at nearly as great an angle. Around the little bay there are
no exposures, but a mile northward begins a very high line of cliffs,
which extend through to the mouth of Soledad Cañon. This fold at La
Jolla brings to the surface fossiliferous strata, bearing a number of
species similar to those at Point Loma; among them is the _Coralliochama
Orcutti_. The strata of the high cliffs north of La Jolla dip northerly
at a small angle, and show only a few fossils of the Eocene age. The
cliffs rise fully 400 feet. At the bottom of the cliffs are shales;
higher up are great beds of conglomerate bowlders, chiefly a reddish
porphyry.
Coal is reported to outcrop above the water at very low tide somewhere
along this stretch of cliffs. It of course must occur in strata of
Tertiary age. The coal vein struck in a boring at La Jolla must be
Cretaceous. About 3 miles up the coast from La Jolla, there appears a
dike of basalt cutting the Tertiary shales. At high tide it is nearly
covered by the ocean. It has a course about 30° east of north, and
stands vertical. It begins on the north, close in under the high cliffs,
but does not extend into them, the only signs being a fault in line with
the dike. It is not more than 2 feet wide at the northern point where it
is exposed. It is dark and compact and so decayed as to be easily taken
for an argillite. The walls of the dike are very smooth and regular,
except near the southern point, where it runs into the water. Here it
swells to a width of 30 feet. The edges of the dike are compact, while
the vesicular portion is in the center, where there is often a flowage
structure developed. The central portion is more or less laminated
parallel to the wall, and thus is generally a well-pronounced columnar
structure developed the whole width. The columns lie horizontally across
the dikes and are 12 to 15 inches in diameter. The cavities are wholly
or in part filled with calcite. Metamorphism of the adjoining shales is
apparent for 2 feet away, but the sea water has so decomposed the shale
that it is not so strongly marked as it would otherwise be. The dike
projects above the water in places for a distance of 1,000 feet, making
its total exposed length about 1,800 feet. In the mesa southeast of Rose
Cañon, and along the San Diego River, and back of San Diego, the
formation belongs almost wholly to the late Tertiary. It is not certain
whether the Miocene is present or not. A number of Miocene fossils have
been found in the county, but perhaps the most of them have come from
Carrizo Creek. Many fossils are given in Dr. Cooper’s list, as being
found in the Pliocene of San Diego, which are more characteristic of the
Miocene in other localities. I see no reason for doubting that the
Miocene is present, but so intimately related to the Pliocene as to be
stratigraphically inseparable from it. In the region between Rose Cañon
and the northern boundary of the county, I do not know that Miocene
fossils have been found, but in Orange County they are well
characterized.
The region occupied by San Diego Bay and the mesa back of it is
composed, as far as we know, of Quaternary, Pliocene, and perhaps
Miocene strata. Sandstones characterize the lower formation, and loosely
cemented conglomerates, increasing in thickness toward the mountains,
the upper. These were deposited in a sort of basin, of which Point Loma
and the Soledad hills formed the northern and western borders. Many
oscillations of level have taken place, the most recent being an
elevation of 40 feet, shown by an old beach line on Point Loma. The
shells in this beach are the same as those now living in the adjoining
ocean. It is a peculiar fact that the mesas are slightly higher near
their western terminations than farther east, indicating a recent uplift
along the ocean. Water is scarce through this mesa formation. At the end
of Point Loma there is a strong sulphur spring exposed at low tide. Its
waters may possess medicinal properties, and should be examined.
On the southern shore of False Bay is a large deposit of calcareous
tufa. The central portion is quite pure and a number of feet thick; just
how thick is not known. It extends along the shore some distance, and
often contains bowlders and shells. This is evidently a deposit from
some former spring of great size. The mesas lying west of the extensive
volcanic tuffs have been derived largely from the decay of the latter,
and have heavier soil. North of Soledad they become more sandy, and
maintain this character to the Santa Margarita Creek. This light soil,
however, is being successfully cultivated in many places and for certain
kinds of fruit, without irrigation. The surface of the higher portion of
Point Loma, as well as some of the mesas north, is covered with
spherical nodules, a quarter to half an inch in diameter, of sand
cemented with red oxide of iron. These literally cover the ground in
places so that it is difficult to walk. The origin of these at first
seemed very puzzling, but on examining the face of a cliff on the top of
which these were found, an explanation was reached. They were seen to
grow smaller away from the surface of the ground until a depth of 2 feet
was reached, when they cease. Their formation is due to the oxidation of
the iron in the sandstone, and its segregation in little nodules on the
same principle as the formation of concretions.
[Illustration: Fig. 13.]
The cliffs of Eocene sandstone along the ocean grow gradually lower
north of Soledad Cañon. At Encinitos the cliffs are higher again and for
a short distance the strata dip south, but toward Oceanside they resume
the northerly dip and disappear several miles south of that place.
Faults grow less numerous the farther we get from Point Loma. The mesa
is low about Oceanside; it was either never very prominent or else the
erosion has been great.
On the north bank of the Santa Margarita Creek, near the ranch house, is
an interesting cliff of Quaternary sands and gravel, showing a number of
strata deposited under different conditions on an old beach. (Fig. 13.)
The Tertiary beds north of the Santa Margarita Creek are very different
in outline from those south. Instead of their extending in a gradual
slope from the older mountains to the ocean, there arises in them, near
their western border, a range of mountains, known as the San Onofre
Mountains. These extend parallel to the ocean at an average distance of
2 miles. They rise north of the Santa Margarita Creek and extend to the
San Onofre Creek. They have a gradual slope on the west, rising to an
elevation of 1,400 feet, but are quite abrupt on the east. Los Flores
Creek cuts through the southern end of this range, showing that while
the soft, clayey sandstones between it and the Santa Margarita Mountains
slope only 5° to 10° southwest, the rocks of the range itself dip west
at an angle of 35° to 40°. The formation is a breccia, the fragments of
which are argillitic, micaceous, and hornblendic schists. Some of these
fragments are of great size, one bowlder of hornblende schist being 8
feet in diameter. Pebbles of white quartz and other hard metamorphics
are also present. The soft, coarse sandstone in which the fragments are
imbedded show no traces of any granitic matter. The range was ascended 2
miles north of the Los Flores ranch house, and found to consist entirely
of fragmental schists, such as those mentioned, dipping southwest at an
angle of 45°. The mountains were also climbed at their northern end,
near San Onofre Creek. Here there is a very abrupt escarpment on the
eastern side. The strata dip toward the ocean at a high angle, while the
irregular hills and ridges of soft, light-colored sandstone lying east
toward the Santa Margarita Mountains are nearly level. After a careful
study of the range the conclusion was reached that its origin was due to
a great fault, represented by the very abrupt eastern slope, tilting the
elevated portion to the west at a high angle. I believe that this fault
took place after the deposition of the Tertiary strata. As far as my
observation went the Tertiary beds on the east do not rise to meet the
San Onofre range, as they would to a certain extent if it were present
when they were deposited; on the contrary, they dip toward it. West of
the range the ocean is bordered by very high cliffs of Quaternary clays,
and in only two or three places do the Tertiary rocks outcrop. Small
patches of sandstone outcrop near the road at the western foot of the
mountains; they also dip west at a high angle. Many of the fragments at
the northern end of the range show their derivation from a massive
crystalline rock. The hornblende schists are generally garnetiferous.
Blue glaucophane schists are also very common. South of Mission Viejo
Creek, Orange County, there is an outcrop of rock, apparently in place,
which greatly resembles these schists. Good outlines of these mountains,
indicative of structure, can be seen to great advantage from the San
Luis Rey Mission. On the west slope of the San Onofre Mountains, 4 miles
north of Los Flores, is an outcrop of a garnetiferous hornblende schist,
which certainly appears to be in place. This rises 10 feet above the
side of a gulch, and is fully 20 feet across. One mile north and in line
of strike with the last is another outcrop of similar rock, which is so
large that it certainly seems that it must be in place. The only point
north of the San Onofre where this breccia appears is at Arch Beach,
Orange County. The Santa Margarita Mountains are bordered by very
extensive bowlder deposits, which rise as high as 1,500 feet on their
western side.
The topography of the northwestern part of the county between Temecula,
Elsinore, and the ocean, is very complicated. This section is occupied
by rugged, brush-covered mountains and narrow, deep valleys, with the
exception of the Santa Rosa plateau, where the configuration of the
county has been entirely changed by extensive lava eruptions, stretching
over a distance of 10 miles. This mountain region narrows toward the
north to form the Santa Ana range. The variety of rock formations is
very large. The northern portion is unsurveyed. On the south are the two
large grants, the Santa Margarita and the Santa Rosa. Between these lies
De Luz Valley. The Santa Margarita Mountains extend north and south,
forming the eastern borders of the grant and rising to an elevation of
3,100 feet. The granite of the region about De Luz Valley is far from
being homogeneous. A part of it is undoubtedly intrusive, and a part may
represent an original sedimentary formation. Bedding planes are present
in much of this supposed metamorphic granite, but generally no schistose
structure. The presence of the De Luz warm springs is perhaps due to a
dike of dark, aphanitic diorite, which has cut through the granite in an
irregular manner. A very interesting breccia outcrops in the bed of the
creek below the warm springs. (Fig. 14.)
[Illustration: Fig. 14.]
The fragments are chiefly granite and an aphanitic rock. They are quite
angular, showing only a slight rounding of the corners. Some of the
larger fragments are a foot in diameter. The boundaries are very
irregular. Long arms of the inclosing granite project into the breccia.
The base or matrix varies from a coarse syenitic rock to an aphanite. It
often seems to present a blending of different kinds of fragments.
Besides the large inclusions there are scattered through the matrix
small angular pieces, which are so regular in outline and distribution
as to give to the rock the appearance of a porphyritic structure. The
granite in the hills west of the valley contains much biotite and quartz
in long, rounded grains, presenting a pseudo-porphyritic aspect. This
appearance is characteristic of much of the granite of this section.
Imbedded in the granite are masses of dark aphanitic rock. The lower
granite hills are covered with considerable sandy soil. There are
isolated peaks of a coarse white granite, much like that of the Sierra
Nevada, arranged in some sort of regularity in north and south lines.
One rugged peak of this coarse granite rises 2,500 feet west of the
valley. At the northern end of the valley the bedding planes in the
finer grained granites are very regular; strike north 45° west, dip 65°
southwest. There is, however, no schistose structure present.
A half mile above the warm springs is another conglomerate or tuff,
which seems so related to the granite that the latter must really be
eruptive. In a little valley southwest of De Luz and just east of the
Santa Margarita grant there is a large outcrop of diabase. It has been
intruded in a fine-grained, jointed granite. Farther down the valley, on
the road to De Luz Station, there is a narrow outcrop of black quartz
feldspar porphyry, followed on the east by a dark felsitic mica schist;
strike northwest, dip 60° southwest. Immediately west of the deep canons
which lead down to De Luz Creek, rises the Santa Margarita Mountains.
They consist of a fine-grained granite, verging at times on a quartz
porphyry. The main crest is 2 miles long, the highest peak of which is
nearly 3,200 feet. The rock is perfectly massive, but shows apparent
bedding planes; strike north 30° west, dip 80° northeast. The
porphyritic facies of this formation occur in the western slope. On the
western slope of the main range, at an altitude of 2,500 feet, there is
a plateau-like area of a thousand acres or more of fine grass land. It
is dotted with white oak trees. The western slope of this plateau is
very abrupt and brushy. The formation is partly porphyry and partly dark
diabase and diorite. The most interesting fact connected with the Santa
Margarita range is the occurrence of sandstone at an elevation of 2,600
feet on its western slope. The sandstone occupies very limited detached
areas in the heads of the gulches, and is evidently the remnant of a
once far more extensive formation. The sandstone is largely kaolinitic,
and has evidently been derived from the adjoining rocks. At the foot of
the southern end of the mountains appears very quartzose rocks, probably
of metamorphic origin. Coarse granite has been intruded into them in
small bunches. Granite extends southwesterly in the form of a wedge as
far as the Santa Margarita ranch house, and is there covered by modern
deposits. Between De Luz and Fallbrook the country is gently rolling,
with knobs of granitic rocks projecting here and there. About Fallbrook,
and for some distance east, the granite does not outcrop much, owing to
its easy decomposition. A little east of De Luz Station is a small body
of mica schist; dip 30° east, strike north 15° west.
The road from Fallbrook to Temecula leads through a long, narrow valley.
On either hand rise high mountains of bare granite. Immense bowlders, 20
to 30 feet across, line the valley, having fallen from the cliffs. The
granite here is a coarse rock, rich in biotite, and though great masses
could be obtained free from checks, yet does not seem durable. The
valley owes its origin to a difference in rapidity of decay along
certain lines. On this section there appears no trace of the schist belt
extending northwest from Julian. This coarse granite is undoubtedly
intrusive and has cut it off.
A wholly different series of rocks is exposed in the Temecula Cañon, not
more than 2 miles north of the country just described. This cañon is
deep and rocky, taking a very direct course from Temecula to the ocean.
At the upper entrance there is a narrow exposure of granite. This is
followed by quartzite, dipping 45° southwest. The rocks shortly become
massive and are replaced by dark syenitic ones with an excess of
hornblende. Two miles down, granite appears for a short distance, and in
it a quarry has been opened. The rock can be obtained in blocks of any
size from great masses which have broken off and rolled into the cañon.
Gneissoid rocks soon replace the granite, and these are followed by
hornblendic rocks, which vary from a schistose to a massive structure.
In places they contain feldspar and pass into syenites; in others the
rock is almost pure hornblende. The greater portion of these rocks are
of metamorphic origin. The dip is generally vertical, strike east and
west to northwest. The syenites are followed by mica schists, and these
by coarse biotite granite about 5 miles above Howe Station. In the
granite are many pegmatitic veins, carrying biotite, garnets, and
tourmaline. Fine-grained granite, varying at times to syenite, forms the
rock along the cañon for many miles below this point.
The most interesting geological feature about this northwestern part of
San Diego County is the long plateau, confined chiefly to the Santa Rosa
grant. This plateau lies near the western corner of the grant, and
extends east nearly to Murrieta. The lava is broken up into detached
tables by erosion, which become very strongly pronounced toward the
western end of the flow. The western body of lava is the highest. It has
a length of nearly 2 miles and is broken into three peaks or ridges,
sloping generally a few degrees to the east; height 2,850 feet. There
are two terrace-like tables lower down its southern slope. The lava is,
perhaps, a hundred feet thick at its eastern end, and has been so much
eroded toward the western portion that the underlying sandstone is
exposed along the crest of the ridge, with lava lying in broken masses
along its sides. The sandstones form quite an extensive bed under the
lava flow, being 200 or 300 feet thick, and horizontally bedded,
wherever bedding is present. The upper part is very soft and granular,
the lower portion is hard and stained reddish. It carries many bowlders
6 to 8 inches in diameter, different from any other rock seen in the
adjacent mountains: quartzite mica schist, aphanitic rocks, and some
granitic ones. These are washed smooth. The sandstones contain much
kaolinitic matter, and at one spot show an incipient crystallization. A
number of contiguous grains, over a space half an inch in diameter, show
the same orientation. Near the bottom the sandstones are impregnated
with iron. The western ridge in particular shows a great amount of
erosion. The lava is nearly gone in places, but occurs southward in
scattered outcrops for half a mile. At the northern end the sandstone
rises fully 300 feet above the lava. Lava is present on its sides. Much
of the sandstone closely resembles a granite decomposed _in situ_.
Fragments of the mica schist resemble that in the hills west of
Temecula. Northward half a mile is the deep cañon of the San Mateo. The
country descends very rapidly from the lava ridge, especially so on the
north, where the cañon is fully 1,500 feet deep. It is a number of miles
in any direction to mountains which are as high as this lava-capped
sandstone ridge, and the amount of erosion must have been enormous since
it was deposited. Mesa Redonda has an elevation of 2,750 feet, and is
separated from the lava just described by a valley fully 800 feet deep,
and nearly a mile broad. Mesa Redonda is formed by a lava table,
probably basalt, 150 to 200 feet thick. It is quite precipitous on three
sides. The lava is bedded, dipping 5° to 8° northeast. Underneath is a
body of coarse, friable sandstone, similar to that just described. Some
pebbles and bowlders of lava lie in the upper portion of the sandstone.
The sandstone consists of angular quartz grains and kaolinic matter, and
often presents the appearance as if it had been partly fused by the
lava. In the top of the sandstones are pebbles of quartz, feldspar, and
mica schist. The sandstone shows no bedded structure, but seems to form
a mantle over the hill, following the irregularities of the underlying
granite. It descends 700 feet on the southern slope of the mountains
which rise so abruptly from De Luz Valley. The lava has spread out in
thin sheets on the southern slope of the mountain, descending more than
a thousand feet on the east side of Cottonwood Creek. These thin beds
are not massive, but are formed of angular lava bowlders. The flows were
so thin that they either broke up on cooling, or later through
atmospheric agencies. Fig. 15 is a sketch of Mesa Redonda from the
north.
[Illustration: Fig. 15.]
[Illustration: Fig. 16.]
Cienega Peak lies east of Mesa Redonda and is separated from it by two
gulches opening in opposite directions. It has an elevation of 2,400
feet, and the mesas east rise still less. Sandstone underlies this as it
does the other lava flows. Near the eastern end of the southern slope, a
lava flow has broken out from a basin-like depression which opens
southward, and flowed down the mountain for a mile, descending a
vertical distance of 1,800 feet. It appears to have broken up entirely
into angular bowlders. The stream was probably very liquid, like the
others, and formed a thin flow. It takes a slightly winding course and
slopes often 30°. One short branch appears on the western side about
half way down, and another on the east near the bottom. The lava
descends in successive terraces, like steps, from the crater depression.
The width varies from 500 to 700 feet, terminating in a straight line
about a hundred feet above the bed of the cañon at the head of De Luz
Valley. This distance may represent the amount of erosion since the
stream flowed. There are also cañons worn to some depth on each side.
The surface of the flow is rounding, and appearances indicate that it
descended over a surface not much different from the one now shown. A
large part of the bowlders in the creek for several miles are lava. Fig.
16 shows this lava stream as it lies on the mountain side, and also
Cienega Peak, from which it flowed. These lava beds appear very
prominent from De Luz Valley. The long, winding flow is known locally as
the Oak Ridge, on account of its being covered with oaks, while the
adjacent mountains are barren and brushy. It is hard to reconcile the
appearance of these isolated peaks, with often precipitous sides, and
deep valleys between them, showing a great amount of erosion, with the
thin sheets spread out on the southern side of the mountain in so many
places, which from their position indicate so little erosion since they
flowed. It is possible that the mesas, with the high precipitous cliffs,
represent remnants of an older flow, and yet the lithological character
of the lava seems to point to a single origin. With the exception of the
long southerly flow and another short one west of it, the lava
everywhere presents bluffs on its southern side, with deep gulches
between them. Toward the northeast and east there is a gentle slope. A
large part is coarsely vesicular; dense massive portions are mixed
irregularly in places with the vesicular. The lava table-lands lie
nearly 2,000 feet above De Luz Valley. This abrupt escarpment extends
east as far as the lava does, though less marked. There has either been
an enormous erosion in the region lying south, or a great fault
elevating the plateau. A detached portion of the lava plateau caps the
hills west of Murrieta, extending in a north and south line for a
distance of 2 miles. Whether these detached portions all had their
source in one great flow and have been separated by erosion, or were
formed from different sources, was not fully determined. It seems
probable, however, that the main portions did belong to one flow, from
the fact that they have a uniform slope and are underlaid by similar
sandstone, which may once have been the bed of a stream.
The range of mountains lying west of the valley which extends from
Temecula to Elsinore, also has the appearance of having been elevated by
a fault. From the entrance to the Temecula Cañon, northward past
Elsinore, and along the eastern base of the Santa Ana Mountains, these
abrupt escarpments and indications of a fault become more pronounced.
The eastern part of the Santa Rosa plateau, with its lava fields, forms
the southern end of the escarpment. The valley in which are located the
towns of Temecula, Murrieta, and Wildomar, rises gradually toward the
east. The western portion is very fertile. Artesian water is found at
Murrieta. The eastern portion, which rises toward the granite mountains,
is more gravelly, while east of Temescal there is a stretch of many
miles of these dry gravel hills, probably of Quaternary age. The town of
Temecula has an elevation of 1,000 feet. Immediately west and north of
the cañon there arise hills of metamorphic rocks, having an elevation of
1,800 feet. They are covered with dense brush on their eastern slopes,
but contain some fertile valleys to the west. For several miles the rock
is almost wholly metamorphic. It extends south to the cañon and north to
the lava plateau. It is chiefly a fine, dark mica schist; strike
indistinct but north 60° west, to east and west, dip vertical. On the
west it changes to a quartzite. Dikes of granite cut this rock; one is
noticeable for several miles by its more pronounced croppings over the
hills.
East of Murrieta the granite begins near the boundary of the grant, and
forms a line of barren hills extending northerly. East of these the
country is less rocky and quite fertile. Many springs abound in the
granite through this section. Near the Hot Springs is a dike of granite
porphyry. Numerous bunches of a dark, coarse, dioritic rock are
scattered through the light-colored country granite. They weather away
more slowly than the granite. The metamorphic rocks of the Santa Rosa
plateau extend north to the entrance to the cañon, up which the road
passes to Parker Deer’s. The strike in the cañon is a little east of
north. The metamorphic rocks terminate in a range of hills which form
the southern boundary of the Rinconada. Northward the country is formed
of rugged granite mountains. The Los Alamos opens westward into deep
cañons, which lead down to the coast. A dark dioritic granite is
included in the usual light-colored variety, sometimes in bowlder-like
masses and sometimes in dike form. The metamorphic rocks extend 2 miles
west of Parker Deer’s house. They include mica felsite and dark vitreous
quartzite. They are often intruded by granite bosses and dikes of quartz
porphyry. The lava table-land lies just south of the ranch house. It is
about 40 feet thick, and has underneath a kaolinic stratum 12 to 14 feet
thick, which is impregnated with bog iron; one assay has shown 10 per
cent. This is quite similar to the sandstone under the table-land
farther west, but is less quartzose. A similar deposit, impregnated with
iron, was seen north of Mesa Redonda. The Santa Rosa grant consists
chiefly of broad, open valleys, having an altitude of 1,700 to 1,800
feet, with rocky ridges between them.
On the trail from Santa Rosa to Howe Station, the metamorphic rocks
extend to within a mile and a half of the latter place. They are chiefly
light-colored, granular quartzites. Dikes of diabase and gabbro appear
in many places on the Santa Rosa grant. Ores of gold, silver, and copper
are found in the metamorphic rocks of the grant, but they have never
been developed. Selected samples of galena assay several hundred dollars
to the ton in silver. The veins are, however, small and bunchy, and it
is not probable that they can be profitably worked. The granite varies
from one with mica, as the only dark constituent, to one with much
hornblende. It is uniformly coarse and of undoubted eruptive origin,
judging from the manner in which it has broken through the metamorphic
rocks.
The table-land west of Murrieta is about a mile broad and fully as high
as that near Parker Deer’s house. It is separated from the lava farther
west by a mile of brush-covered hills. The lava was supposed to extend
no farther than the big cañon west of Murrieta, but a close examination
revealed a small outcrop on the hills about a mile south of Wildomar.
The elevation is about 600 feet above the valley. It is perhaps one
fourth of a mile across. In places it extends down the hill nearly one
third of its elevation above the valley. It presents the appearance of
having flowed out of the summit of the hill when it had much the same
form as now, and down over its sloping surface. This eruption is
fine-grained, and not vesicular. Much of it has a conglomeritic
character, appearing to have been broken up when almost solid, and then
cemented. The fragments are more or less rounded and elongated, and are
at times almost blended in the matrix. There are signs of former
solfataric action on the summit, there being a considerable deposit of a
light yellowish material, consisting chiefly of alumina and magnesia.
Under the lava is sandstone 10 to 20 feet thick, composed of quartz
grains and kaolinic matter, exactly similar to that under Mesa Redonda.
It would be easily taken for granite decomposed in situ, but for the
large quartz grains. The sandstone has an apparent southerly dip. It is
very difficult to account for the presence of the sandstone under the
lava, unless we suppose it covered the adjacent country, and was only
preserved by the greater permanency of the lava. Another hill of lava
was found 200 feet lower, about a mile south of this, and west of the
cañon leading up to Parker Deer’s. It occupies a sort of depression
between three hills, with gulches cutting into it between them. The lava
is very similar to that just described.
A long, high ridge running northwest and southeast, adjoining the lava,
is very interesting. It is about 200 feet higher, and covered with dense
brush, as is all the country in this vicinity. The greater part of the
hill is formed of a coarse tuff, whose fragments stand out in sharp
relief on the surface of the huge projecting, bowlder-like masses. The
matrix, which is darker and softer, weathers out, leaving the surface of
the rock covered with a great variety of fragments. Some are scoriaceous
or amygdaloidal, others are very coarsely crystalline and porphyritic
with feldspar or hornblende. Some of the fragments are themselves
tufaceous, containing large masses of hornblende in a dioritic matrix.
On the southern end of the hill, a great variety of dikes intersect the
tufa. This is in all probability the neck of an old volcano, but appears
to have no relation to the modern lavas near it. Owing to the exceeding
difficulty of traversing these hills, the exact relations of the
formations were not ascertained. About 10 miles east of Temecula, near
the point where the creek takes a turn to the southeast and enters a
cañon, there has been another lava eruption, but the time at my disposal
did not permit me to examine it. A great variety of rocks are exposed
along the road from Elsinore to Menifee Valley. For nearly 2 miles east
of the station the rock is a white, glassy diorite, with an excess of
feldspar. At the point where the road crosses the railroad, metamorphic
schists appear. The rock is a fine, dark mica felsite. It is so compact
that it breaks with a conchoidal fracture. A great irregularity in
strike and dip exists; the average strike is a little west of north, dip
northeast. As far as the top of the grade, the rocks are in part
metamorphic and part dioritic. There are many dikes; some fine-grained
granites, others micaceous diorite porphyrites. The hills along the west
side of Menifee Valley seem to be mostly metamorphic, with some bunches
and veins of granite.
All of the mountainous region lying south of the road from Temecula to
San Jacinto is granite or diorite, excepting a strip of micaceous schist
and gneisses near Glen Oak Valley. These strike northwest toward
Menifee.
No opportunity was given me to examine the mountainous regions comprised
in the San Jacinto range. The line of hills lying northeast of the town
and having a northwest direction are composed largely of gneiss and mica
schist, with some bodies of white crystalline limestone. In the line of
strike these hills finally disappear north of San Jacinto Lake, under
Quaternary clays and gravels, which form rather an abrupt rise from the
San Jacinto Valley and extend northerly to the San Bernardino Mountains.
The deposits show a great deal of disturbance. A part of them may be
Tertiary. Dikes of dark, heavy diabase and diorite are common about the
sulphur springs north of San Jacinto.
The hills for a distance of 3 miles north of Elsinore are formed of
slate and mica schists; strike north 70° west, dip vertical to 45° east.
This is a continuation of the same series of rocks exposed on the road
to Menifee. A lenticular body of limestone occurs in these slates about
3 miles north and east of Elsinore. It is highly metamorphosed, has a
gray to dark color, and is traceable for 500 or 600 feet. At one spot a
stratum of quartzite divides it. It was carefully examined for fossils,
but none were found. About 4 miles from Elsinore granitic rocks appear,
followed by dark diorites in the vicinity of the Good Hope Mine. The
Pinacate district, taken as a whole, is rather peculiar. At first sight
it seems to be formed of granite, dark diorite, gneiss, mica schist, and
other metamorphic rocks, arranged in the most irregular manner. The belt
of metamorphics northeast and east of Elsinore is terminated on an
irregular east and west line by these granitic bodies, which inclose
portions of the schists, and extend into the main body as long,
dike-like arms. In the vicinity of the Good Hope Mine the strata of
metamorphic rocks inclosed in the granitic rocks have a north and south
strike, and are traceable for a mile or more. The veins of the Good Hope
Mine are in a dike of light-colored biotite granite. It has considerable
width on the surface, 100 feet or more, but below ground some distance
it is not over 12 feet. On the surface it is greatly decomposed and cut
by numerous small veins, which are so scattered that they hardly pay for
working. Below they unite to form larger veins, generally one on the
foot and another nearer the hanging wall. The latter is more irregular,
often running out at a small angle. The foot wall, a dark compact
diorite, is very regular. The walls are separated by well-defined clay
seams from the vein matter, the decomposed granite. Clay seams also
separate the different veins. The foot wall diorite forms the country
rock indefinitely eastward. The hanging wall is a fine, dark brown mica
schist. The quartz is generally friable, and the granite vein matter
much decomposed. The quartz at a depth of 300 feet carried one third of
the gold in the sulphurets. A small amount of silver is also found. This
vein is located for over a mile; direction a little east of north, dip
65° west. It is remarkable that there is no barren quartz; all the ore
pays for working. In the lower workings the veins become more regular.
The Good Hope is the first mine in this district to reach a paying
basis, and that has succeeded in finding a regular, defined quartz
ledge. About 3 miles northwest is another vein, which has an east and
west direction. It seems to lie wholly in granite, save for a narrow
stratum of mica schist on one wall. The vein is located for a mile, but
no extensive development has yet been made on it. On the hills, a short
distance southwest, is an old Mexican mining camp. Many small veins are
found in the vicinity, generally in a dark micaceous diorite. There are
also bunch-like masses of coarse white granite, blending into gneiss and
the gneiss into mica schist. The strike is exceedingly irregular,
changing from north and south to east and west in the course of a few
feet. Toward Elsinore mica schist, quartzose, and feldspathic rocks
replace the greater portion of the granite. For some distance the
schists are cut up by dikes of fine-grained granite, running in
different directions, and small bunches of the same rock, often only a
few feet across, but sharply differentiated from the schists. Judging
from the exposure here, I think we might say that at the time of the
metamorphism the action was so intense as to change the sedimentary
rocks to mica schists and gneisses and through these to squeeze the
liquified portions of the same formation in dikes and fissures. Small
fragments of mica schist were noticed in the eruptive masses. In a
region like this it is often difficult to draw the line between eruptive
and metamorphic rocks. It has been shown before that lamination is no
sure indication of sedimentary structure.
Lake Elsinore is bordered on the west by a high and rugged granite
range. In the mountains west of Elsinore the granite which cuts off the
metamorphic rocks on the Santa Ana range is again replaced by the
Metamorphic Series, which are here very greatly altered. They strike a
little west of north and dip vertical or at a steep angle to the east.
The boundary of the granite is very irregular, and masses outcrop in the
metamorphics near the main contact line. Much of this crystalline rock
perhaps more truly belongs to the diorites.
The new silver mines lie just north of the San Diego County line, and
west of the divide, a position which brings them into Orange County. The
formations in which the veins occur vary from a dark brown felsite,
often micaceous, to a finely banded quartzose rock. The latter is very
compact, and often almost massive. In places hard, blocky argillites
appear. The two or more veins found here carry galena bearing silver,
and also much magnetite and iron sulphurets, with some of the baser
metals. The veins are characterized by a dark red gossan cap on or near
the surface. Carbonates are found in this. These deposits exist as
impregnations along a fissure, which is not very strongly pronounced.
The ore is usually quite massive. The little gangue present is calcite.
Not enough development has been made here at the time of my visit to
show how extensive the deposits are. The metamorphic rocks extend north
along the mountains, forming the summit and eastern slope for a number
of miles. Granite borders them on the west toward San Juan. It is
probable that this belt of mineral-bearing rocks runs continuously
through to Silverado Cañon. At some time this Elsinore basin opened out
through the Temescal Valley, but now a low divide separates it from the
head of Temescal Creek. Gravel-topped hills lie along the mountains west
of the creek. At the terra cotta works a drill was sunk over 600 feet
without reaching the bottom of the basin. The Cheney Coal Mine is
located 5 miles northwest of Elsinore, in the same basin. The beds dip
to the west and southwest, having clay below, and sandstone followed by
clay above. The coal is 7 to 8 feet thick, generally solid, but in
places showing a parting in the middle. A great deal of faulting has
taken place, but there seems to be no system about it. The throw of the
faults sometimes amounts to 30 feet, often disturbing the pitch of the
vein, and making it greater. The strata evidently belong to the
Miocene-Tertiary, for a little farther down the valley fossils of that
age are found. This old Tertiary valley, undoubtedly an arm of the sea,
opened into the large valleys of San Bernardino and Los Angeles
Counties, and extended southerly to Temecula; though south of Elsinore
the Tertiary is covered by Quaternary gravels. The depth is unknown, but
the width is quite narrow, being from 1 to 2 miles. Not more than a
quarter of a mile northeast of the coal mine the metamorphic rocks,
quartzites, and hard, blocky argillites outcrop. East of Temescal Creek
northward the mountains are formed of a quartz feldspar porphyry of a
dark gray color; at times it blends into portions not distinctly
porphyritic.
Three miles north of the San Diego County line granite appears again on
the east flank of the Santa Ana range and extends north to Cold Water
Cañon. Between Temescal Creek and the mountains is a broad, sloping
gravel and bowlder deposit of great thickness, resting on the Tertiary.
Two miles south of the Temescal Post Office there is an outcrop of soft
sandstone carrying Miocene fossils. It dips southwest at an angle of
30°. Extensive clay banks of various colors and nearly horizontally
bedded lie along the flanks of the mountains both east and west of the
creek. An interesting series of rocks is exposed up Cold Water Cañon.
This cañon has been eroded near the northern termination of the granitic
portion of the Santa Ana Mountains. The first rock exposed is a
micaceous diorite, decomposed to a great depth, but very tough when
fresh. This is followed by syenite. A mile up the cañon, near the
western edge of this rock and wholly inclosed in it, is a small mass of
jasper schist and a lenticular body of semi-crystalline limestone. No
traces of fossils were found in it. West of the syenite is another
diorite dike. Then follows banded jaspery rocks, sometimes verging on
micaceous felsite or quartzites. There are also some slates, and all are
often greatly contorted; strike north to northeast. North of the cañon
these rocks extend to the summit, while south the Santiago Peak, the
highest of the range, and the ridges leading up to it from the east,
consist of a coarse quartzose granite, with but little if any triclinic
feldspar. A variety of dikes occur near the summit north of the cañon,
among them hornblende porphyry, porphyritic granite, and syenite.
Fossils were found on the ridge leading up to the summit, north of Cold
Water Cañon. They occur in a grayish rock, apparently a fine micaceous
felsite. They are poorly preserved, on account of the extreme degree of
metamorphism to which the rocks have been subjected. The rocks have
become so altered by pressure that they will not break on the lines of
bedding, but perpendicular thereto. The fossils consist of impressions
of a small bivalve shell. Only about a dozen specimens could be found.
The rocks are more altered than any others I have ever seen carrying
fossils. These are the first fossils reported from the metamorphic rocks
of the Santa Ana range. These fossils when determined will give a clue
to the age of the metamorphic gold-bearing rocks of this portion of the
State, and also of the granite, concerning which much diversity of
opinion has existed.
Dawson Cañon, which heads in the Temescal Mountains, was explored and
found to contain interesting geological features. A fine opportunity is
given for the study of the relation of the granite to the extensive
porphyry intrusives. For 2 miles east of Temescal Creek no eruptives
appear; the rocks being wholly of the Metamorphic Series, with exposures
of highly altered sandstones, clay shale, conglomerates, etc., striking
northwest and dipping southeast at 45° to 50°. About 3 miles up the
cañon the argillaceous rocks are replaced by a coarse granite, rich in
mica and quartz. This is the prevailing rock up the cañon for 2 miles,
and it apparently extends much farther east. It shows a great variation
in appearance; much of it contains large crystals of flesh-colored
orthoclase. In this granite, particularly on the north side of the
cañon, there are dikes of many kinds of rocks. Large dikes of beautiful
diorite porphyrites, both light and dark colored, appear in places. At
one spot 4 miles from the mouth of the cañon, there are rectilinear
dikes of fine-grained granite, intersecting each other like artificial
stone fences. For the distance of a mile east, after the granite begins
in the cañon, the hills north show nothing but metamorphic schists. The
porphyry in the mountains south does not reach the cañon. The great mass
of this rock is dark, but in the vicinity of the granite it is lighter
colored and more feldspathic, sometimes assuming a granitic structure.
In places it is a gray, hard rock, of almost conchoidal fracture, and
faint feldspar crystals. The granite near the contact is usually sharply
defined, and has a faintly porphyritic appearance at times. The line of
junction of the two formations is sharply defined, not only
lithologically, but physically. It is difficult to say which is the
older. No granite appears in dikes in the porphyry, but there are many
dikes of a porphyry-like appearance, resembling the light-colored
porphyry in the granite itself. The line of junction is very irregular,
and it is certain that the two formations do not belong to the same
eruptive mass.
East of the head of Dawson Cañon there is another outcrop of
considerable extent of metamorphic rock, micaceous felsites, and other
dark schists. A mile west of the Gavilan Mines is a high conical peak
formed of coarse, dark diabase. The mines of this district are in white
biotite granite, continuous with that of the Pinacate district. The
metamorphic rocks south of Dawson Cañon strike east and west, dip north,
and extend in a westerly direction nearly to Temescal Post Office. North
of the cañon, 2 miles from its mouth, there are a number of outcrops in
vein-like forms and in bunches, of a black crystalline material,
evidently tourmaline, identical with that at the Temescal Mine. These
occur in the metamorphic sandstone and shales. The next large cañon in
the Santa Ana range north of Cold Water Cañon shows highly disturbed
Tertiary strata at its mouth, dipping away from the range at a high
angle. They are soft, white clayey deposits, containing small nodules of
selenite. The first of the older rocks exposed in the cañon is a
hornblende porphyry, with variations to a granular diorite. For 3 miles
up the cañon the only rocks seen are crushed and silicified ones of the
Metamorphic Series. They dip, as a usual thing, at a high angle to the
east, though in spots it is to the west or horizontal. Quartzose
sandstones prevail, with blocky argillitic rocks and conglomerates. Near
the summit there are dikes of green tufaceous porphyries.
Temescal Valley is underlaid by clays of a great variety of colors. They
are being used very extensively for the coarser kinds of pottery and
drain pipe. The Miocene deposits of the valley dip westward from the
Temescal range, and instead of also dipping away from the Santa Ana
Mountains basin-like, they dip west into the latter range. The dip of
these beds 3 miles south of South Riverside is 5° to 10° southwest, and
as the Santa Ana Mountains are approached the dip increases, and at a
distance of a fourth of a mile, up to the metamorphics, it varies from
45° to vertical. The strata are not exposed all the distance across the
valley, but there is no sign of a fold or overthrow; everything seems to
point to a gradually increasing dip. This is indicative of an elevation
of the region toward Temescal, or a sinking of the Santa Ana Mountains.
This is undoubtedly the fault line which follows the range for such a
long distance south. Several thin seams of coal outcrop for a distance
of 10 miles along the base of the mountains. They have been opened in a
number of places and all the strata found dipping into the mountains.
The coal seams are often only a few hundred feet away from the
metamorphics, and dip toward them at a very regular angle of 45°.
Judging from the position of the strata it is not probable that the coal
underlies the valley, and as it is so close to the mountains, formed
wholly of rocks of the Metamorphic Series, it cannot be of great extent.
I believe that appearances point to the whole of the coal beds having
been eroded, save the limited, steeply inclined portion at the foot of
the mountains. A crowding of the strata against the mountains during the
movements along the fault line, have given rise to the steep dip. The
highest portion of the Tertiary beds has an elevation of 1,500 feet. A
very even, gently sloping plain extends from this elevation toward South
Riverside. It is formed of unconsolidated wash from the mountains,
deposited on and dipping in the opposite direction from the Tertiary.
The Tertiary formation consists of clays in various conditions of
consolidation, others chalky in appearance, and a great thickness of
argillaceous quartz-sand loosely cemented. Poorly preserved fossils are
found in places. Near the southeastern corner of Mr. Hoag’s ranch is a
hill with hardened concretionary sandstone outcropping around it. Nearly
every portion of this contains fragments of bones supposed to be
cetacean. Artesian wells are obtained near Temescal Post Office at a
depth of 300 feet. The water is abundant and of excellent quality, and
is flumed to South Riverside.
Bunches of granite outcrop in the metamorphic rocks along the east side
of Temescal Creek north of Dawson Cañon. At the dam of the San Jacinto
Company there is a large outcrop of the beautiful diorite porphyrite,
similar to that seen in Dawson Cañon. This extends northwesterly along
Temescal Creek toward South Riverside, where it is quarried. It makes an
excellent and durable building stone, being compact and free from much
mica or hornblende. West of this is a narrow strip of coarse granite,
followed by diabase. North of Mr. Hoag’s ranch, and west of the dam, is
a dike of black porphyry. Westward the crystalline rocks are overlaid by
the Tertiary.
The dam commenced across the Temescal Creek at this point, where it
enters the cañon, was intended to have been extended down to the
bedrock, and thus bring to the surface the water which flowed beneath
the surface channel. The diorite porphyrite is followed on the east by
black porphyry.
_Geology of the Temescal Tin District._—The Temescal Tin Mine is located
in the northern part of the San Jacinto grant, and about 5 miles
southeast of South Riverside. This portion of the grant consists of
rolling hills. On the west is a large body of porphyry, extending nearly
to the Temescal Creek.
The first rock exposed along the road to the mine east of the creek is a
dark flinty one. This is followed by a body of black porphyry, with
white feldspar crystals. The porphyry is about a mile across, and is
followed on the east by massive black crystalline rock, and that by a
felsite. These rocks are soon replaced by granite, in which there are
dikes of fine-grained, highly quartzose granite. Little black veinlets
of tourmaline aggregates are very numerous in the granite, extending
through all the rock up to the porphyry. They have a northeast
direction. The material forming them is the same as the gangue of the
tin veins. A half mile south of the road is a cañon. Here the porphyry
is seen extending up to the granite. The granite is greatly broken near
the contact, and though there is no blending of one into the other,
there is a confused mixture of broken portions of both rocks. Bunches
and dike-like bodies of granite are inclosed in the porphyry. The little
veinlets of tourmaline seem to have replaced the feldspar and mica,
leaving the quartz. These veins grow larger toward Cajalco Hill. Just
west of the works is a great mass of the black veinstone, the gangue of
the tin ore. This rises in high, rugged croppings, and covers an area of
about 300 by 250 feet. This is the greatest body of vein matter to be
seen in the district. The tin deposit worked lies in an eastern
prolongation of this cropping. The course of the veins is north 45°
east, dip 65° to 70° northwest. The country rock is a coarse hornblendic
biotite granite. The vein has the usual character of mineral deposits,
swelling at times to a width of 8 feet, and then contracting to much
less. The highest grade ore is found in the narrower portions, where it
is sometimes almost pure tin oxide, running as high as 70 per cent. The
vein matter does not consist wholly of tourmaline, but contains quartz
grains scattered through it in about the same proportion as in the
granite. The tin is not found in the quartzose part of the gangue to any
extent, but in the irregular vein-like deposits of pure tourmaline,
which lie in the quartzose gangue. The tin occurs in this in bunches and
stringers of nearly pure ore, or disseminated through it. This is
particularly the case where the width of the vein is 6 to 8 feet. Where
it pinches, the whole vein is sometimes formed of the tourmaline
aggregate and tin ore. The vein has usually clay seams on both walls;
sometimes it is frozen to one wall; wherever the walls come together and
cut out the vein matter, the seams remain. The tourmaline vein matter is
an aggregate of needle-like crystals. There are two varieties of tin
ore: the yellow, appearing in thin layers in an uncrystalline form; the
brown, in granular form in the massive specimens, or in small, clear,
reddish brown crystals lining cavities. In the latter case it forms
handsome specimens. A small amount of arsenical pyrites is present in
places in the vein, and iron pyrites in the granite. The quartzose
portion of the vein matter often blends into the granite walls, and
there are bodies of evidently granitic origin wholly inclosed in the
vein matter. A careful study of the vein matter, and its relation to the
walls, shows that it is simply a portion of the granite, in which the
feldspar and dark silicates, hornblende, and mica have been removed and
tourmaline substituted. The quartz has the same character and color as
that in the granite, and many transition stages in the process are
shown. Where the action has been more intense, near and along the
fissures, the quartz has been wholly removed and the tourmaline
deposited, together with the tin. Cajalco was the center of this action.
The veins decrease in size farther away.
At the time of my visit the mine had been opened to a depth of 180 feet,
by two working shafts. The total length opened on the vein is 300 feet.
Two levels have been run and work was in progress on the third. The main
ore body lies in the center of the workings, and extends downward in the
dip of the veins. The ore milled averaged 5 per cent of tin oxide,
though large portions, as before stated, are very high grade. The
company has also prospected Cajalco Hill by tunnel and open cuts, and
one or more of the veins south by shafts. At the time of my visit two of
Husband’s pneumatic stamps were in operation. They weigh 900 pounds
each, and drop one hundred and thirty-five times per minute.
South and southeast of the works are many bunchy veins of the black tin
gangue. They often carry considerable iron. They extend, generally
nearly parallel, in a northeast and southwest direction. Some appear as
mere bunches on the surface. These veins closely resemble the main vein
at Cajalco Hill, and are due to the same action, and it has been
supposed that many of them will be found to carry tin, though it is not
present on the surface. About 2 miles south the granite is replaced by a
banded feldspar porphyry. This cuts off the tourmaline veins. The
granite about the works, and especially toward the contact with the
porphyry, is cut by many dikes of a fine-grained granite, having an
excess of quartz and feldspar. Associated with the porphyry are strata
of metamorphic rocks, of a hard, dark, quartzose character. A quarter of
a mile northwest of the mine is a bunch-like outcrop of porphyry,
carrying silver and copper carbonate. The black veins outcrop for a
distance of 2 miles northwest from the mine, extending into the
porphyry, which replaces the granite in that direction. The granite
extends eastward for many miles.
The general geological features which obtain here are: A semi-circular
area of granite over 2 miles in diameter, surrounded on the northwest
and south by porphyries and joined on the east to a great body of
granitic rocks extending indefinitely in that direction. Around the
border of this granite protuberance are many dikes of a fine-grained
granite. Cutting through the granite in a northeast and southwest
direction are the black tourmaline veins, which form the gangue of the
tin ore when it is present.
Tin occurs here under conditions different from any other known deposit.
Tin veins are almost always found in granitic formations, but such an
extensively developed tourmaline veinstone is remarkable. The direction
of the fissure system shown here is an uncommon one in California. The
veinstone, together with the associated metals, has probably resulted
from a process of sublimation along lines of fracture, removing those
portions of the granite easily affected, over a large area, as at
Cajalco Hill, and in the immediate contact completely replacing it with
the massive aggregate of minute tourmaline crystals.
Thanks are due to the manager, Captain Harris, for the facility freely
afforded me for examining the mine, the works, and the country about.
North of South Riverside the Tertiary beds dip at a small angle to the
north. The Santa Ana River has cut its course through the hills at the
northern end of the Santa Ana Mountains. No outcrop of the metamorphic
rocks appears in the cañon. The Tertiary strata no longer dip toward the
west, but in the Chino hills north of the river show a great anticlinal
arch. Along the south side of the river the beds dip 70° northeast;
farther west, near the heart of the range, they dip 60° southwest,
strike north 30° west. Near the upper end of the cañon there are fault
lines dipping toward the range, which show an elevation of the hanging
wall. Bedrock Cañon is the first large one which opens to the Santa Ana
River from the western slope of the mountains. Opposite the mouth of
this cañon the greatest amount of water appears in the bed of the river,
indicating the presence of hard rock only a little distance below. Coal
veins are located near the head of this cañon, one 700, the other one
1,300 feet above the river. They dip only a few degrees to the
southwest. They are exposed on cliffs facing the mountains to the
northeast. Below them are very hard sandstones carrying fossils,
probably Cretaceous. One prospect shows a number of seams within a width
of 8 feet. The widest is 29 inches, the others much smaller. The other
prospects show only one 39-inch seam. Their position, lying so flat high
on the mountains, indicates an uplift without great disturbance, while
the gypsum mines farther down on the flank of the mountains dip at a
high angle to the southeast. It may be that all of the coal deposits of
the western slope of the Santa Ana Mountains belong to the Cretaceous,
and have been greatly separated by faulting and folding. A deposit of
white, granular gypsum has been opened in Gypsum Cañon, 2 miles south of
the river. The beds have a thickness of 8 or 10 feet. At one spot a
large mass of crystalline dolomite was found. The deposits run with the
strata, north and south, and dip west 60°. As we approach Olive, the few
croppings seen still dip south or southwest, but at a less angle. South
of the mouth of Silverado Cañon a line of hills extends north and south,
bordering the Santa Ana Plain. The western portion of these hills is
formed of basalt considerably decomposed. The basalt varies from
scoriaceous to fine-grained and compact. Its eastern edge was seen to
rest on Miocene sandstones, and it dips west at a small angle, perhaps
10°. The lava seems to have been squeezed up in fissures, judging from
the way in which it outcrops. Its greatest elevation is 800 feet. At
some places the sandstones, where not covered with lava, have been
silicified, turned to quartzite, or rendered granitic in appearance.
This may be due to an intrusive neck of lava, or more probably to the
action of thermal springs.
An interesting fold of the Tertiary strata was observed at the entrance
of Santiago Cañon. The sandstones and conglomerates on the eastern side
dip to the northeast at an angle of 30°, while those on the west side
dip in the opposite direction. The valley has been eroded in the summit
of an anticlinal. The rocks of the eastern side rise again against the
side of the mountains, thus forming a synclinal. Up the cañon the
sandstones on the west maintain a southwest dip of 45° to 50°, and
strike north 40° west. The cañon finally leaves the anticlinal, and the
rocks dip southwest on both sides. Toward the summit of the hill, north
of the Harris Coal Mine, the dip increases to 70°, but on the top they
turn so that the strata lie horizontal. Here they consist of clay shale.
The strata at the coal mine swing around, and one mile northeast they
strike north and south and dip west. This hill seems to form the
southern termination of the anticlinal ridge north of Santiago Cañon.
Southeast of this point there is a simple monoclinal fold or slope away
from the older rocks of the high mountains. There has apparently been a
fault extending northwest in this anticlinal ridge, bringing up the clay
shales which farther south were shown to belong to the Cretaceous.
Harris Coal Mine shows a seam 18 inches wide, shale forming the foot
wall and sandstone the upper. There is a fault of 200 feet cutting this
coal seam. The sandstone at the mouth of Silverado Cañon dips south 30°,
forming bold cliffs. A half mile up the cañon there are heavy beds of
clay shale inclosed in the sandstone. Cretaceous fossils appear in the
shales, as well as in a coarse sandstone which underlies them. This
sandstone is replaced by conglomerates near the contact with the
underlying Metamorphic Series. The sandstone rises to a height of 2,500
feet, with bold, almost perpendicular cliffs facing the mountains.
Portions of the sandstone containing the fossils are often very much
hardened.
The first crystalline rocks met are dark and fine grained, with traces
of bowlder-like inclusions, and are evidently eruptive tuffs. Above
these are green, dioritic rocks. These intrusives are followed for
several miles by sandstone and shale, in which the stratification is
often obliterated. In other places thin layers of sandstone and shale
are wonderfully contorted. The dip is at a high angle either east or
west. In the vicinity of the old Silverado Camp there are dikes and
bunches of a green dioritic rock. The mines in the Silverado district
are again being developed to some extent. The mineral belt is about 2
miles wide, and extends nearly north and south. The country is formed to
a great extent of dikes of greenish to blackish rocks, often showing
distinct hornblende crystals. The dip of the metamorphic rocks is east
about 45°. There is one main mineral vein located, beginning about a
mile north of Silverado, and extending in a southerly direction for 7 or
8 miles. The Quincy Mine is one of the most northern ones. The vein has
a width of 2 feet; the ore, silver-bearing galena in a calcite gangue.
It carries but little base metal of any kind. The ore has a peculiar
appearance, the galena being distributed through the gangue in little
leafy crystals or aggregates. The fissure is well defined and regular,
with a pale green syenitic rock on the hanging wall, and a dark diorite
on the foot wall. This hanging wall rock weathers to a light gray color,
producing a rock known as porphyry among the miners. South of this mine
a side vein carries much antimony. The Quincy has been opened along a
length of 500 feet. The ore is quite uniform, producing one eighth in
concentrates. The Quincy camp has an elevation of 2,300 feet. South of
Silverado Cañon, in Silver and Pine Cañons, a great amount of work was
done during the former excitement. The sides of the steep, rocky cañons
are fairly honey-combed with tunnels, which were undertaken without
sufficient prospecting, and, of course, never struck anything. The New
York Mine spent much money, but did not prove a success. West is the
Princess, and farther still, about 1,200 feet above the cañon, is the
Blue Light Mine, on which much work has been done and rich ore taken
out. The mines south of the cañon are in a feldspathic rock, which
weathers white. It is undoubtedly an intrusive porphyry, for traces of
feldspar crystals are to be seen. The mines are characterized by a large
amount of zinc-blende, iron pyrites, and not a large percentage of lead,
making them more difficult to reduce. The porphyry is mineralized in
many places where no traces of the precious metals occur. Litigation and
poor management seem to be the chief factors in stopping work in this
district. Though some of the ore runs into hundreds of dollars to the
ton, the most of it is medium to low grade.
About a mile and a half down the cañon from the old Silver Post Office,
and in a cañon coming in from the south, is a cropping of dark, somewhat
argillaceous limestone inclosed in shales. The limestone does not seem
to have been highly metamorphosed, yet the fossils which it contains are
almost obliterated. The faint impressions are those of coral stems,
stromatopora, and some other low forms of life. Half a mile farther down
the cañon is a cropping of brecciated marble. At many points,
particularly on the north side of the cañon, there are great masses of
apparently conglomeritic character, but with a crystalline structure.
The matrix has a green to brown color, and in it are imbedded pebbles of
the same degree of fineness, but often distinguished by much brighter
red, purple, and green colors. In this cañon, as in others of this
range, the water holds much lime in solution, and extensive tufas are
frequently to be seen. The basal members of the Cretaceous at the mouth
of Silverado Cañon consist of conglomerates passing up into sandstone,
and those into shales; dip 55° away from the mountains in the highest
ridge, but in the course of a quarter of a mile becoming much less. The
change in dip is very sharp, giving the appearance of a fault.
A cañon which enters Silverado Cañon from the northeast near its mouth
was followed up nearly to its head for the purpose of investigating some
limestone outcrops. The first outcrop in this cañon is the usual dark
porphyry. Beyond this the stream has cut a deep cañon through an immense
conglomerate of porphyritic and quartzose pebbles. The porphyritic
pebbles are dioritic and part red and black porphyries. Some of them are
similar to dikes farther up the mountains. Through this great
conglomerate bed there are dikes of black porphyry, with pale feldspar.
Tufaceous porphyries form a large part of these dikes. The base is
purple and the pebbles light green, or the reverse. These conglomeritic
porphyries differ from the great beds of sedimentary origin, in having
all the pebbles of a uniform character with a crystalline matrix. The
sedimentary beds contain pebbles of all sizes and description, in a
matrix of small pebbles or coarse sand. Farther up the cañon the great
body of the rock is crushed shale and black to gray sandstone; dip
vertical, inclining most generally to the east, strike north and south.
Two miles up the cañon is a dike of diorite porphyrite, coarse in the
middle and fine on the edges. Four miles up is a stratum of gray
limestone. More outcrops appear on the north side of the cañon, but
whether they belong to the same stratum or different cannot be told, on
account of the crushing undergone. These deposits are bunchy, swelling
in one case to a width of 100 feet. It is not crystalline. The color is
from black to gray. It contains fine specimens of a bivalve shell, and
faint traces of corals and univalve shells. These beds are said to
extend to the summit, and undoubtedly further examination would reveal
more fossils. Several specimens were sent to the National Museum and
pronounced Carboniferous in age. We have here, then, the first
announcement of the age of the Santa Ana range. This range stands in
such intimate relation to the granite and crystalline schists farther
south, that an approximate determination of the latter is made possible.
Professor Whitney and others following him have classified this range as
Cretaceous. Their grounds are utterly untenable stratigraphically, but
this discovery of Carboniferous fossils makes the evidence of greater
age certain.
On the eastern side of the range, near its northern end, the sandstones
were observed to be silicified, being filled with a network of quartz
veinlets, exactly similar to the silicification of the metamorphic rocks
of the Coast Range proper.
The lowest Cretaceous beds are between Silverado and Williams Cañons,
but the fossils are very similar to those generally found through the
Cretaceous of this region. The lowest beds are a duplicate of those seen
at the mouth of the Silverado Cañon, though apparently a thousand feet
lower in the series. Between the two beds there is another conglomerate
stratum, carrying bowlders different from any seen in the Santa Ana
Mountains. Many of the porphyry bowlders in the basal Cretaceous
conglomerates resemble the porphyry about Temescal. The Santa Clara Coal
Mine, at the mouth of Silverado Cañon, is undoubtedly in strata of
Cretaceous age, though no fossils are found in the strata above till the
Miocene is reached. Up the Santiago Cañon as far as Madame Modjeska’s
place, the fossils are chiefly confined to the eastern side of the
cañon, but the character, dip, and strike of the strata on both sides
are the same. The Cretaceous is separated from the metamorphics by a
line of cross cañons. The highest portions of the Cretaceous terminate
in a line of hills with sharp eastern escarpments.
Shrewsbury Cañon comes in just below Modjeska’s. After passing the
Cretaceous, through which the cañon has cut, the Metamorphic Series was
seen to consist of slates and sandstones, followed by light-colored
granitic rocks, chiefly hornblende and triclinic feldspar. The
metamorphic rocks generally dip to the east.
A mile above Modjeska’s the Santiago Cañon cuts through bold cliffs of
Cretaceous sandstones and conglomerates, and higher up still has eroded
a cañon in the Metamorphic Series. The cliffs, a little back from the
stream, rise probably 1,000 feet. Sandstone belonging to the Metamorphic
Series outcrops in the cañon for some distance, and is followed by
conglomeritic porphyries, containing purple and red bowlders. In some
places the inclusions are angular. These rocks are extensively developed
about the Santiago Mines and higher up in the mountains.
Just below the Alma Mine, on its western side, the creek has cut through
dikes of diorite, which are coarse in the center and fine on the edges.
On the hill south of the Alma Mine the diorite is on the opposite or
eastern side of the vein. Although the exposures are poor, all the
crystalline rocks have the character of intrusives. No blending into the
metamorphic rocks has been noticed. About 3 miles up Shrewsbury Cañon
several claims are located on the southern continuation of the veins of
the Silverado district. Near the head of Santiago Cañon these again
appear well defined, and considerable active work was going on here at
the time of my visit. The veins here have a direction a little east of
south, dipping to the east somewhat less than 45°. It appears that the
mineral belt follows a certain line, generally quite regular, without
any particular reference to the dikes and bunches of intrusives
scattered irregularly here and there. The fissure system has taken a
comparatively regular line and the walls may or may not be intrusive.
The Alma Mine, the northern claim of the Santiago Silver Mining Company,
has been worked by the former operators in a very irregular manner. The
veins and stringers as far as exposed lie wholly in the crushed
quartzose argillites. A tunnel is now being run to open what is supposed
to be the main vein, several hundred feet farther east, and which has
syenite on the hanging wall. Other tunnels are being run along the
creek, one near a body of granitoid rocks, others in slate or quartzite.
The ore is a leafy galena, arranged often in narrow bands with the
crystalline orientation different in alternating bands. Sometimes it
occurs in the form of large solid bunches. Thus far the ore deposits
have been found quite irregular, but it is thought that the main body
has not yet been reached. Very little base metal is present. South, on
the hill, the Morrow claims have been bonded by the same company and are
being opened. On the summit of the hill diorite lies on the eastern side
of the veins. Down the southern slope the veins lie wholly in
metamorphic rocks; the foot wall being more silicious, the hanging
argillaceous. The mineralized portion has a width of 20 feet. A small
vein occurs in the upper side, and a heavier one, sometimes reaching 3
feet, on the foot wall. There are occasional stringers in the crushed
portions between. The gangue is calcite, with some quartz, all mixed
with the broken clayey slates. More zinc-blende and iron pyrites are
found here, also a little antimony. In the Alma Mine the richest galena
is very fine, and sometimes resembles antimonial ores.
South of these mines, toward Trabuco Cañon, another claim is being
worked. South of the Trabuco the conglomerates hide the lode, except at
one spot, where it appears and has been worked. The Cretaceous formation
grows lower as the Trabuco is approached, and does not appear prominent
south of it. In all probability it is covered by the Tertiary, for the
elevation and consequent erosion have not been as great in this
direction.