By Harold W. Fairbanks, F.G.S.A.
The topography of this region has been quite thoroughly described by W.
A. Goodyear, in former reports of the State Mining Bureau. The structure
of San Diego County is comparatively simple. Three main divisions might
be made: the desert on the east, the Peninsula range of crystalline
rocks in the middle, and the nearly level mesa on the west. The
Peninsula range is supposed to represent the southern continuation of
the Sierra Nevadas, but in just what relation it stands to the Sierras
has been a matter of dispute. The Peninsula range in San Diego County
forms one main mountain chain. It maintains this simplicity of structure
southward, forming the backbone of the peninsula of Lower California.
Northward it becomes broader and more complex, rising in the lofty San
Jacinto and San Bernardino ranges on the east, and the Santa Ana range
on the west, while the region between is filled with mountains and
valleys irregularly disposed.
Complex as is the topography of this region, the geological problems,
though often difficult to solve, are quite simple. The higher mountains
are formed wholly of ancient crystalline schists and massive rocks,
respecting the age of which a great diversity of opinion has existed;
while the region bordering the coast consists of unaltered Cretaceous,
Tertiary, and Quaternary deposits.
Owing to the very limited time given me to prepare my field notes for
the press, they will be given substantially as they were taken in the
field, without any attempt at systematic arrangement.
The crystalline rocks of San Diego County are varied in character, and
of much interest. No opportunity has been given me to study the large
collection made, and the determinations given are simply the result of
superficial examination, and are subject to correction.
The bay of San Diego is bordered on the east by gently sloping mesas of
modern Tertiary and Quaternary age. These unaltered strata are
characteristic of the western slope of the Peninsula range through its
whole extent. They sometimes rise as high as 3,000 feet; though in San
Diego County they do not exceed 1,500 feet. The upper portion of these
beds consists to a great extent of coarse, loosely cemented
conglomerates. The rivers issue from the higher mountains through narrow
valleys or cañons, and have cut valleys, often quite broad and with very
steep sides, through the mesas to the ocean.
The Otay mesa has a height of about 500 feet, the western portion being
somewhat higher than the eastern, indicating a recent elevation near the
coast. The soil of the mesa is adobe, due to the decay of porphyry
mountains to the east. Under the adobe there is a calcareous marl, often
many feet thick.
The first exposure of the older rock seen as one goes up the Otay River,
is in a hill rising through the mesa about in the center of the grant.
It is a part of the extensive porphyry intrusives, which, in southern
San Diego County, form a number of high mountains between the granite
and the mesa. To this formation belong the San Miguel and Otay peaks.
This exposure on the Otay River is a felsitic breccia. It contains a
felsite base (intimate mixture of quartz and feldspar), in which are
imbedded fragments of felsite and chlorite. No more rocks appear for
about 2 miles up the river. Then we reach the base of the long ridges
which lead up to the Otay Peak. Some interesting rocks are exposed where
the stream issues from the cañon. The greater portion are fine dark to
greenish aphanitic rocks, with green chloritic or epidotic nodules.
Bunches and dikes of coarse to fine grained porphyritic rocks
occasionally appear. They probably belong to the diorite porphyrites.
The rock continues very much the same for several miles farther east; at
times it is almost wholly feldspar. In the cañon above El Nido Post
Office it changes to a light green feldspar porphyry. Near the western
edge of the Jamul grant a dark-colored porphyry takes its place, and a
little farther east it becomes jet black, with small white feldspar
crystals, producing a very pretty effect.
The mesa conglomerates extend along the top of the low hills bordering
the valley nearly to the eastern edge of the Jamul grant. A great
variety of rocks appear along the Campo road between the Jamul grant and
Sheckler’s, on the Cottonwood. Near the eastern end of the grant the
porphyry is followed by fine-grained granitic rock, frequently becoming
schistose. Numerous dikes and bunches of dark diorite cut through this
rock. As Dulzura Post Office is approached, these rocks change to mica
and hornblende schists, and are filled with intruded dikes of diorite
porphyrites. Bodies of massive syenite and coarse granite were also
seen. About Dulzura many of the dikes have the appearance of diabase.
Between Dulzura and Sheckler’s the country rock is largely micaceous and
chloritic schists. Massive granite forms the high, rugged mountains
east, extending in an arm westerly across the road. The schists have a
northwest strike, vertical dip, and are evidently of metamorphic origin.
They form a strip of country extending in the line of strike from near
Sheckler’s to the Sweetwater River, and are situated between the wide
belt of porphyry on the west and the coarse intrusive granites on the
east, which rise to form Lyon’s Peak and other rugged mountains.
The first rock met east of Sheckler’s, on the Campo road, is coarse
hornblendic granite, so decomposed that a fresh specimen could not be
obtained. Dikes of fine-grained granite intersect it in every direction.
Three miles west of Potrero, mountains of olivinitic diabase rise on the
north side of the road. This rock is very similar to many large bodies
of intrusives through the mountains between Julian and the Tia Juana
River. It has evidently been intruded into the granite, for dikes extend
out, intersecting the latter rock.
Potrero is located in a valley of several hundred acres in extent, and
surrounded by granite mountains. It has an elevation of 2,400 feet.
South of Potrero, along the boundary line, the mountains show large
areas of the dark dioritic and diabasic rocks. The hills immediately
south of the valley consist of hornblendic gneiss; strike east and west.
Eastward, toward Campo, the rock is chiefly a coarse white granite, very
easily decomposed. It shows a slightly gneissoid structure for a number
of miles. It does not seem to represent the bedding of a sedimentary
rock, but of parallelism of the constituents, induced in the magma by
movement or pressure. Long, drawn out, lenticular inclusions are often
present, and are arranged parallel to the schistose structure. These
consist largely of hornblende, with little feldspar.
In the vicinity of Campo the topography of the country changes from that
of high mountains and deep, narrow valleys, to an elevated mountain
plateau with meadows and rounded granite ridges. The mountains are
covered with brush, while live oaks are numerous in the valleys. The
country maintains these features while gradually rising to the divide 8
miles east of Campo. The granite is so deeply decomposed along the
summit region that no good samples could be obtained. Campo has an
elevation of 2,600 feet. The bare, rounded ridges closely resemble those
left by glacial action, but their slope is produced simply by the
cleaving off successively of the more angular portions in great slabs.
Many fine examples of this manner of decay appear about Campo. The
corners are decomposed faster than the smooth surfaces, and thus finally
a shelly concentric structure results. The fresh massive central portion
weathers out like water-worn bowlders. The presence of rugged angular
ridges results either from a less inherent tendency to decay, or to a
comparative freedom from crushing. Four miles northeast of Campo is an
outcrop of coarse hornblendic granite, with large six-sided mica scales
and numerous yellow crystals of titanite. The height of the divide is
3,800 feet. Near the summit the rocky ridges all disappear and the
country becomes covered with granitic sand. Erosion here is evidently
very slight. The country descends gradually on the east to Jacumba
Valley, being sandy for some distance. This finally gives place to bare,
rocky ridges and cañons. Veins of fine granite, and others of feldspar
and quartz, are abundant on the eastern slope.
Before reaching Jacumba Valley a body of mica and hornblende schist is
encountered. The schists do not form a regularly defined belt, but often
appear as inclusions in the granite. These inclusions have a very
variable strike, and from their relation to the granite it is evident
that the latter is intrusive.
Jacumba Valley empties northward into the desert through a narrow gorge.
It has an elevation of 2,600 feet, the same as that of Campo. It is
several square miles in extent, the greater part of which is in Lower
California. The warm springs here are considered quite medicinal. The
schists just described occupy a large area west and north of the cañon
through which the valley empties. They are cut in every direction by
dikes of granite and others, consisting of a very coarse aggregate of
quartz and feldspar with a little muscovite mica. A high mountain
several miles north of the valley is distinctly ribbed all over by them.
The schists extend northward toward those which outcrop on the eastern
slope of the Laguna Mountains and at Julian, but are cut off by a body
of intrusive granite. They undoubtedly belong to the same series.
Gold-bearing veins have been found in them a little north of Jacumba
Valley.
At the north end of Jacumba Valley, and on the west side of the outlet,
is an area of volcanic rock, probably basalt. It forms a table-land,
gently sloping toward the valley, and rising 600 or 700 feet at its
northern end. It is underlaid by gravels and conglomerates. Just east of
this is a black butte, rising perfectly symmetrical to the same height.
It consists of bedded lavas, with tufa at the bottom. In spite of the
fact that it is shaped like a crater, its structure is different, and it
is probably a remnant of the flow which once covered the outlet to the
valley.
The high range of mountains between Jacumba Valley and the desert has an
altitude of something over 4,000 feet, but where the road crosses it, it
is only 3,100 feet. Basalt outcrops also on the eastern side of the
valley. North of the road to Mountain Springs it forms a series of
plateaus, the highest of which reaches a height of 3,900 feet. It forms
the summit of the range, being 800 feet above the granite forming the
pass. South of the pass several miles the granite rises much higher and
the lava lies along its western slope, extending an unknown distance
below the line.
Large deposits of water-worn bowlders and gravels lie along the eastern
slope of Jacumba Valley. Among them are pebbles of porphyries, black
quartz, and others not seen in place in this part of the county. A short
distance west of the summit they are found in beds with gravel and
sandstone, dipping southwest. These late Tertiary deposits are overlaid
by the volcanic beds. The volcanic plateau which rises so high north of
the pass has a thickness of 500 to 600 feet. Massive and bedded lavas
form the upper half of this thickness, the lower portion consisting of a
volcanic breccia. The beds lie nearly horizontal. On the west are two
lower terraces, also capped with lava and abutting against the higher.
The whole is underlaid by sand rock of granitic origin. It is nearly
level in places, in others it dips to the southwest. It is very strange
that these lava beds, with nearly level flowage lines, should be found
at such greatly varying elevations about Jacumba Valley, and be
underlaid everywhere by such similar tuffs and sandstones. My
investigations disclosed no volcanic vent, and it is possible that the
lava issued from fissures, as was noticed elsewhere in the county.
Another interesting question is the origin of the sandstones and
conglomerates. The sandstone underneath the high plateau is higher than
the divide at that spot, and the only granite within miles that exceeds
it in height, is the narrow ridge which rises on the southeast. The
erosion must have been very great along the ridges since the sandstones
were deposited, but the valley cannot have changed much. There may have
been great elevation along the crest of the range bordering the desert
since the deposition of sandstone, tilting up the sandstone and lava on
the eastern slope, but elevating without great disturbance those near
the summit. Southeast of Mountain Springs is a body of bedded tufas
reaching an elevation of 2,300 feet, and dipping to the east away from
the range at a considerable angle.
The presence of these modern sandstones at so great an elevation nearly
on the crest of the Peninsula range is a very interesting fact. Either
Jacumba Valley was a lake, or a great elevation has taken place in
comparatively recent times, raising the valley from the sea-level.
Appearances indicate that during late Tertiary times this range was
almost submerged beneath the sea.
The rocks between the summit and Mountain Springs are chiefly gneissoid,
at times granitic. They contain bodies of fine dark mica schist, and
many dikes of very coarse muscovite granite. The descent to the desert
is very abrupt over bare granite ridges. Mountain Springs, an old stage
station, is located on the side of the mountain at an elevation of 2,300
feet. From the springs the road descends along the dry bed of an arroyo
to the desert. The most of the distance is through a rocky cañon, where
there is an excellent opportunity to study the relations of the gneiss
and granite. For some distance down from the springs the rocks continue
to be gneissoid, but through the lower end of the cañon they become more
massive and coarse, and all the veins characteristic of the gneisses of
the higher mountain region disappear. At the upper end of the cañon is a
dike of very coarse granite, with large biotite crystals instead of
muscovite. This is the only instance in which biotite was seen in one of
these coarse dikes. Banded gneiss, varying from very thin to very thick
bedded, alternate with other rocks, to all appearances massive granites,
but in surface decay the latter break up into slabs of varying
thickness, parallel to the schistose structure of the gneisses. The
banding is caused by an excess of mica or hornblende, chiefly the
latter, arranged in parallel layers. These strata are often very thin,
varying from one fourth to one half inch and upwards in thickness. They
are very regular, but often discontinuous; stop, and in course of a few
feet begin again. These features are generally supposed to indicate
metamorphic origin, but at one spot a body of dark mica schist is cut by
a dike a foot wide or more of this dark banded gneissose rock. This dike
cuts across the stratification of the mica schist, showing conclusively
the intrusive nature of at least a part of these gneisses; and it is
quite possible that the inclusions of mica schist are the only really
sedimentary rocks present. In places the rocks which show this banding
have the constituents arranged in the bands independent of any
direction. At one spot a distinct, well-defined mass of mica schist, 15
feet across, is imbedded in a granitic rock. At one side this gneissoid
structure extends through the inclosing rock and abuts sharply against
the mica schist. The banding shows no constant direction; in the cañon
it is northeast. The bands sometimes become wavy.
As the cañon opens out to the desert, hills appear on either side formed
of volcanic tuffs. They dip northeast 30°. Underneath is a sandstone
wholly unconsolidated and dipping in the same direction 40°. This
contains no lava pebbles. The fragments of the tuff are quite varied in
character and generally quite angular. They are imbedded in a volcanic
mud, free from granitic detritus. In some of the strata appear thin beds
of lava, seeming to represent a flow. These tufa hills extend
northwesterly along the base of the granite mountains for 10 miles or
more. It is not known how far they go in a southerly direction. In
places they form mountains of considerable size high up on the side of
the range. The range of mountains between this point and Carrizo Creek
appears also to have some volcanic beds on its southern slope. The open
desert at the foot of the mountains has an elevation of 1,200 feet. It
slopes gently for miles in an easterly direction and consists largely of
loose sand.
Between Mountain Springs and the summit is another illustration of the
fact that lamination in a crystalline rock is no proof of its
sedimentary origin. A small dike less than 2 inches thick cuts across a
coarse biotite gneiss at an angle of 30°. It is separated from the
gneiss by a thin layer of quartz and feldspar. It is made up of the same
constituents as the gneiss, arranged so as to show a well-pronounced
gneissoid structure. This is very similar to the large dikes seen in the
cañon.
The road was followed back to Campo, and from there the Laguna Mountains
were climbed. The road ascends a long, narrow cañon on the southern
slope. At the entrance to the cañon, 4 miles southeast of Buckman’s
Springs, the mountains are high and rocky, being formed of thin-bedded
gneisses, which, in many places, blend into mica schists. They strike
parallel to the mountain axis, a little west of north; dip 70°
northeast. Three miles up the cañon the gneiss becomes thick-bedded and
is finally replaced by mica diorite, which forms the hills on both
sides. Granitic dikes outcrop near the junction and sometimes apparently
in the diorite. There is often a blending between the two, as if the
intrusion of both took place nearly at the same time. The region east of
the southern end of the mountains is formed of coarse granite,
decomposed to a considerable depth. The mica diorite extends northward,
forming the whole central and western part of the mountains. On the east
it is bordered by a slightly higher ridge, forming the crest of the
mountains. This rock does not decompose as easily as the granite and
gneisses on the west, and there consequently remains a mountain plateau
having an elevation of about 5,500 feet. There is a considerable amount
of pine timber and open meadows. The dark diorite forms one of the
highest peaks of the Laguna Mountains, rising 6,250 feet. The highest
portion of the mountains lies to the northeast, and is formed chiefly of
a quartzose mica schist. From the eastern crest of the range a most
magnificent view of the desert is obtained. The strata on the crest
strike north 15° west, dip 70° northeast. The descent of 4,000 feet to
Vallecitos is very abrupt. Near the crest it is almost as steep as the
dip of the rocks. The mica schists in places approach gneiss in
composition, but all of this series of rocks forming the crest and
eastern slope very probably belong to the Metamorphic Series. South of
this point the Laguna Mountains do not terminate so abruptly, but extend
out in long, gradually descending ridges for many miles. The mica
diorite extends north of the Laguna about a mile, when the schists on
both east and west sides unite and extend north toward Banner. They are
intruded by granite and diorite in many places. The body of diorite
forming the Laguna plateau is about 8 miles long and 1½ to 2 miles wide.
The highest peak is not over 200 feet lower than the Cuyamaca, rising
6,300 feet. The diorite seems to have been intruded in the middle of a
considerable area of mica schists, for this rock outcrops on all sides.
On the west, toward Pine Valley, they carry the gold veins of the Pine
Valley district. The descent is very abrupt to Pine Valley and Buckman’s
Springs. The schists and gneisses extend about 3 miles south of the main
portion of the mountains, when they are replaced by coarse hornblendic
granite.
Buckman’s Springs has an elevation of 3,400 feet. Here are some very
excellent soda springs, the only ones I know of in this section of the
State. A coarse, dark diabase outcrops in the edge of the mountains just
north of Buckman’s and also a little farther south, on the west side of
the valley.
A narrow cañon leads up to the divide which separates the valley of the
upper Cottonwood from Pine Valley. The western prolongation of the
diorite of the Laguna Mountains appears near the road on the divide. The
prevailing rock is, however, of a granitoid nature and filled with many
large bunches of massive white quartz. Pine Valley has an elevation of
3,800 feet. Gneisses and hornblendic and micaceous schists outcrop
between the valley and the divide east of Descanso. The strike is north
and south. One mile southeast of Descanso there is another outcrop of
the coarse diabase or gabbro which forms so much of the Cuyamaca peaks.
Descanso has an elevation of 3,400 feet. The rock which outcrops for a
number of miles along the road to Stonewall is a coarse, easily
decomposed granite, rising in rounded knobs over a rolling,
brush-covered country.
The Pine Valley district lies in a belt of gneissose, mica schist, and
quartzose rocks, which extend in a direction a little west of north.
They begin about 2 miles south of Pine Creek and extend, probably
unbroken, through to Banner and Julian. It is 4 miles north to the Deer
Park district. The metamorphic schists widen as this district is
approached. They extend from the desert slope to Deer Park, where a body
of diorite has been intruded, and from there westward 2 miles to the
Cuyamaca grant. A half mile west of the camp the slaty mica schists and
quartzites are well defined. A vein of gold-bearing quartz has been
traced for several miles in these rocks, and a number of locations have
been made on it. The strike is north and south, dip 80° to the east. A
body of white crystalline limestone lies in this formation, about 1½
miles southwest of Deer Park. The body of micaceous diorite which
extends through this camp is about a mile long, and is quite gneissoid
in places. Near its southern end it incloses narrow bands of hornblendic
and feldspathic rock, containing garnets. The veins are numerous in this
diorite, and extend in every conceivable direction. The diorite has been
greatly crushed at some time, with fissures in every direction; hence,
the irregularity of the quartz veins. The veins are generally bunchy,
and not often very large. They seem simply to follow the exceedingly
irregular fracturing of the diorite, which is decomposed to a
considerable depth. The work thus far in this district has been confined
to the surface, so that nothing can be said in regard to the permanency
of the veins. The elevation of the camp is 4,600 feet.
Two miles northward an outcrop of coarse biotite muscovite granite was
met. It is about 2 miles long and 1 mile wide, having schists on all
sides of it. This is the only example of typical granite (according to
Rosenbusch) that was seen in Southern California. A broad valley,
occupied by Mr. Harper’s ranch, has been eroded in the center of this
granitic mass.
Northward, along the crest of the range overlooking the desert, mica
slates appear. They are so little metamorphosed as to closely resemble
argillaceous slate. They strike a little west of north, dip 70° to 80°
east, and maintain the same character north to Banner and Julian.
The road from Julian to Stonewall crosses mica schist and gneisses for
about half the distance. Then we met outcrops of a dark basic rock,
ranging from fine to very coarse texture. It extends southward and
covers a large extent of country. The three Cuyamaca peaks, the highest
in the southern part of the county, are formed of this rock. This rock
was again met about half a mile west of the Stonewall Mine. It extends
westward across the mountains for at least 2 miles, and some miles south
of the main peak. In places it has a schistose structure. Near the
outlet of the Cuyamaca reservoirs it is filled with vein-like aggregates
of coarse hornblende crystals, which are probably the result of
secondary crystallization in fissures or cracks of the almost
consolidated magma.
The formation in the vicinity of the Stonewall Mine is biotite gneiss.
Toward the east it is not sharply defined from the mica schists. On the
west it is generally more granitoid, though at one spot near the lake
there are finely laminated schists, dipping 70° southwest. Southward,
also, the gneisses become more massive and coarse. At the northeastern
base of the main peak a body of coarse granite is partly inclosed in the
norite. This norite assumes a dioritic habitus near the granite, with
the development of biotite mica.
The valley of the Cottonwood was followed down several miles, when it
was left and the Morena Valley traversed to its head. The mountains
south of the valley are formed of a dark syenite. North and northwest
the higher mountains consist of granite, with a great development of
mica and hornblende schists along the slope facing the valley. Many
granite dikes have been intruded into the schists. North of Mr.
Candler’s there is a great dike of pegmatite, carrying small garnets,
tourmaline, mica, and large masses of quartz. This pegmatite dike cuts
across the cañon, forming a precipice on its lower side. On the upper
side the soil has been retained, forming a small but fertile valley. The
strike of the mica schists on the north is quite unusual, being north
75° west, dip nearly vertical. Dark syenite lies on the west of this
little valley. Morena Valley has an elevation of 3,400 feet. A rough
road leads over the mountains west to McClain’s ranch, a distance of 10
miles; the highest point reached having an elevation of 4,000 feet. Near
the summit there outcrops a body of coarse eruptive rock, probably a
diorite. It consists of coarse hornblendic aggregates in a light-colored
feldspathic matrix. This is followed by syenite, and that by coarse
white granite as far as McClain’s. The granite about the valley is
coarse, with dark, fine crystalline inclusions. It has every character
of a truly eruptive rock, even to the minute spaces between the
components, left at the time of crystallization. The road now descends
to a branch of the Cottonwood, and from the creek there is a long,
gradual ascent to the pass north of Lyon’s Peak. The north side of the
pass is formed of another high and rugged granite range. This section is
one of the wildest to be found in San Diego County. The mountains are
bare granite, often precipitous; the valleys small and covered with
brush. Crystalline limestone is reported to have been found in the
granite 5 or 6 miles northeast of Renney’s Pass. A very interesting
eruptive rock outcrops east of the pass, on the north side of the road.
In places it resembles the norite from Cuyamaca peaks. The rock consists
largely of large, dark, cleavable crystals filled with small granular
crystals of a green color, probably olivine. A large portion of the dark
crystals are undoubtedly pyroxene, though there are some showing
hornblende cleavage. This is one of the most peculiar and interesting
rocks seen in the county, but, from lack of any opportunity to make a
microscopic examination, no more definite definition can be given.
Coarse granite extends some distance west of the pass. It is decomposed
to such a depth that no specimens could be obtained. A mile west of the
pass a fine granite, apparently metamorphic, replaces the other. It
contains much quartz, little mica, and yellowish feldspar. Some distance
down the new grade a large body of diorite outcrops. It is one of the
southern arms of a great mass of coarse, dark rock which forms the high
mountains about Dehesa Post Office, on the Sweetwater. A large portion
of this rock very closely resembles that forming the Cuyamaca, and is
probably a gabbro.
Near the foot of the grade, 2 miles east of Jamul Post Office, this rock
is very coarse, with large hornblende crystals. It extends out in the
form of arms or dikes into the adjoining granitic rocks. This rock is
very tough and heavy. Gneiss outcrops for 2 miles westward. It varies
between thin-bedded micaceous strata and thick-bedded, almost granitic
forms. It strikes north 15° east, dips 70° to 80° east. Farther down,
toward the Oakdale House, this is replaced by coarse eruptive granites.
Just below the Oakdale House there is a very interesting contact between
granite, quartz porphyry, and diabase. The first rock exposed below the
house is a rather dark micaceous syenite. Beyond this the rock becomes
coarser, containing large grains of quartz and glassy feldspar, with
inclusions of a very dark diabasic rock. In a little cañon which comes
down to the road from the east, this rock comes into contact with one
which varies from a feldspathic mica schist, through a gneiss, to a
quartz porphyry. The junction is very irregular and the two rocks are
slightly mixed; sometimes branches of the syenite are partly inclosed in
the porphyry. Some portions of the syenite show gas pores, or spaces
left at the time of consolidation, one fourth to one half an inch in
diameter. They are partly filled with secondary quartz. The next rock
exposed up the cañon is a mica schist; strike 15° west, dip vertical.
Quartz porphyry follows this, then an irregular dike of granite, in
which are imbedded nodules of quartz porphyry, some nearly a foot in
diameter. Above the granite there appears a granitoid gneiss, with many
cavities, arranged with their longer axes parallel to the schistose
structure. This changes into a coarse, knotty granite, containing large
nodules of dark petrosilex. A slight blending is noticeable. Farther up
this is succeeded by a micaceous quartz feldspar porphyry, showing a
somewhat gneissoid structure. It is out at right angles to this
structure by jointing planes lying thickly together. It has a milky,
vitreous luster, and contains inclusions of very coarse granite. The
next rock exposed is a diabase 200 feet across. It is coarse in the
middle and aphanitic on the edges. Adjoining it are bunches of granite
and a fine, dark, compact mica schist, showing traces of little pebbles
in places. The schist changes to a petrosilex, which comes in contact
with a dike of very coarse granite. At the upper edge of this granite
outcrop, and inclosed in it, is a stratum of fine, dark mica schist and
a dike of quartz porphyry. At one end these inclusions are hidden, but
at the other they have been bent, fractured, and the pieces separated
some distance in the granite. (Fig. 9). This is a most interesting
example of the intrusive nature of the granite.
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
The granite dike is bounded on its upper side by one of aphanitic
diabase 2 feet wide, followed by granite again. Nodules of the diabase
appear in the edge of the granite. A stratum of wavy quartzose gneiss 10
feet wide follows the last dike of granite. It changes across the strike
into a coarser gneiss, and that into irregular and partially blended
masses of coarse granite, filled with long fragments of porphyry similar
to a dike on the east. (Fig. 10.) The quartz porphyry is very compact
and brittle, and has a conchoidal fracture. The porphyry dike has a
width of 8 feet. About it is still another dike of granite 12 feet
across, and containing many long fragments of a diabasic rock, which
appears next in the succession.
The diabase is cut by small, interlacing granite seams for 6 or 8 feet,
and also contains some large, irregular, lenticular masses of the same
rock. Still farther is another dark dike cut squarely across by granite.
Fine granite and quartz porphyry dikes occur still farther east. The
porphyry dikes all run about north 35° west, average dip 70° southwest.
They extend along the summit of a ridge for more than half a mile, when
the coarse granite becomes the country rock. On the summit of the hill
at the head of the cañon are strata of metamorphic rock, and also some
veins of pegmatite, cutting across the porphyry. This wonderfully varied
succession of rocks does not cover a width of more than a quarter of a
mile. The relations exhibited here prove that the porphyry and diabase,
as well as the schists, are older than the granite. The strip of country
between the ancient porphyries of San Miguel and the coarse granite of
Lyon’s Peak and the ranges north is remarkably rich in its variety of
intrusive rocks.
On the north bank of the Sweetwater, just above where the Spring Valley
road crosses, is a large exposure of coarse granite, containing so many
inclosures of a dark dioritic rock as to present the appearance of a
conglomerate. It is cut by bunches and ramifying dikes of fine granite
and diorite.
At Dehesa the granite is replaced by norite, which forms a high mountain
on the north side of the river, and extends southeasterly 3 or 4 miles,
forming two high peaks. In this are veins of hornblende aggregates, and
in general appearance the rock bears the closest resemblance to that on
the Cuyamaca.
The crystalline rocks exposed between the Sweetwater and El Cajon Valley
generally show more of a dioritic composition than granitic. They have a
glassy feldspar, much hornblende, and little quartz. There are two
varieties: one is light colored, and forms most of the country; the
other is darker, occurring in bunches and dikes. Southwest of the valley
the rocks are more granitic. On the road to Spring Valley they give
place to a hard, light—colored felsitic rock, containing specks of
chloritic matter. Masses of granite appear in places, intruded into the
rock. As it decays it becomes jointed, and seems to be reduced mostly to
kaolin. Near the railroad, northwest of Spring Valley, rocks outcrop
which belong to the series of dark intrusives, tuffs, and porphyries so
extensively developed west of the granite. The rock here has a
fragmental appearance in weathered specimens. It has a dark green color,
and is of uncertain composition. The most western exposure of this rock,
before it becomes covered by the mesa, has much the same character. It
is without doubt an ancient intrusive, very greatly altered. It contains
bunchy dikes of white feldspathic composition, which easily reduce to
kaolin. The whole exposed width of these rocks west of the granite at
this point is over a mile, about half of which is tufaceous. By
tufaceous is not necessarily meant a fragmental intrusive deposited in
water, but for lack of any other term it is used to designate those
fragmental intrusives of plutonic origin which are so abundant in
California.
Between the Oakdale House and the Sweetwater River, on the road to the
dam, dark quartz mica diorites outcrop. They have been intruded by
coarse granite in veins and bunches. A body of dark aphanitic rock, of
uncertain origin, is inclosed in the diorite near the river. It may be a
portion of the aphanitic rocks associated with the porphyries farther
west. A little distance down the river the diorite is succeeded by
conglomerate rocks, containing small garnets. The fragments are
feldspathic in composition. Dark feldspathic porphyry then forms the
prevailing rock for some distance. It shows great variations. Much of it
contains no distinct crystals of feldspar, but is mottled by light green
felsitic bodies of irregular outline. These contain chloritic
substances, which decay out, leaving holes. There is a great variety of
these ancient intrusives exposed along the sides of the river north of
San Miguel Peak. None of the other crystalline rocks in San Diego County
appear so old or show so much alteration.
Farther down the cañon the rock assumes a fragmental appearance, having
masses of feldspar in a dark matrix. It finally becomes a pronounced
breccia, being formed largely of angular, felsitic fragments. The base
in which these fragments lie decays away and leaves them standing out on
the surface. This may be a tuff formed in water, but the matrix has
undergone such alterations that nothing certain can be said about it.
This breccia continues to the lower end of the cañon. An interesting,
light-colored dike was observed here. It is so filled with minute
spherical amygdules as to resemble in structure an oolitic limestone.
The breccia continues as far as the Sweetwater Dam, where it is replaced
by feldspar porphyry. The porphyry shows much variation; in places no
feldspar crystals are present, chlorite taking their place. These rocks
disappear a quarter of a mile west of the dam, and mesa-like hills
extend down to Chula Vista. These consist of soft sandstones and some
calcareous deposits. Dark rocks, belonging to the series just described,
outcrop near the mesa road from San Diego to El Cajon. It forms a long
ridge south of La Mesa. The rocks are in part tufaceous, and in part
dark green and massive, with felsitic inclusions. Near the Eleven-Mile
House the granite is filled with dark, bowlder-like inclusions.
Hornblende is an important constituent of the granite. It is more than
likely that many of these so-called granites are really diorites. The
mesa formation terminates in an abrupt escarpment on the west of Cajon
Valley. The valley comprises many thousand acres of very fertile land.
Granitic knobs rise in many parts of the valley, showing that the
alluvium deposit is underlaid by it at no great depth. North of the
valley, for a number of miles, the crystalline rocks are covered by the
mesa conglomerates. Granitic rocks are the only ones exposed for many
miles east of Lakeside. Just above Forster’s there is a small outcrop of
gneiss and mica schist; strike north 35° west, dip 70° northeast. The
rock exposed along the grade is a coarse biotite granite, with much
quartz and glassy feldspar. A dike of quartz porphyry varying to granite
porphyry outcrops by the road at the top of the grade. This does not
seem to be sharply defined from the adjoining granite. A dark hornblende
granite or diorite begins here and outcrops along the road for several
miles toward Ramona. Three miles from the town there appears a dark rock
with a somewhat resinous luster. It is probably a diabase. The Santa
Maria is a large, plateau-like valley, with low granite hills
surrounding it. Light-colored granite, becoming somewhat hornblendic,
extends some distance east of Ramona, when it is replaced by a dark
diorite. The latter extends along the road for 2 miles. It has glassy
feldspar, and in places much mica. Four miles west of Ballena the
granite is filled with pegmatitic veins, running in a northwesterly
direction. They carry brittle crystals of black tourmaline, garnet, and
muscovite mica. East of Ballena the rock becomes coarser and somewhat
gneissose, with an abundance of mica. There are no bedding planes,
simply a parallelism of constituents. This schistose structure has a
northwest direction. The gneiss changes to mica schist on the grade
above Santa Ysabel. The schist alternates with occasional strata of
quartzite; strike north 25° west. The quartzite is a white, fine,
granular variety, containing a little mica. Coarse gneiss outcrops again
farther up the grade; at the top it becomes finer, carrying some
hornblende, and alternating with strata of fine mica schist; dip
northeast. The gneiss often becomes granitoid. The lamination has no
constant direction, and resembles a flowage structure. This granitoid
gneiss extends to within a mile of Julian, when there is quite a sharp
transition to a gneissoid mica schist. That gives place to a fine
schist, and a quarter of a mile west of Julian to a pearly hydro-mica
schist. The schists strike north 20° to 30° west, dip 70° northeast. At
Julian the rock is a dark, thin-bedded mica schist, usually termed a
slate. There are also dark felsitic rocks and some gneisses. The belt of
dark slaty mica schists is about half a mile wide. Toward the Balkan
Mountain the rocks become more gneissoid. This mountain seems to be
formed wholly of gneiss and mica schist. In some places the rock is
nearly massive, and contains a large amount of quartz.
The rocks forming the mountains near the road from San Felipe to
Vallecito are nearly all gneissoid. The real desert begins on leaving
San Felipe Valley. The road extends southeast for 5 miles and then turns
southwest, descending a narrow, rocky gorge to the head of Vallecito
Creek. Near where the road turns to the southwest, the gneisses are cut
by innumerable dikes of coarse granitic composition. Many of these run
parallel, cutting across the strike of the gneisses, and give the
appearance of a bedded formation. In the cañon, before reaching
Vallecito Creek, mica schist appears. It is bent and twisted into every
conceivable direction, and varies exceedingly in structure and
composition in the course of a few feet. The dip is northeast, and often
at a small angle. The granite dikes often carry garnets, being quite
similar to those between Julian and Banner. They were probably intruded
after the eruption of the granite, and the related metamorphism.
At Mason’s granite and gneiss both appear as intrusives in the schists,
and are themselves cut by coarse dikes similar to those described.
Between this point and Banner, in a direct line, gneiss is the
prevailing rock. The high mountain east of Banner is more granitoid.
There is a sandy valley of considerable extent about Mason’s; eastward a
low ridge is crossed, and the road descends to Vallecito Valley between
barren granite mountains, gradually descending until the open desert is
reached. When the granite is hard these mountains are very rugged, but
in places they are covered so deeply with soft, decomposed material as
to closely resemble the Tertiary deposits farther east. The mountains
often show pale purple to brown tints. On examination they appear to
consist of a white granite, which has been so shattered as to break up
into pieces, averaging not over 2 inches in diameter. This rock presents
a very similar appearance for miles, particularly on the north side of
the creek.
At Palm Springs the soft clay beds of late Tertiary age begin to appear.
They dip in all directions, though that to the south and southwest is
the most common. Toward Carrizo Station these are often covered by
granitic detritus. The Tertiary beds widen out where the road strikes
Carrizo Creek, and at Carrizo Station they have a width of nearly 10
miles. They rest against a granite range north of the creek and south
along a long ridge which terminates in Carrizo Mountain. They also
extend a long distance up Carrizo Creek. Four miles below the station
harder sandstones and strata of shells cap the clay hills. This shell
layer near the creek is 10 feet thick and contains pectens and oyster
shells, belonging to the Miocene-Tertiary. The clay hills which surround
Carrizo Station form a veritable Bad-Land. They show many different
colors, and are perfectly devoid of any vegetation. They have been
eroded into a most confused network of hills and cañons, and are so soft
that it is difficult to travel over them. Six miles below this station
and a mile north of the creek there rises a rugged granite range, facing
the open desert. It rises from the desert quite precipitously nearly
2,000 feet, but blends westward into the ridges which run east from San
Felipe. The granite is coarse and deeply decomposed. Sharp, angular
grains of quartz stand out so prominently as to tear anything with which
they come in contact. The thin-bedded shell strata rest against the
slopes of the mountain, and near the southern end rise against the
eastern slope at a very high angle in sharp, jagged points. These strata
rise above the granite ridges at the point where they occur, and dip
fully 70° to the east. (Fig. 11.) Resting on the summit of this granite
ridge, and immediately west of the steeply inclined and jagged points of
the Tertiary strata, is the southern outcrop of a bed of coarse, hard
sandstone, which rises along the summit of the ridges to the north and
dips west, extending down the western side of the range for a thousand
feet or more, dipping at an angle of about 30°. This sandstone outcrops
along the crest of the ridge for nearly a mile. It is, however, not
absolutely continuous, the granite rising through it in places. Near the
southern end it is so highly metamorphosed as to be with difficulty
distinguished from the granite at the contact. In fact, the two
formations have become so intimately united, that a fracture of the
granite made near the sandstone, instead of stopping at the contact,
extends into the sandstone, so that the two rocks break off together.
The contact is so exceedingly irregular that it seemed at first sight as
if the sandstone had been intruded by the granite. Deep, narrow crevices
extend into the granite, and are filled with sandstone. Some granite
bunches are almost inclosed in the sandstone. The most interesting
feature of this sandstone is the presence in it of an abundance of
well-preserved corals of a type fully as old as the Cretaceous.
Fragments of two species of a large oyster and poorly preserved
specimens of univalve and bivalve shells are also abundant. There are at
least two species of coral, many specimens being at least a foot in
diameter. The sandstone is literally filled with them toward the
southern portion of the outcrop. They also extend down into the crevices
and cavities in the granite. The sandstone shows somewhat less
metamorphism toward the north, where it contains some strata of pebbly
conglomerate. The granite is intersected by many veins of coarsely
crystallized feldspar and quartz. From these veins many of the pebbles
in the conglomerate have been derived. At the highest point of the ridge
the sandstone has the greatest thickness. Here it is broken up into
great blocks 15 feet square, piled in the greatest confusion, as if by
an earthquake. One great mass overhangs the almost precipitous granite
escarpment at an elevation of 1,000 feet above the desert. Many
fractures extend from the sandstones through into the granite. The
sandstones, as well as the granite mountains north, have been baked a
dull red color by the intense heat of the sun. To account for the
peculiar position of this sandstone, as well as the steep inclination of
the Tertiary beds at the southern end of the mountain, we must suppose
that an extensive fault has taken place along the edge of the range
facing the desert. The general inclination of the Tertiary beds to the
west and southwest is also evidence of an uplift along this line.
[Illustration: Fig. 11.]
Carrizo Mountain was ascended from the north by following up a wash
through the clay hills. At the foot of the mountain there is a small
outcrop of ancient volcanic rock greatly altered. This rock rests
against the limestone which forms a large part of the north end of the
mountain. The limestone is crystalline in every portion. The strike, as
well as that of the schists and quartzites by which it is inclosed, is
north 30° to 40° west, dip 75° to the southwest. The color of the
limestone varies from white to streaked and variegated. It was carefully
examined for fossils, but no traces of them could be found. The
limestone is, however, filled in many places with the holes of borers,
and one small incrusting coral and one barnacle were found. The
elevation of the north peak is 1,700 feet, and these were found nearly
at the top. Everything points to a great elevation here in times so
recent that the atmospheric agencies have not yet had time to remove the
surface exposed to the ocean waters. Another factor enters into this
time computation, however, and that is that in this climate, where the
rainfall is so slight, a great many years are required to effect slight
changes. Toward the south peak the rock is almost wholly micaceous and
quartzose schists. Judging from the great amount of metamorphism shown
by these rocks, fossils, if they ever existed, must have been destroyed.
At various points on the northern end of the mountain are horizontal
deposits of a soft, shaly sandstone full of fossils, similar to those
found north of Carrizo Creek. The corals are very large and perfect.
Some nearly entire specimens of large oysters were also obtained, but
most of the bivalve shells are represented only by casts. With all the
exploration which has been done, it seems probable that these beds have
never before been seen.
Professor Blake, geologist of the Pacific Railroad Survey, passed up
Carrizo Creek in 1853, but, from the statements made, it seems his
observations were confined to the immediate vicinity of the road. His is
the only geological party that has visited this region up to the present
time. The vicinity of Carrizo Mountain is a favorite one for
prospectors, and every winter it is visited by a number of parties. As
yet no important discoveries have been made, and I cannot say that I
think the region is a favorable one. Silver has been reported from the
southern slope of Carrizo Mountain, and at one time there was
considerable excitement. The mountain, though formed wholly of
metamorphic rocks, does not appear to have been mineralized to any
extent. Only one dike was seen on the mountain, and that was of a
coarse, granitic nature.
There is not the slightest doubt as to the presence of a non-conformity
between the Miocene-Tertiary and the coral-bearing sandstones. Specimens
of the coral were sent to the National Museum, and were pronounced
similar to some from the lowest Cretaceous of Texas. As to the age of
the limestone and associated metamorphic rocks, they are unquestionably
Carboniferous or older. A float piece of silicious limestone was found
containing some shells, but no opportunity for their investigation has
yet occurred. The Tertiary beds are covered, wherever any of their
original surfaces yet remain, by a great variety of washed bowlders.
With regard to the structure of this eastern slope of the Peninsula
range, I can hardly agree with the views before expressed, that there
are to be seen here evidences of an enormous fault, to which the steep
escarpment toward the east is due. The eastern side of the range, so
precipitous in places, has been compared to that of the Sierras in
structure and general features. It is true that this descent is very
abrupt in places, but in others it is almost as gradual as the western
slope. For instance, the gradually descending ridges which extend east
from Banner for nearly 30 miles, show no indication of any fault, save
at the mouth of Carrizo Creek, where there has undoubtedly been a fault
of considerable importance. The very abrupt descent east of the Balkan
and Laguna Mountains is due solely to enormous erosion, for both north
and south ridges extend past them for many miles into the desert.
The San Jacinto Mountains also send out long arms into the desert, and
below the boundary Signal Mountain and a connecting range seem to be
merely a spur of the main system. The rocks of the metamorphic belt at
Julian and Banner, and farther south, dip to the northeast, indicating a
great fold rather than fault, with the most strongly pronounced
intrusive granites and diorites at some distance on each side.
It is not generally known that an ancient auriferous gravel channel
exists in the county. It begins about a mile north of the old stage
station, and 3 miles west of Ballena Post Office, where there rises a
hill shaped like a whale’s back (hence the name Ballena), covered with
washed gravel and bowlders. The main portion of the channel which has
escaped erosion begins south of the stage station, capping a hill which
has an elevation above the sea of 2,400 feet, being a little lower than
the so-called Whale Mountain. The gravel is 50 to 100 feet thick, and
has a width of 2,000 feet or more. It rises 300 to 500 feet above the
valleys and cañons on its sides. It extends in a direction a little
south of west for about 4 miles, terminating on the south of Santa Maria
Valley. A granite ridge runs 2 or 3 miles farther in the same direction,
probably preserved by the gravels, which are now gone. A pretty valley,
a mile long, has been eroded in the eastern end of the gravels, down to
the underlying granite. Placer mining has been carried on for years here
in a small way by Mexicans. Gold is said to be scattered everywhere
through the gravels, which are often very firmly cemented. Lack of
water, for the ridge is higher than any of the surrounding country, has
prevented work on a large scale. Lately a mining district has been
organized, and it is proposed to bring water 7 miles in pipe. In the
gravels are washed bowlders, many of them being 2 feet in diameter and
well polished. The remarkable thing about them, however, is that they
are nearly all porphyries. The most abundant is a red feldspar quartz
porphyry. Quartzite bowlders of all colors are numerous, and there are a
few of the basic diorite so common in portions of the county. Garnets
are said to be very abundant in the gravels, and many bowlders of a
schist carrying them are also present. The matrix of this rock could not
be made out in the field; it is very tough and heavy, and has never been
seen in place. The red porphyry bowlders resemble those on the mesa
farther west, but have never been found in place. Never, in the
mountains east or north, has porphyry of this kind been seen, either by
myself or described by others. From the old stage station the upper
course of the stream was north and south as far as it can be traced.
There are indications that one branch extended easterly toward Julian.
These gravels appear on a hill surrounded by deep cañons, about 2 miles
east of the top of the grade above Foster’s. At the top of the grade the
hills on the west are flat-topped, and covered with gravels to a depth
of 150 feet. These have much the same character, and probably belong to
the same channel. More investigation is needed to determine whether the
course of the old stream was down toward the San Diego River, in Cajon
Valley, or west toward the high mesas south and southeast of Poway
Valley. It seems probable, however, that the stream flowed west, and
that the mesas have been formed partly from the bowlders which they
brought down. This mesa, as well as the gravels at the head of the
grade, has an elevation of 1,500 feet. The source of the porphyry
bowlders and the garnetiferous schists of this old river is a matter of
great perplexity. The gravel deposit has every characteristic of an old
river channel, and not that of an elevated arm of the sea; besides, the
presence of gold in the gravels indicates their derivation from the
country farther east. The gold may have been derived from Julian or Mesa
Grande, or some more remote point. The river must have flowed across the
gold belt, but then the question arises, how could a river of such
magnitude have existed so near the summit? The only way out of the
difficulty is to suppose that a great uplift has taken place along the
crest and western slope, coupled with an enormous amount of erosion; and
that this stream once, before this great change took place in the
configuration of the country, headed many miles to the northeast, far
beyond the drainage of the western slope. The bowlders consist largely
of hard rocks, and are very smoothly rounded and polished, indicating
that they have been transported a long distance, and subjected to
attrition through a protracted interval. It is quite possible that this
river emptied into or near San Diego Bay, and that the immense beds of
bowlder conglomerates about the bay owe their formation largely to this
river action.
The first outcrop of crystalline rocks in Mission Valley is about 3
miles above the old Mission, where the San Diego River enters a cañon.
It is a volcanic tufa, consisting of grayish to greenish fragments of a
fine-grained rock imbedded in a brown matrix. This has a width of about
half a mile. Along the cañon, dikes of a greenish amygdaloid have been
intruded in the rock, and are particularly numerous north of the river.
One of these dikes in the cañon was observed to be amygdaloidal in the
center. Farther up the cañon there is a great variety of tufas. The
first contains feldspathic and hornblendic fragments nearly blended in a
base consisting of crystallized feldspar and dark chloritic particles.
Above this is a dike of brownish crystalline rock, much altered; the
only distinguishable mineral being feldspar, in small crystals. Then
follows another tufa, with nearly blended micaceous fragments. The next
rock is a fine crystalline one with very regular bedding planes, a foot
or more thick; dip 30° to 40° southeast, strike north 35° east. Then
follows a dark, aphanitic, structureless rock for some distance. At one
point a branching dike of almost pure feldspathic material spreads out
into this aphanitic rock in radiating arms. Apparent bedding planes run
through them, as well as the country rock, showing that these planes are
not those of sedimentation, but are due to some secondary cause. These
rocks occupy the cañon for 1½ miles, and are all undoubtedly of volcanic
or intrusive origin. A series of rocks of metamorphic origin outcrops a
thousand feet along the cañon. The first of these is a micaceous
felsite. That is followed by fine-grained granitic rocks carrying
garnets, and this by a hornblendic felsite. The latter finally becomes
mixed and blended with a coarse micaceous diorite, containing a glassy
feldspar. This rock is the chief one exposed through the cañon. It has
granitic and syenitic facies. The tuffs exposed at the mouth of the
cañon extend in a direction a little east of south for 3 miles, until
covered by the mesas which extend west from Cajon Valley. They show a
comparatively uniform character, the fragments being generally nearly
blended. The ridge which these rocks form is separated from the granite
by an elevated mesa a mile wide. The tuffs are exposed along Chaparral
Cañon to within 2 miles of the mission.
The granite ridge at the lower end of Cajon Valley does not extend more
than 2 miles north of the cañon, when it becomes covered with bedded
deposits and bowlders of late Tertiary age. Granite does not appear in
Sycamore Cañon until the northeast boundary of the Cajon grant is
reached. High hills of gravel and bowlders lie east of the cañon and
extend toward Foster’s. The main body of granite is met at the head of
the cañon. It extends along the east side of the road to Poway Valley.
Bunches of dark, coarse diorite occur in it in many places. The gravel
mesa south of Poway Valley has an elevation of 1,200 feet. Small areas
of gravel also remain on the hills northeast of the valley. The granite
ridge, flanked by porphyries, does not outcrop very prominently south of
Los Peñasquitos Cañon. The ridge southwest of Poway Valley seems to be
formed largely of gravels, rising 1,500 feet.
The usual brecciated tuffs outcrop in the gulches and along the creek
just above Los Peñasquitos ranch house. They appear along the old road
to Escondido for 2 miles. A body of chloritic granite appears in the
center in the form of a long dike, extending from the Peñasquitos Creek
a mile or more north of the road. Toward the east the breccia becomes
finer and almost loses its fragmental character. Between this formation
and the granite a dark micaceous felsite, probably metamorphic,
outcrops. The fragments in the tuff are diabasic at times and at others
largely petrosilicious and feldspathic. The crystalline rock on the east
is, perhaps, more nearly diorite than granite, as the feldspar is
chiefly a glassy one. Black Mountain is formed of this dark breccia,
while the high range of mountains which rises on the north and extends
northwesterly between San Bernardino and the ocean, is formed partly of
granitic rocks and partly of the tuffs and porphyries, the latter lying
on the west.
A rolling, hilly country, containing much good land, stretches north
toward Escondido. Remnants of the mesa conglomerates remain in places on
the eastern edge of the Peñasquitos grant. The granite is coarse and
rises in huge, rounded knobs along the road. A little south of San
Bernardino Post Office there rises a conical peak of micaceous diorite.
A somewhat similar rock, but more diabasic in appearance, forms the
mountain immediately west of the Post Office. This formation extends
northwesterly for several miles, having a width of about a mile. The
rock over much of this area closely resembles the gabbros and olivinitic
diabases from the southern part of the county. It is penetrated by
dioritic and granitic veins, in which the structure is often pegmatitic.
At the point where the road stops at the entrance of the cañon of Diablo
Creek, this basic formation is replaced on the west by a massive,
jointed quartz rock, containing a little feldspar and chlorite, and in
places becoming granitoid. It often has a fragmental character, with the
quartzose bodies imbedded in a matrix more granitic, or simply darker
and chloritic. This rock is quite uniform for 3 miles down the cañon,
quartz being the predominating constituent. It is very probable that
this represents an original sedimentary terrain. It is followed on the
western slope of the range by the dark tuffs before described. Here the
matrix is often porphyritic, with a fluidal structure. Portions are real
porphyries. A mile east of Olivenheim it resembles diorite porphyrite.
The last exposure seen on the west was of the usual tufaceous character.
This formation narrows northward, and on the road to San Marcos shows
the width of a mile.
Northwest of San Marcos there is a large body of metamorphic rock,
chiefly felsite schists and feldspathic quartzites. These extend in a
northwest direction toward Buena Vista, but there are not many
exposures. Dark diorite outcrops south of Buena Vista, and extends west
for a mile and a half, when it sinks under the Tertiary deposits. The
last outcrop seen was a dark micaceous diorite. A mile west of this
point there is quite an outcrop of diabase containing an excess of dark
feldspar. The sandy clays extend west from this point to Oceanside. Near
Buena Vista station the diorite is impregnated with green copper
carbonates, and a considerable amount of work has been done, but
evidently no paying bodies of ore were found. Syenite outcrops near
Kelly’s ranch house, and in the hills east. The crystalline rocks come
nearer the ocean here than at any other place in the county.
Between Escondido and Moosa Cañon, granites, with bunches of dark
diorite, are the only rocks seen. A broad, sandy valley extends up San
Luis Rey River to within 5 miles of Pala, when the high granite
mountains close in, forming a cañon. The valley widens at Pala, and for
many miles a broad, sloping deposit of bowlders and gravel borders the
river, and rises high against the foot of Smith’s Mountain. It is often
2 miles wide and represents a great amount of erosion. A mile northeast
of Pala is a high hill of diabase, similar to that in the southern part
of the county. On the eastern slope of this hill is an enormous
pegmatite vein, carrying a very interesting set of minerals. This vein
is twenty or more feet wide, and dips west at a small angle. There are
masses of great size of almost pure mica and feldspar, or quartz and
feldspar—in the latter case very fine specimens of graphic granite have
been formed. Near the southern end of this vein is a deposit of
lepidolite mica, 10 feet thick at the widest part, and appearing in
detached bodies for several hundred feet. It is fine-grained and shows a
pale purple color. In places it is pure, in others filled with large
radial aggregates of pink tourmaline (rubellite). Some of the aggregates
are a foot across, others are long and slender, with arborescent forms.
North of the main deposit it is found in quartz in fan-shaped
aggregates, the crystals being more than a foot long, but greatly
decomposed. Black tourmaline is abundant in the pegmatite surrounding
the lepidolite, but in poor, brittle crystals. Green tourmaline is
present in places in the form of minute grains. Garnets are also to be
found in places. The vein as a whole is inclosed in the diabase.
The western end of Smith’s Mountain shows many bodies of dark dioritic
rocks. The major part of the rock is, however, gneiss and mica schist,
all very easily decomposed, leaving an immense amount of bowlders and
gravel along all of the gulches. Mica schists cover an extensive area
along the southern slope of Smith’s Mountain, on the Pauma grant. These
are undoubtedly a continuation of the schists of the Julian belt, but
carry no minerals. The belt of schists extends nearly if not quite
through to Julian. Warner Valley is located at the head of San Luis Rey
River. It is entirely surrounded by granite mountains. There is not as
great a variety of intrusive rock here as in other parts of the county.
Point Loma forms a peninsula, the greatest length of which is about 6
miles, and greatest breadth, 1½ miles. During Quaternary times it was an
island, but owing partly to an elevation of the coast, and partly to the
detritus brought down by the San Diego River, it becomes joined to the
mainland. It rises 400 feet in almost perpendicular cliffs at its
southern end, gradually lessening in height toward Old Town. The rock of
which it is formed consists of soft shales and sandstones, the latter
often quite consolidated. The strata at the extreme end of the point dip
south, but in a short distance turn and maintain a quite uniform dip to
the northeast nearly the whole length of the peninsula. This abrupt
elevation evidently owes its origin to a fault accompanied by an uplift,
and not to erosion. Beginning at Ocean Beach, and following along the
base of the cliffs to the light-house, hundreds of faults can be
counted. Near Ocean Beach fifteen can be counted in the space of 200
feet. The direction of these faults is nearly at right angles to the
strike. The most of them are nearly vertical and clean cut. The throw
varies from a few inches to many feet. Sometimes the north and sometimes
the south wall has risen. An interesting overthrust fault is exposed in
the cliffs north of Ballast Point. (Fig. 12.)
[Illustration: Fig. 12.]
A conglomerate of late Tertiary age overlies the Cretaceous rocks
unconformably on the extreme end of the point. These conglomerates are
firmly cemented and form high cliffs. They dip at an angle of 30° to the
southeast. The pebbles are in part derived from the sandstone of the
point, and in part from the crystalline rocks east of the mesa. Near the
top of this conglomerate are immense, semi-angular bowlders. These have
rolled down to the beach and are strewn around the end of the point.
Many large ones are to be found a little west of the new light-house,
but the greatest of all is on the eastern side. It is fully 10 feet in
diameter, and formed of the same kind of rock as that on which the mesa
rests 8 miles east of San Diego, viz: a green volcanic tuff. It is a
very interesting question as to how these immense bowlders have been
transported so far and left in the beds near the top of the cliffs. I
can account for it only by supposing that the point with relation to the
country back of San Diego was several thousand feet lower at one time,
and that a river of great volume, flowing over a steep channel, entered
the bay at this place, depositing irregular beds of bowlders. This old
river may have been the same one which formed the auriferous gravel
channel before mentioned. The fault planes on Point Loma extend through
the conglomerate beds, indicating that the elevation took place after
the bowlder beds were formed.
An interesting collection of fossils was gathered from the lowest strata
exposed, and from the bowlders in the conglomerates. This collection
numbers something over sixty Cretaceous species, many of which are new.
The fossils are not abundant, nor are they well preserved. Nearly all of
these are characteristic Chico (Upper Cretaceous) fossils. There is,
however, one species found here in considerable abundance, but rather
poorly preserved, which Dr. White has described under the name of
_Coralliochama Orcutti_, and which he has made the chief foundation for
a new division of the Cretaceous, termed the Wallala Beds. The name was
given on account of the occurrence of this fossil, together with several
other species, at or near Fort Ross, Sonoma County, and also at Todos
Santos Bay, Lower California, where the best specimens were found. These
beds stand in an unknown relation to the other Cretaceous deposits
stratigraphically, but have been supposed, on account of the fossils, to
indicate a division between the Chico and Shasta groups. I believe,
however, that the occurrence of the most important fossil of this
supposed division on Point Loma, in the same beds with undoubted Chico
fossils, destroys the validity of the supposed Wallala Beds.
In a bluff at the northeastern end of the Point Loma peninsula, west of
Old Town, there is a stratum of calcareous sandstone, carrying many
fossils belonging to the Eocene, or lowest Tertiary. The strata dip
northeast at a small angle, and though they cannot be traced
continuously west to the outcrops of Cretaceous rocks, yet from the fact
that they have the same dip, leads me to the belief that the two beds
are conformable. This younger deposit corresponds to the Tejon, or
Division B, of Professor Whitney. Everywhere in the State there exists
the closest relation between the Chico and the Eocene. Here on Point
Loma they are undoubtedly also conformable, but each is distinct as
regards its fauna, for they are separated by nearly a thousand feet of
unfossiliferous strata.