Part 4

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.)

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.

False Bay occupies the basin of a synclinal, for the strata dip 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.

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.)