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.
Fig. 15.
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.