Part 2

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.

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.