USA > California > Contra Costa County > History of Contra Costa County, California, including its geography, geology, topography, climatography and description; together with a record of the Mexican grants also, incidents of pioneer life; and biographical sketches of early and prominent settlers and representative men > Part 3
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About one mile farther north is the highest point north of the pass; called "Rocky Mound ;" it is nineteen hundred and twenty-one feet high, forming a rounded hill, having a distinct stratification, although very Trappean in its appearance, and a dip to the northeast. Between this point and the ridge spoken of in the last paragraph, there is a mass of Trappean rock, finely crystalline and very hard, in which no planes of stratification can be observed. On the northeast of San Pablo, the unaltered strata rest on these metamorphic rocks and dip northeast.
The ridge between Wild Cat and San Pablo creeks is made up of strata dipping northeast from 30° to 35°, and having a strike of about N. 52° W. The north end of this ridge is of quite unaltered strata, while the southern portion is highly metamorphosed.
On the east side of Carlisle creek, a metamorphic limestone occurs, in which all traces of stratification have been obliterated, the mass of the rock being traversed by veins of quartz, resembling semi-opal in appearance.
South of the pass from Oakland to Lafayette, several high dome-shaped hills rise, having an elevation of about eighteen hundred and fifty feet, made up of highly metamorphic rock having a Trappean aspect, but strati- fied and dipping northeast. Intruded in this are masses of rock which appear to be of decidedly eruptive origin, as the metamorphic strata ale displaced in their vicinity. Here, as in many other localities in California, it is difficult to draw the line between eruptive and sedimentary, as both have undergone extensive metamorphism since their formation.
A short distance south of the pass the metamorphic strata suddenly con- tract to about one and a half miles in width, an arm of unaltered sandstone and slates extending up between two branches of the metamorphic. In this region the slates are little metamorphosed, appearing white and easily decomposed, although much contorted. Portions are highly silicious, but soft and fiable, and, under the name of " Kaolin," are used to mix with clay in making pottery at San Antonio. This belt of slates and shales may be traced southeast as far as Suñol valley, beneath which they dip, rising again probably and appearing in a highly metamorphic form in the mass of the Mount Hamilton group. In the places where they are not metamorphic they have all the lithological character of the strata known to be of Cretaceous age, which have been described as occurring near Martinez, and which will be noticed farther on as so well developed near Monte Diablo.
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History of Contra Costa County.
Lying to the west of this are massive sandstones, entirely unaltered, which as yet have furnished no fossils, but which are believed to be of Cretaceous age. They form an elevated ridge, of which a culminating point is Redwood peak, sixteen hundred and thirty-five feet above the level of the bay. The strike of these sandstones at this point is about N. 69° W., but they curve more to the south on the southeastern side of the ridge. Their usual dip is to the northeast, but near Redwood point the strata are much broken, and three miles southeast they sometimes stand vertically or have a very high dip to the northeast.
Beneath this mass of sandstones, and extending to the southwest, there is a body of coarse conglomerate, forming a series of ridges of considerable altitude. Northeast of San Leandro it appears in the range of hills form- ing the eastern boundary of the San Antonio Ranch. Ten or twelve miles farther to the southeast it appears in Suñol peak, which rises to an elevation of over two thousand feet, on the southeast side of which it dips to the southwest. It passes through the Suñol valley and becomes a portion of the great metamorphic belt of the Mount Hamilton Range.
Although no fossils have been found in place in the belt of slates and shales alluded to above as exhibiting so well marked a resemblance to rocks elsewhere determined to be of cretaceous age, yet a few boulders have been picked up which contained shells undoubtedly of this epoch. A more careful search will hardly fail to furnish some farther evidence on this point. One of these, boulders was found near the entrance of Suñol valley, in a locality where it is hardly possible that it should have come from any other belt of rocks than that indicated above.
The metamorphic band before alluded to, as beginning near San Pablo, after narrowing near Redwood peak, extends along the western slope of the hills, forming the lower ridges at their base. It does not, however, form a well defined belt parallel with the strike of the strata, nor does it appear to represent an axis of elevation. In a section examined from San Leandro across the summit of Monte Diablo, it was seen conformably underlying the conglomerates and sandstones before spoken of ; but farther south its rela- tions to the adjacent rocks become very obscure, owing to the almost entire obliteration of the lines of stratification consequent on the increased meta- morphism of the mass. As observed in the foot-hills of the range between San Antonio and Alameda creek, this metamorphic belt has all the charac- ters which are so often exhibited by the altered cretaceous rocks. Serpen- tine is abundant in it in large irregular masses, and jaspery slates like those of Monte Diablo. East of San Antonio large patches are to be seen, having all the characters of the quicksilver bearing rock of New Almaden and New Idria, exactly like those noticed as occurring near San Pablo. Considerable masses of chromic iron occur in this position, one of which was formerly worked to some extent. Stains of copper are not unfrequent, and have led
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to several attempts at mining, none of which have proved successful, or are likely to repay the labor and capital invested.
In the neighborhood of Alameda Cañon this metamorphic belt appears to be almost lost; but traces of chemical action, commenced and partially completed, are exhibited in narrow streaks visible among the highly inclined and broken strata; these, however, do not appear to connect through with the metamorphic mass of Mount Hamilton.
Monte Diablo Group .- Monte Diablo itself is one of the most conspicuous and best known landmarks in California. But few persons in the State can have failed to recognize it from some point either of the Coast Ranges or of the Sierra Nevada. It is not its great elevation which has given it its pre- eminence among the innumerable peaks of the Coast Ranges; it is just the height of Mount Bache near New Almaden, a point hardly known by name to those who have not made a special study of the geography of California, and it is overtopped by Mount Hamilton, San Carlos, and some nameless peaks to which no public attention has ever been attracted. The reason why Monte Diablo has so marked a pre-eminence among the peaks of the Coast Ranges is, that it is, comparatively speaking, quite isolated, especially on the northwest, north, and northeast, the directions from which it is most likely to be seen. To the traveler passing up Suisun bay and the Sacra- mento or San Joaquin rivers, it presents itself in all its symmetry and grandeur, rising directly from the level of the sea, and easily recognizable from a great distance by its double summit and regular conical outline, re- sembling that of a volcano, which it was generally supposed to be by the early settlers.
If the mountain is made such a conspicuous landmark by its isolated position, it becomes itself, in turn, a point from which a vast area of the State may be observed and studied. Rising as it does among the Coast Ranges, there may be traced from its summit from Mount Hamilton on the south to unnamed peaks in the vicinity of Clear Lake on the north, and from the plains of the Sacramento and the San Joaquin to the Pacific, east and west. The great interior valley of California lies spread out like a map, extending as far as the eye can reach. To the east the view seems illimitable, and it is believed that there are few, if any, points on the earth's surface from which so extensive an area may be seen as from Monte Diablo. This is due to the peculiar form of the Great Valley of California and the gradual rise of the Sierra, which brings higher and higher points to view as the distance becomes greater. The eye can range over an extent of four hundred miles from north to south, and back to the east, or towards the summit of the Sierra, as far as the crest of this range, the farthest northern point visible being Lassen's Butte, and the most extreme southernsome point near Owen's Lake probably, thus affording a range along this snow-crested
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History of Contra Costa County.
line of mountains of over three hundred miles in length. The whole area thus spread out before the eye can hardly be less than forty thousand square miles, not much less than that of the whole State of New York.
In describing the geology of Monte Diablo and its surroundings, it will be convenient to begin with the central mass of the mountain itself as a starting point. This central mass is made up of metamorphic rocks ; it is about six miles long, and one and a half miles in width, and is surrounded on all sides by entirely unmetamorphosed strata. It is of an irregular cres- cent form, the concave side turned to the north-northeast. The material of which it is composed is extremely variable in its lithological character; but it consists essentially of a central portion of very hard metamorphic sand- stone, containing considerable epidote, flanked on both sides by jaspers, silicified shales and slates. The former constitutes the north peak, the latter the main peak, or Monte Diablo itself. The central crescent-shaped mass of altered sandstone commences on the northeast, about a mile and a half in that direction from the north peak, sweeps around to the south and passes with its southern limit about a quarter of a mile north of the main peak, then bends around to the north so as to include the precipitous spur which runs off to the northwest, lying to the east of the head of Mitchell's creek, and to the highest point of which has been given the name of Eagle Point ; this is two thousand three hundred and ninety-three feet above the Bay. Extending still farther to the northwest, it crosses the creek, and forms the high north and south ridge which makes up the most northwest- erly portion of the mountain mass. The southern extremity of this ridge is named Black Point ; the northern, Pyramid Hill ; the former is about eighteen hundred feet in elevation, the other a little less. The rocks of this ridge are, in part, an exceedingly dark-colored, fine-grained, crystalline material, destitute of traces of stratification in the central portion of the mass ; but which appears to be a metamorphic sandstone, although at first sight it might be taken for an eruptive rock. Its relations to the surround- ing rocks indicate rather a metamorphic than an eruptive origin. It would be difficult to consider this part of the mountain as being of purely igneous origin, without including with it the rest of the crescentic mass, which, however, we know from its connection with the adjacent sedimentary strata, and from the fact that portions of it have partially escaped the metamorphic action, to be made up of detrital materials deposited from water.
Between the north peak and the main peak, or Monte Diablo itself, along the narrow ridge of a little more than a mile in length which connects the two summits, the variable character of the metamorphic rock of the mount- ain may be well observed. Portions of it consist of jaspery material, or silicious slate, distinctly stratified; these have resulted from the meta- morphism of the purely silicious strata. Here and there are patches of imperfect serpentine, formed from the more argillaceous sandstones ; while
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in places the rock becomes so highly metamorphosed as to be converted into a well-characterized mica-slate in which numerous small garnets occur, and also zircons of minute size. The north peak has an elevation of three thousand five hundred and ninety-three feet, or two hundred and sixty- three feet less than Monte Diablo proper. Here all traces of stratification are lost; but by a careful examination of the rock, where it is well exposed in all its relations to the surrounding strata, leads irresistably to the con- clusion that it is not of eruptive origin. The gap between the two peaks is excavated in the soft, imperfect serpentine ; it is about eight hundred feet below the summit of Monte Diablo.
One of the best points for observing the gradual passage of the argil- laceous sandstone into the hard dioritic or Trappean rock, is along the flanks of the ridge of which Eagle Point is the culmination. The strata here may be traced in all stages of passage, from the soft sandstone to the hardest and most crystalline rock, to which in hand specimens an eruptive origin would readily be assigned by most geologists.
On the outside of this great central metamorphic mass, both on the north and south, but not entirely surrounding it, are heavy accumulations of jaspery rock, one of the most peculiar features of the mountain, and the material of which the culminating point itself is made up. On the north side of the North Peak, these beds are finely exposed, forming a lenticular mass about two miles long and half a mile wide. They have a nearly east and west strike and dip to the north. They are here, as elsewhere, of a red color, varying from a dull brick-red to a brilliant vermilion hue. The strata are usually thin, an inch being about their average thickness, and they are much folded together and twisted. These jaspery strata on the north side of the North Peak do not extend around so as to pass to the north of the Eagle Point Ridge, but may be traced in the ravines in which Bagley creek heads, passing into the unaltered shales of undoubted Creta- ceous age, in which ammonites inoceramus and other fossils have been found, and which are largely developed to the north of the mountain as well as to the south. On the north side these may be traced high up into the mountain mass along the branches of the Arroyo del Monte Diablo. No one making an examination of this part of the mountain could doubt that these jaspers are the result of the alteration of the Cretaceous shales.
The rock of the summit of Monte Diablo is the same jaspery material, filled with fine reticulations of quartz, running through it in every direc- tion ; but, in some places, containing a large amount of epidote, which has been formed where the shale contained originally more lime than usual. The dip of these metamorphic strata is distinctly to the north, and the strike along the ridge leading to the summit is nearly east and west. At many points on the south side of the mountain near the summit, and for a thousand feet below, these masses of contorted jaspery strata may be seen.
3
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History of Contra Costa County.
At one locality, just two miles west of the summit, there is, in a narrow ravine, a most beautiful exposure of this kind. The strata of jasper are alternately brilliant red and light green, contrasting finely with each other, and are folded together in a manner which is rendered very attractive from the thinness and regularity of the different layers. The strike here is, in general, about N. 54° W., and the dip to the north from 50° to 70°. In tracing these strata to the west, they appear to give place to other meta- morphic varieties of rock, of which serpentine is the most prominent, and we soon reach the entirely unaltered shales as on the north side of the mountain.
Serpentine is found, on both the north and south sides of the mountain, in considerable quantity. The largest mass is met with on descending the North Peak towards the north, where it forms a lenticular deposit about two miles long and half a mile wide, lying next to the jaspery shales. It also occurs in the gap between the two summits, and around the head of the Arroyo del Cerro, two and a half miles west-northwest of the summit. Here as in other localities, the serpentine is seen in every stage of passage from the argillaceous sandstone to the perfect serpentine itself. The bound- aries are very irregular in all these localities, especially on the Arroyo del Cerro, where we come into the unaltered shales and sandstones on going a short distance in either direction.
The metamorphic region, thus indicated as forming the central mass of Monte Diablo, covers about twenty square miles, and from it a great variety of rocks might be obtained. The red and green jasperry rocks, however, are the most characteristic forms, and having been here so unmistakably traced to their origin as Cretaceous shales, they have been of great service in recognizing this formation in other localities, where the facilities for tracing it out in all its connections, and of determining its age by fossils are less than they are found to be in this vicinity.
This metamorphic region has been, at various times, assiduously ex- plored for minerals and metalliferous ores of various kinds. Gold is reported to have been obtained in small quantities, and was at one time the object of expensive mining research. Cinnabar occurs at several points, especially on the northeast side of the North Peak, where quite handsome specimens have been obtained, associated with a silicious rock in which this ore usually occurs ; it is also found on the ridge of Eagle Point. Copper ore has also been the object of much excitement in this region, as it is frequently found in small quantities, and occasionally in rather large masses, in the dioritic variety of the metamorphic rock. In and about Mitchell's Cañon, where this kind of rock is most developed, a considerable number of com- panies were at work in 1862 and 1863; but nothing had been discovered which could properly be called a regular vein, or worked with profit. It is interesting to notice, however, the occurrence of these ores in a rock of so
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late a geological epoch, so evidently associated as they are with the existence of metamorphic action in this region.
Near the northwestern extremity of the metamorphic mass, about two miles northwest of the summit of Black Point, is the largest mass of traver- tine or calcareous trefa which has been yet observed in the State. It ex- tends north and south for a distance of over half a mile, forming low ridges running northwest and southeast, and having sandstone both to the north and the south. Its width east and west is fully one thousand feet. It is almost white, much of it quite so, made up of a very pure carbonate of lime, and possessing the concentrically-aggregated structure so often exhib- ited by masses of stalagmite. It undoubtedly owes its existence to depo- sition from a hot spring, which once came to the surface at this point. This deposit has been quarried and burned for lime. 1721995
It may be mentioned that. there are other deposits of this calcareous material in this region. The most extensive, next to the one just noticed, is on the other side of the San Ramon valley, where it forms a very heavy mass on the side of the hill, about five hundred feet above the valley. Flanking the whole north side of Monte Diablo are unaltered cretaceous strata, having everywhere a northerly dip, and a general strike of about east and west magnetic ; the dip of the mass is irregular, in some places vertical, but usually from 45° to 35°. These cretaceous strata consist of shales and sandstones, the former containing frequent beds of argillaceous limestone, which are generally less than a foot in thickness and rarely con- tinuous for any great length. The shales are very soft and disintegrate easily, hence they are usually found occupying valleys between the ridges of sandstone, which latter rock resists the weather better. The valley at the base of Monte Diablo which separates the mass of the mountain from the hills farther north, in which are the coal mines, is occupied by these shales, which may be traced in the beds of the two branches of the Arroyo del Monte Diablo, which unite at the village of Clayton.
The same shales may be observed on the south side of the mountain, especially in the Cañada leading to Curry's house, and in the ravines run- ning up to the south from this Cañada. As on the north side, so here, quite a number of characteristic Cretaceous fossils are found in this belt of rock, among which are: Ammonites Newberryanus, Ammonites Batesii, Bacu- lites Chicoensis, Fusus Mathewsonii, Amauropsis Alveata, Dentalium Cooperii, Dentalium Stramineum, Venus Varians, Cardium Annulatum, Eriphy la Umbonata, Pina Brewerii, Trigonia, Evansii, Cucullœa Trun- cata, Pecten Operculiformis.
These strata, as exposed in the bed of the creek in Curry's cañada, have a very irregular dip, although usually at a high angle, and to the southeast, south, or southwest, near the mouth of the cañada near Curry's, the dip is from 80° to vertical, and the strike nearly east and west magnetic.
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History of Contra Costa County.
Next above the Cretaceous shales with argillaceous limestones intercal- ated, as just noticed, comes a very thick and heavy bedded mass of sand- stones which, on the north side of the mountain, form the elevated ridge just south of and facing the coal mines. These sandstones contain a few Cretaceous fossils, such as Axinca, Natica and Dentalium.
The Cretaceous strata curve around to the south as they pass to the east of the mountain, running out into the plains of the San Joaquin in long, low, and almost exactly parallel ridges. The counterpart of these Creta- ceous sandstones of the north side is found also on the south side of the mountain, forming an elevated and conspicuous ridge, sweeping around parallel with the general strike of the rocks in this vicinity, but not form- ing so distinct a feature of the topography of the region as the Tertiary ridge next south of it. Its culminating points rise to the height of from two thousand to two thousand two hundred feet. To one of these points or ridges where the white soft sandstone was, at the very summit, curiously worn into cave-like hollows, is given the name of Cave Point. This is two thousand and seventy feet in elevation. Although these sandstones, in this vicinity, are very barren of fossils, enough are found to determine the fact that they belong to the Cretaceous series.
A little south of Cave Point, in the depression between that and the next ridge south, the sandstone is worn into curious tower-like forms, com- monly known as Tower Rocks.
Coal has been found in the Cretaceous shales noticed above as lying under the sandstone, but the only extensive workable beds yet discovered are included in the sandstones belonging to the upper part of the Cretaceous.
Of the Mount Diablo coal-beds, the only workable deposits of this inval- uable material yet discovered in the State, a full account will be given in . our chapter entitled " The Mount Diablo Coal Field." Leaving this section of our theme, therefore, we will follow the geology of the region so far as it concerns the limits of this county.
The exact limit of demarcation between the Cretaceous and the Ter- tiary, in this vicinity, has not been exactly made out. Resting on the coal-bearing strata above described, there is a heavy mass of sandstone, with some shales interstratified, which, however, are more silicious than the truly Cretaceous beds of otherwise similar character. These beds appear to be beds of passage between the Cretaceous and the Tertiary ; but fossils are so extremely rare in them that it is not easy to come at their precise relations. They hold the position which should be occupied by the Eocene Tertiary ; but have yielded no forms recognized elsewhere as of this particular age. This mass of sandstones occupies, on the north side of the mountain, a con- siderable width on the surface, apparently not less than a mile to the north of Mine Hill. On the south side it appears less distinctly marked ; in fact, there seems to be but little room for this body of strata between those of
JOEL CLAYTON.
Wittemarin
١
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Geography, Name, Topography, Etc.
undoubted Cretaceous age at Tower Rocks, and the high ridge of Miocene Tertiary directly south of it.
Rocks of both Pliocene and Miocene age are extensively developed to the north of the strata first spoken of, on the northern slope of the range in which are the coal-mines. Those which are referred to-the Miocene division of the Tertiary-consist chiefly of sandstones, which are very heavily bedded towards the base of this part of the series. They are succeeded above by thinner and more fossiliferous strata, which not only contain large numbers of marine fossils, but also impressions of leaves and considerable fossil wood, the latter silicified and lying upon the surface, the rock having decomposed around it. These upper strata are referred by Mr. Gabb to the Pliocene division of the Tertiary, from a consideration of the number of living species which they contain, as well as from their stratigraphical position.
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