and sandstones, the former containing frequent thin layers of hydraulic limestone. These rocks, however, exhibit but few fossils. The dip and strike are variable, but generally about east and west magnetic, and the dip is also irregular, but almost always to the southwest, and at almost every angle from nearly horizontal to vertical; the strike is nearly parallel with the line of the Straits. Near the upper limit of the cretaceous, are sandstones very like those of Monte Diablo which accompany the coal, and they contain a considerable quantity of carbonaceous matter, but no regular coalbed, so far as yet discovered. Near these carbonaceous strata, and above them, is a narrow belt, partly altered and folded, and from one hundred and fifty to two hundred feet in width. The Rodeo valley marks the limit of the cretaceous, going west from Martinez, the tertiary succeeding in that direction, and resting conformably on the strata beneath, and having the same general southwestern dip. South of Martinez the cretaceous strata have a higher dip, but in the same direction.

Southwest of the Rodeo valley lies a broad belt of tertiary rocks, which extends from San Pablo bay to Amador valley, forming the mass of the Contra Costa hills, for a distance of about thirty-five miles northwest and southeast, and having a breadth of from six to eight miles. The rocks are chiefly sandstones, and in places highly fossiliferous. San Pablo creek heads in this belt, and flows between two parallel ridges, in the line of the strike of the rocks. On the west side of the creek, about four miles a little south of east from San Pablo, the rocks contain considerable bituminous matter, and a well had been bored here in 1862 to the depth of eighty-seven feet, at which point oil was struck, which it was proposed to purify by distillation, and works were erected for this purpose, as also to obtain oil from the highly saturated sandstone.* At these springs the rock has a high dip northeast; but farther northwest it dips to the southwest, while the hills in the vicinity are too deeply covered by soil and decomposed rock to admit of the general position of the strata being determined satisfactorily.

To the north of San Pablo are low hills of very recent strata, which are nearly horizontal and which rest uncomformably on the edges of the Tertiary. Whether these beds contain any extinct species of shells has not yet been determined; at all events, they are no older than the Post Pliocene.

In the valleys between San Pablo and Walnut creeks, many sections made by the rains of 1861-62 in the superficial detritus are observed. The beds are horizontally stratified, and made up of light and darker-colored materials, the lighter ones being darker near their upper surfaces, and growing lighter downwards to the depth of from six to twelve inches, as beds usually do when acquiring a color from decaying vegetable substances.

*The quantity of oil obtained seems to have been too small to pay, as the work was not profitable, and had been discontinued previous to the oil excitement of 1865; whether resumed between that time and the present, 1882, we have been unable to discover.

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This would indicate that the rate of deposition of this detritus has been exceedingly irregular, long periods having sometimes elapsed without much addition to the detrital deposits, and then, again, a heavy mass of materials being suddenly spread over the surface, just as takes place at present during a Winter of extraordinary storms, like those of 1861-62. The appearances indicate sometimes a heavy deposit during one year only; at others, a succession of them for several years. The same or similar facts are observed at many points in the Coast Ranges.

The whole range under consideration is denuded into a great number of hills and valleys, the latter running parallel with the strike of the strata. The valleys are excavated in the softer materials, and are frequently drained by streams running in two opposite directions, which connect at their sources by very low divides, so that one hardly recognizes the fact that he is passing over them. When streams cut across the strike of the strata, as they occasionally do, the valleys become mere canons, or narrow rocky defiles.

To the southeast of Martinez there is a good exhibition of the folding of the strata exhibiting in synclinal axis, which runs from a point one mile north of Pacheco, southwest to the Cañada del Hambre, a distance of about four miles.

Walnut creek (Arroyo de las Nueces) heads in the divide between the valley of this name and that of the San Ramon; it separates the Contra Costa hills from the Monte Diablo group proper. High hills of Tertiary sandstone rise to the west of it, attaining an altitude of from eighteen hundred to two thousand feet. The high group of hills north of the head of the San Ramon is also of sandstone, and has about the same elevation. The strike of the strata here is about N. 50° W. to N. 55° W., and the dip 65°, to the southwest. The San Ramon, heading in this group of hills, runs southeast, then turns and runs parallel with its former course in the opposite direction, having a high and steep range of fossiliferous sandstones between the two parallel portions.

The foot-hills along the eastern base of these higher ridges are of strata very much broken, with every possible dip and strike, the latter frequently at right angles to that of the strata in the main ridge, and standing vertical. There are indications of a line of quite recent disturbances of the rocks through the San Ramon and El Hambre creeks, which line crosses the general direction of the stratification at an angle of 35°. There are fissures in the soil along the west side of the San Ramon valley, which were formed during the earthquake of June, 1861, and which may be considered as strengthening the probability of the recent formation of this valley. That extensive disturbances have taken place in the Monte Diablo chain within the most recent geological epoch will be seen farther on.

Near the head-waters of the San Ramon, the hills of Tertiary sandstone rise to the height of about two thousand feet; the strata having a

strike of about N. 39° to 41° W. and they have a high dip to the southwest. The same strata, as followed along a few miles farther to the northwest, near Moraga valley, become more nearly vertical, and the strike curves around more to the west. The same belt of rocks extends southeast from the head of the San Ramon, through the range of hills west of Amador valley, and they have a lower and more uniform northwesterly dip. These hills sink into the plain near the eastern end of the pass leading from Haywards to Amador valley.

Near the "Walnut Creek House," a small patch of cretaceous occurs, extending over a few acres, from which the overlying Tertiary, forming the crown of a low anticlinal, has been denuded.

A belt of metamorphic rock may be traced along the western side of the Contra Costa hills, beginning near San Pablo, thence following the west side of Wild Cat creek, and appearing in a southeast direction along the foothills of the range, for a distance of about thirty-five miles. It generally forms a narrow belt, not over two miles wide, and often not half that; but in some places there is more or less metamorphic action observable over a width of four miles. The northwestern portion of this band of altered rock curves to the northwest, and seems to form the isolated metamorphic hills lying near the Bay, and west of San Pablo and islands of similar rock in the Bay, apparently connecting with the range of high hills which run out at Point San Pedro and extend back of San Rafael.

Near San Pablo a great variety of the results of metamorphic action may be observed; as, for instance, in following a line extending from the house of V. Castro back to the top of the ridge. The original rock seems to have been a more or less bituminous slate or shale, and patches of it have almost entirely escaped metamorphism, while others in the immediate vicinity are very much altered and converted even into mica slate. The dip of the strata, when it could be made out, was to the northeast, 30° at the base of the hill, and gradually getting higher towards the crest of the ridge, where the metamorphism is most complete. Here the rock is traversed by small quartz veins, and has evidently been acted on by water containing silica in solution, as it is, to a large extent, converted into that mixture of ferruginous, jaspery and chalcedonic material, which is so well known as frequently containing cinnabar, that we have become accustomed to call it the "quicksilver rock." Considerable masses of actinolite have been found lying on the surface in this vicinity, evidently derived from the rocks of this ridge. The specimens resemble exactly those obtained from the very much older metamorphic rocks of New England.

The widest and highest portion of this metamorphic belt lies near the pass leading from Oakland to Lafayette, the summit of which is thirteen hundred and eleven feet above high tide. About a hundred rods west of the summit metamorphic slates stand vertical, having a close lithological