GEOLOGY AND MINERALOGY OF SHASTA COUNTY.

By HAROLD W. FAIRBANKS, F.G.S.A.

Shasta County occupies an important position in the structure and topography of the State, being situated at the head of the Sacramento Valley, and having its surface chiefly occupied with rugged mountains; the union of the Sierra Nevada and Coast Range. It is interesting alike for the great variety of its rock formations and the range of geological time represented by them, as well as for its mineral resources, whose richness is just beginning to be appreciated. It is in this county that the Carboniferous was discovered by Dr. Trask, the first State Geologist. Since then the county has been the subject of geological investigation by the old State Survey under Professor Whitney, and particularly of late by the members of the United States Geological Survey. The geological problems here presented for investigation are of uncommon interest. Among them may be mentioned the disputed point concerning the relation of the Coast Range to the Sierra Nevada, that of the Cretaceous to the Metamorphic Series, and the possibility of the presence of older Paleozoic rocks as well as those of early Mesozoic age. As a mining county it first came into prominence on account of the extensive placers which existed along nearly all the streams flowing into the Sacramento from the west. As the placers failed the gold and silver deposits were opened, but on account of the rebellious character of the ores the operations were not generally profitable. It is only within the last few years that quartz mining has been carried on with any degree of success. Now the county is rapidly becoming one of the foremost producers of the precious metals in the State.

In undertaking an investigation of the county for the State Mining Bureau, I have thought best to give particular attention to geology. Too much emphasis cannot be laid on the necessity of a thorough knowledge of the underlying geological features of a mineral district before attempting to give a description of its ores. In the following report I have described as accurately as possible, with the time at my disposal, the general geology of the county and the geology of the ore deposits, gained by a visit to one or more of the typical mines in each district. All the mines could not be visited in the limited time, but especial attention was given to those districts not heretofore described in the Mining Bureau reports.

The map which I have prepared to accompany the report is intended to show the areas occupied by the various geological formations and also the positions of the important mineral belts.

As yet I have had no opportunity to study the large collection of rocks. made in the county, nor have the many new species of fossils from the older formations been classified, consequently the following notes are based almost wholly on field observations.

The Modern, or Quaternary, deposit of the Sacramento Valley has a

width of about 20 miles on the southern boundary of the county, stretching from Gas Point on the west, to the lava a short distance east of the Sacramento River. It consists of stratified sands, gravels, and coarse conglomerates derived from the crystalline rocks of the surrounding mountains. These unconsolidated deposits narrow northward and disappear before reaching Pitt River. In the vicinity of Anderson the deepest wells do not penetrate through them. Between Redding and Millville the drilled wells in many places pierce the underlying Cretaceous shale, the water from which is generally somewhat brackish. The Cretaceous beds inclose the modern deposits, basin-like, on three sides. The Cretaceous is covered by lavas on the east; on the north and west it rests against the Auriferous Series, which is in part Carboniferous.

I will now take up the detailed description of the county, beginning east of the Sacramento River, following the Sierra around to its junction with the Coast Range, and tracing the latter southward.

A hasty trip was made from Anderson east through Shingletown over the old Fort Crook road, to Fall River Valley, passing close by the northern base of Lassen's Butte.

The whole of this southeastern part of the county is a great lava plain, gently sloping to the Sacramento River, and reaching an elevation of 5,000 feet near the summit of the range. South of Bear Creek no deep cañons have been eroded, though in scattered spots the thin lava crust has been cut through, exposing the sedimentary beds. The surface is that of broad ridges, generally well timbered, and with more soil than is generally found on the lava.

The South Fork of Bear Creek, about a mile above its mouth, falls over the edge of the lava plateau to join the main fork where the underlying sandstones are exposed. Shales and sandstones are also exposed along the creek in the vicinity of Clark's Mill for about 2 miles. Gold has been found in the gravels underlying the volcanic tufas in several places along Bear Creek, associated with metamorphic pebbles. This indicates a former exposure of the Auriferous Series, higher in the mountains, which is now covered with lava. A thin seam of coal is reported from Snow Creek, and also some Upper Cretaceous fossils.

The elevation of Whitmore Post Office, on the Tamarack road, is 2,150 feet. North Cow Creek, 1 mile away, is 600 feet lower. It has cut down to the Cretaceous beds, which are exposed first about 3 miles above. In the north side of the creek, near Whitmore, a coal seam has been exposed; it is 14 inches thick, dipping north 30°. Up the creek thin seams of coal outcrop for several miles. One of these appears to be 10 inches thick and dips to the southeast 40°. Another aggregate of seams with clay layers between them has a thickness of 4 feet.

The

Where the Tamarack road strikes North Cow Creek there are large expanses of undisturbed Cretaceous sandstones, with many fossils. ridge between this creek and Clover Creek is not high and is formed entirely of lava fragments and bowlders imbedded in a matrix of volcanic ash or sand. About 6 miles east of Millville this ridge is replaced by hills of Cretaceous shale, dipping slightly to the east and south. The adobe hills formed by this shale are about 3 miles across. On the west they are replaced by volcanic ash having a very barren surface. Near the western edge of the Cretaceous beds is a conglomerate of bowlders derived from the old crystalline rocks resting unconformably on the Cretaceous.

The Cretaceous (Chico) beds appear on Clover Creek, about 3 miles east of Millville, and are continuous along the creek and hillside, up to where the creek divides; then they continue to appear in the valleys of the different branches, the total surface area being considerable.

About a mile east of the point where the creek divides, the old Metamorphic or Auriferous Series outcrops. It is a dark green rock, generally. massive, though jointed. The strike is north 70° west, dip vertical. This extends easterly about a mile, when it becomes capped by the lava. In a southeast direction it crops out for 2 miles, and on the eastern edge of the exposure is replaced by a quartz porphyry.

An interesting series of rocks appears near Mr. Whitmore's place, Sec. 3, T. 32 N., R. 1 W. At the base of the hills near Dry Clover Creek there are cliffs of undisturbed sandstone containing strata of pebbles and good examples of false bedding. A short distance west, in the same formation, are several thin seams of a good quality of coal, the thickness of which, as far as it has been explored, is not great enough to pay for working. A fourth of a mile up Galt Gulch the soft sandstones are replaced by highly altered rocks dipping north 20° to 30°, strike north 70° west. These consist of thin layers of apparently crystalline material closely resembling a fine-grained granite. Alternating with these are softer strata, largely feldspathic, which form soft white clays on decomposition. The peculiar thing about these strata is the presence in them of a layer 18 inches thick of black carbonaceous matter, containing a little coal. The strata cannot be traced all the way to the coal-bearing formation below, and it seems hardly possible that they belong to the same series, unless there has been local metamorphism. Coal has never been found in the older Metamorphic Series, and should this prove to belong to that series it would be a most interesting fact. A microscopic examination is necessary to determine positively the nature of the strata containing this coaly layer, but it is certainly very peculiar that the carbon should remain in rocks so perfectly crystalline.

Farther up this ridge, north of the Dry Fork of Clover Creek, the old metamorphics crop out; the rock is massive, light green in color, and greatly jointed, so that the bedding cannot often be distinguished. A serpentinoid diabase has been intruded into this rock, as shown in a small exposure. Patches of unaltered sandstone rest on the rocks just described, but the latter are generally covered by lava breccias; the cementing paste appears to be a ground-up rock similar to the fragments, showing the action of water.

Clover Creek Falls are about 70 feet high and very picturesque. The water falls over a cliff of the older series. The rock contains large grains of quartz in a felsitic matrix, and is somewhat similar to that south of the town of Shasta. The ridge between the falls and the Oak Run road is formed of volcanic breccia.

The Chico shales form an extensive valley for many miles along Oak Run, terminating about 3 miles southwest of Morley Post Office. They rest against the Auriferous Series, which is then exposed along the creek for a mile or more, finally being covered by the lava. A very interesting succession of rocks is shown. As we go up the creek, the first is a feldspar porphyry, in which the feldspar crystals are not generally prominent. The rock is in some places perfectly massive, in others laminated, closely resembling a metamorphic one. It often shows traces of a tufa

Fig. 1.

ceous character, with rounded or semi-angular bowlders a few inches to a foot in diameter and sometimes concentrically colored. The matrix appears to be of the same material. This porphyry-like rock is generally more or less crushed, presenting a flowage structure; the laminated portion of the matrix bending and winding among the pebbles. This same structure is sometimes present in the fresh portions. The lamination is north 60° west, dip vertical. In connection with this lamination there is a strong mineralization and silicification of the rock mass, sulphurets being so abundant that the surface is stained a strong yellow color. Placer gold has been found along the creek, and some prospecting done, but without success. Diabase dikes are numerous, and quite irregular in extent and direction, often cutting across the schistose lines of the porphyry. They are coarse in the middle, with prominent feldspars, and become fine-grained toward the edges, where they are often amygdaloidal with calcite. One contains inclosed a thin slab of porphyry. (Fig. 1.) One half mile up the creek the porphyry is followed by rocks of undoubted sedimentary origin, slates, and sandstones. They have a very variable strike and dip, due to the intrusion of dikes. The slates are often finely cleavable. The most prominent dike is amygdaloidal near the slate; in fresh spots it closely resembles the porphyries; a little distance from the slates it becomes laminated and somewhat mineralized, with traces of a conglomeritic structure. If it were not for the amygdules it would certainly be taken for a sedimentary rock.

The rock forming the country northwest toward North Cow Creek is chiefly of a feldspathic nature, often bordering on the porphyritic with quartz or feldspar. In one spot there is shown a plain conglomerate structure, in which the pebbles are not all alike; some are diabasic, others strongly porphyritic. Seldom is there any appearance of bedding seen, but when it is present it is very irregular. The quartzes in the porphyry do not generally show idiomorphic boundaries, but resemble those in the quartz felsites. It is probable that the greater part of this rock consists of old eruptives. There is no true bedding, and the uniform character and direction of the schistose structure indicate that it is due to dynamic action.

The ridge north of Oak Run is capped with basalt; it terminates a

half mile east of North Cow Creek, having near the end a height of 1,700 feet. Where the road crosses Oak Run above Morley's an amygdaloidal serpentinoid rock outcrops. North of the creek the road passes for a mile over shaly rocks, with occasional patches of sandstone. The slates strike north 60° west, dip 50° to northeast. The most of this rock has a porphyritic aspect and the lenticular, shaly structure characteristic of dynamic forces. The high ridge to the north is capped with basalt.

Chico sandstones outcrop in a narrow strip for several miles along Oak Run, in T. 33 N., R. 1 W. It also appears in greater mass along Little Cow Creek, north of the high ridge separating the two creeks, and it is probable that it extends continuously through underneath the lava capping. Coal prospects have been found along the sides of this ridge for several miles, but the only locality in which any development had been made at the time of my visit is on the northern slope of the ridge, about half a mile east of the Donkey Mine, in Sec. 12, T. 33 N., R. 2 W. Here the coal seams are shown in a shaft sunk a little above the outcrop. There is a total thickness of 23 feet exposed; this is cut up into numerous seams by the layers of clay. There are two seams of coal 2 feet thick, one 15 inches, one a foot, and several others slightly less. The aggregation of so many seams in a small space makes it easy to mine, and if the beds are really as extensive as the outcrops indicate a valuable coal field exists here. The coal is apparently of good quality, though some iron pyrites occur in it at the spot where the opening has been made. A quarter of a mile to the west and a little lower than the coal beds, feldspar porphyry outcrops. This shows that it is probably only a little distance down from the coal to the Auriferous Series, and development only will show whether underneath the ridge this series rises high enough to cut off the coal. It is probable, however, that this series sinks toward the east and becomes buried under thousands of feet of the Cretaceous beds. The elevation of the coal beds is 1,900 feet. The strata are nearly level, but north of Little Cow Creek they outcrop 300 feet lower, showing a slight dip to the east. The coal is overlaid by sandstone; underneath is clay and shale.

The coal beds are exposed along the southern slope of the ridge north of Little Cow Creek. They begin on the Iles place, near the stage road, and extend up the creek for several miles. The older series outcrops just below where the road crosses the creek, but does not appear farther east; the Cretaceous beds themselves are covered by lava tufas about 6 miles up the creek. No developments have been made here, but there are apparently three sets of coal seams occurring within a perpendicular distance of 200 feet. The beds dip 5° to 10° east and northeast. The lowest is near the bed of the creek, and its thickness cannot be seen. The middle set evidently corresponds to the beds south of Cow Creek just described. The thickness is about 20 feet; elevation 1,600 feet. It consists of two main parts: one 6, the other 8 feet, divided by several feet of clay. Each division contains several thin clay seams, the thickest solid coal seam being about 24 feet. There is no doubt but what coal is present here in sufficient quantity to mine, though the set of beds just described being inclosed in shale, much timbering will be necessary. The heavy bedded sandstones which outcrop along the side of the mountain on the north are conformable with the coal beds, and evidently belong to the same formation, though they have been reported to be Miocene. North