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1906 - California Slate Industry (up to 1906)

Excerpts from

Slate Deposits and Slate Industry of the United States, Bulletin No. 275,

by T. Nelson Dale with sections by E. C. Eckel, W. F. Hillerbrand, and A. T. Coons,
Slate Deposits and Industry of the United States - California,
by Edwin C. Eckel, pp. 56-59, Department of the Interior,
United States Geological Survey, Washington: Government Printing Office, 1906.

Map - Geologic Map of Placerville Slate District, California. Plate X
Map - Geologic Map of Placerville Slate District, California. Plate X
(From Slate Deposits and Slate Industry of the United States, Bulletin No. 275, 1906.)

Slate Deposits and Industry of the United States - California, by Edwin C. Eckel, pp. 56-59.

"Location and general relations. - Though roofing slate has at different times been quarried on a small scale in other parts of California, the only important slate-producing area in the State is located in Eldorado (sic) County. The quarries which have been opened in this district are located along a line running about N. 15 W. from Placerville, at distances of 1 to 6 miles from that town. The location and geographic and geologic relations of the slate deposits and quarries can best be understood by reference to the maps included in the Placerville folio of the United States Geological Survey. The workable roofing-slate deposits of this district occur in a belt of the Mariposa slates, of late Jurassic or early Cretaceous age. The quarries which have been opened are all situated near the western boundary of this belt of Mariposa slates, where it is bordered by a large area of diabase. This diabase, according to Lindgren and Turner, is 'of the age of the Mariposa slates, or older.' A number of linear areas of amphibolite occur in the Mariposa slates. These amphibolites are described as being derived from diabase or gabbro. They are in part altered to serpentine.

"Previous work on the slate deposits. - The Placerville folio, No. 3, United States Geological Survey, published in 1894, contains the results of detailed geologic work by Lindgren and Turner in the area in which the roofing-slate deposits occur. At that date the roofing-slate industry had not assumed its present importance, though all the quarries now worked (circa 1906) had then been opened. The existence of roofing-slate deposits is noted in the text of the folio, and the locations of the quarries are indicated on the map showing the economic geology of the area. No reference is made to the 'green slates,' or to the dikes cutting the Eureka quarry.

"Excellent though brief descriptions of the different quarries and of the condition of the slate industry at various dates are to be found in the reports of the State mineralogist of California, particularly in the eighth and twelfth reports. The geologic relations of the slate-producing areas are shown in Pl. X.

"At present (circa 1906) the most important quarry is that of the Eureka Slate Company, and this is now being worked on a large scale. This quarry is at Slatington, about one-half mile southwest of Kelsey.

Structural relations in Eureka quarry. - The overturning planes of the slates in the Eureka quarry strike N 25 W. The dip of the cleavage is practically vertical, with slight local variations to 80 E. or 80 W. The upper weathered beds in the quarry are overturned by local pressure so as to give dips of 40 to 60 to the east or west, according to local conditions. This overturning is evidently due merely to the weight of the overlying soil and decomposed slate, and the effects are shown only for a depth of from 3 to 15 feet. It is of interest, however, as of warning against accepting dip readings taken from surface beds of the slate.

"The slate body shows rather frequent, but narrow, 'ribbons.' These ribbons are bands (from one-sixteenth to one-half inch thick usually, but occasionally as thick as 2 inches) of material differing in composition from the mass of the slate. They are in general more siliceous than the normal slate, and do not furnish merchantable material. Their geologic interest arises from the fact that they represent differences of original sedimentation. The plane of the ribbons in a slate quarry is, therefore, the plane of the original bedding. In the Eureka quarry, and, indeed, throughout the roofing-slate belt, the plane of original bedding seems to be usually within 10 of the plane of slaty cleavage.

"The slate mass is cut by a series of joints parallel to the 'grain' of the slates, striking N. 55 E. and dipping from 70 to 80 NW. Joints across the 'grain' of the slate, which would be practically horizontal, do not occur in this quarry, but many of the thin quartz seams occupy this position.

"Quartz and calcite occur in thin layers, filling joint spaces and occasionally cleavage spaces. Pyrite also occurs in very much flattened nodules, which were apparently parallel to the original bedding.

Character of the normal slate. - "The mass of the Eureka quarry product is a dense, deep-black slate, splitting very finely and regularly, with a smooth glistening surface, much like that of the Bangor and Lehigh slates of Pennsylvania. The frequency of the ribbons and of the pyrite nodules prevents the slate from being serviceable as mill stock, but as a roofing material it is excellent.

"A specimen of the black slate, free from ribbon, was selected for analysis in the laboratory of the United States Geological Survey. The results of this analysis by Mr. W. T. Schaller, follow:

Analysis of black slate, Eureka quarry, Slatington, Cal.
Analysis of black slate, Eureka quarry, Slatington, Cal.

"Prof. T. Nelson Dale has reported as follows on a sample of this slate:

"This slate is very dark gray. To the unaided eye it has a fine texture and a smooth and lustrous cleavage surface. It contains considerable carbonaceous or graphitic material. There is no effervescence in cold dilute hydrochloric acid. It is sonorous and has a high grade of fissility.

"Under the microscope it shows a matrix of muscovite (sericite) with a brilliant aggregate polarization and a general evenness of texture, interrupted, however, by lenses up to about 8 mm. long by about one-half mm. wide, consisting chiefly of quartz fragments (surrounded by a rim of radiating secondary quartz) with muscovite scales, plates, and rhombs of carbonate, and rarely a grain of plagioclase. These lenses have their long axes parallel to the cleavage, but in sections parallel to it some of them have such irregular outline and are so large as to appear like minute beds. The matrix contains many quartz fragments, measuring up to 0.09 by 0.03 mm., much less carbonate than the lenses, muscovite scales, chlorite scales, a little pyrite in spherules and crystals, carbonaceous or graphitic matter, ruttile needles, a few grains of tourmaline, and rarely one of zircon.

"This ought to prove a serviceable slate. It will be noticed that carbonate, although present, is insufficient to produce effervescence.

"Eureka quarry green slate.* - A band of green slate several feet wide crosses the Eureka quarry. On examination it is found that the borders of this band are not parallel to the 'ribbon' of the black slate. The green band can not, therefore, be interbedded with the black slates. The probability that it represents a dike of massive igneous rock which has been changed to a slate by pressure subsequent to its intrusion is strengthened when the chemical composition of the green slate is considered. Two analyses of the green slates are presented on the next page. The first is of a sample selected by the writer and analyzed in the laboratory of the United States Geological Survey by Mr. W. T. Schaller; the second was given by Mr. C. H. Dunton, manager of the Eureka quarry, but the name of the analyst is unknown. As the analyses show a close agreement in essential features, it is probable that they are fairly representative of the composition of the green slates, and that their average, which is given in the third column of the table, may be regarded as typical of this interesting and apparently unique type of roofing slate.

(* Page 57 footnote a: For a more detailed discussion of this interesting "igneous slate" the reader is referred to the paper by the writer in the Journal of Geology, vol. 12, 1904, pp. 15-29.)

***Analyses of igneous green slate, Eureka quarry, Slatington, Cal.
***Analyses of igneous green slate, Eureka quarry, Slatington, Cal.

"These analyses have been compared* with a series of 36 analyses of American roofing slates derived from clays by pressure, and remarkable differences in composition are apparent. The green slate, on the other hand, approximates closely in composition to certain 'basic' igneous rocks of the district, and it is probably that it was derived from a gabbro or similar rock.

(* Page 58 footnote a: Jour. Geol., vol. 12, 1904, p. 26.)

"The 'green slate' is in reality grayish green in color. It splits readily, though with not so smooth a surface as the black slate. It stands punching and trimming well, and is sufficiently strong for roofing use. Considering its origin and composition it is probable that it will be a highly durable slate, holding its color well. At present (circa 1906) it is sold entirely for trimming and lettering on black slate roofs, for which purpose it is particularly well adapted, giving a strong but pleasant color contrast.

"Chili Bar Slate Company quarry. - This quarry is located about 3 miles north of Placerville, in sec. 36, T. 11 N., R. 10 E., on the south side of the South Fork of American River, a few hundred yards east of the Placerville-Kelsey stage road.

"This is the oldest quarry in the district, having been opened about twenty years ago. It has been shut down since 1897.

"Several openings were made in a bluff forming the river bank at this point. In the easternmost of these openings, which is about 40 feet high and 30 feet wide, a rather poor slate with irregular joints is shown. The cleavage strikes N. 20 W., and dips 75 E. The westernmost opening is small, with a tunnel which was apparently run in on a band of better slate. The slate piled in the yard has kept its color fairly well.

"It seems possible that this quarry may be flooded at high water. Both it and the one next mentioned (San Francisco quarry) are badly located, having no large dumping area available near the quarries. Neither quarry has gone deep enough to get really good slate, which might have been found at a greater depth.

"San Francisco Slate Company quarry. - This quarry is located in T. 11 N. R., 10 E., within a quarter mile of that of the Chili Bar Slate Company, but on the north side of the river and west of the Placerville-Kelsey road. The principal opening was located about 600 feet north of the river, at an elevation of 150 feet above its bank. A tramway led down to the dressing yards, which were situated at the river bank.

"The cleavage of the slates in the large opening strikes about N. 30 W., and has an almost vertical dip. No slate has been quarried here since 1897. A large stock of trimmed slates is still piled in the dressing yard, and many of these have already discolored badly.

"Transportation and market. - The Eldorado County slates have practically no competition on or near the Pacific coast, while the Eureka quarry has recently placed large shipments in Hawaii and Guam. Until recently the principal problem has been the transportation of the slates form the quarry to the railroad. This was formerly done by wagon hauling over a 6-mile stretch of very hilly road. During the last season, however, the Eureka Slate Company has installed an aerial tramway system from its quarry to a point near Placerville. This tramway is an engineering feat of no mean order, the crossing of the South Fork of American River being the principal difficulty encountered.

"Production of roofing slate in California. - The following table, compiled form figures given in various volumes of Mineral Resources of the United States, shows the amount and value of California slate production for a number of years."

California production of roofing slate, 1890-1901.
California production of roofing slate, 1890-1901.

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