56 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
CALIFORNIA. 
By Edwin C. Eckel. 
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 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 Geolog- 
ical Survey. The workable roofing-slate deposits of this district occur in a belt of i 
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 la rue area of diabase. This diabase, according to 
Lindgren and Tinner, 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 Geo- 
logical 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 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 indi- 
cated 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 min- 
eralogist of California, particularly in the eighth and twelfth reports. The geologic 
relations of the slate-producing areas are shown in PI. X. 
At present 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 cleavage planes of the slates in the Eureka 
quarry strike X. 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 a 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 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 
