CALIFORNIA. 57 
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 cleav- 
age spaces. Pyrite also occurs in very much flattened nodules, which wore appa- 
rently 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. 
Silica (Si0 2 ) 63. 52 
Alumina (A1 2 3 ) and titanic oxide (Ti0 2 ) 16.34 
Iron oxides (FeO, Fe 2 3 ) 6. 79 
Lime ( CaO ) 98 
Magnesia ( MgO ) 2. 50 
Carbon dioxide (C0 2 ) - ) 
Water } 4 ' 86 
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 matter but extremely little mag- 
netite. The sawn edges show pyrite and lenses, and streaks of lighter gray 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 polariza- 
tion and a general evenness of texture, interrupted, however, by lenses up to about 3 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, rutile needles, a few grains of tourmaline, and rarely one of zircon. 
The main constituents of this slate, arranged in order of decreasing abundance, appear to be mus- 
covite (sericite), quartz, chlorite, carbonate, pyrite, carbonaceous matter or graphite, rutile, and 
magnetite, besides accessory tourmaline and 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 — 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 mas- 
sive 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 con- 
sidered. 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 aver- 
« 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. 
