ROOFING SLATE OF IGNEOUS ORIGIN I J 



The slate body shows rather frequent, but narrow, "ribbons." 

 These ribbons are bands (from -^^ to |- inch thick usually, but 

 occasionally as thick as two inches) of material differing in com- 

 position 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 ten degrees 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 So° to the northwest. 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 tiormal 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 

 very satisfactory. 



A specimen of the black slate, free from ribbon, was selected 

 for analysis in the laboratory of the U. S. Geological Survey. 

 The results of this analysis, by Mr. W. T. Schaller, follow: 



ANALYSIS OF BLACK SLATE, EUREKA QUARRY, SLATINGTON, CALIF. 



Silica (Si02) ------ 63.52 



Alumina (AI2O3) and titanic oxide (Ti02) 16.34 



Iron oxides (FeO, Fe203) - - - - 6.79 



Lime (CaO) ------ 0.98 



Magnesia (MgO) - - - - - 2.50 



Carbon dioxide (CO2) ) 



h - - - - 4-86 



Water '^ 



