1 6 EDWIN C. ECKEL 



bordered by a large area of diabase. This diabase has been 

 described^ by Lindgren and Turner as being "of the age of the 

 Mariposa slates, or older." A number of linear areas of amphib- 

 olite 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 OH the slate deposits. — The " Placerville Folio," 

 No. 3, U. S. 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 impor- 

 tance, though all the quarries now in existence 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 cut- 

 ting 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 Mi?ieralogist of California; 

 particularly in the eighth and twelfth reports. 



At present the most important quarry is that of the Eureka 

 Slate Co., and this is now being worked on a large scale. This 

 quarry is located at Slatington, about one-half mile southwest 

 from the point where Kelsey is shown on the Placerville atlas sheet. 



Structural re latio7is in Eureka quarry. — The cleavage 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 40° to 60° dips to 

 the east or west, according to local conditions. This overturn- 

 ing 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. 



^" Placerville Folio," U. S. Geological Survey; legend of " Areal Geology" sheet. 



