212 University of California Publications in Geology [Vol. 13 



Minor faulting, like that seen in the Friday Mine and described 

 in the chapter on that mine, is probably widespread throughout the 

 region. 



THE FRIDAY MINE ORE BODY 



The Friday Mine is situated immediately south of the road between 

 Cuyamaca Reservoir and Julian, at a point four miles southeast of 

 that town. It lies half a mile southwest of the divide between drain- 

 age to the Pacific Ocean and that to the Colorado Desert. The eleva- 

 tion at the mine is about 4700 feet above sea level. It is situated in 

 an embayment of the main area of the Cuyamaca Basic Intrusive. 



Discovery and Development 



The original location of the Friday claim was made on an outcrop 

 of gossan lying 55 feet northwest of the shaft now in use. The shaft 

 there sunk was abandoned, apparently because ore was not found, 

 and a new one started in gabbroic rock. At 50 feet depth it entered 

 a body of schist which dips south at an angle of 75°. At 127 feet 

 depth the lower side of the schist body was reached and a drift was 

 run to the northeast along the contact of schist and basic intrusive 

 rock. The drift entered sulphide ore at 50 feet from the shaft and 

 was continued to the northeast, reaching the southwest or footwall side 

 of the ore body at a point 85 feet from the shaft. From this point 

 the footwall of the ore body was followed for 20 feet, and then a north- 

 west crosscut was driven to the northern limit of ore, where a winze 

 was sunk 16 feet on the contact. A working to the northeast from 

 the junction of crosscut and main drift failed to find any ore. At a 

 later date an incline was put down from the bottom of the shaft, along 

 the schist body, to a point 45 feet measured vertically below the first 

 level. Drifting along the footwall of the schist on this lower level 

 failed to disclose any ore. After considerable blind work the down- 

 ward continuation of the ore was found, as a thin wedge, almost im- 

 mediately beneath the winze on the upper level. 



General Geologic Description 



The underground workings of the Friday Mine penetrate five dis- 

 tinct formations, the most abundant being the Cuyamaca Basic In- 

 trusive, which here is predominantly a norite, but includes gabbro 

 and peridotite facies. Included in the basic rock are several bodies 



