1922] Hudson: Geology of the Cuyamaca Region of California 213 



of schist, which differ in no way from the typical rock of the Julian 

 schist series. A body of massive sulphide ore lies, for the most part, 

 entirely within the igneous rock, but on the upper level of the mine it is 

 also in contact with the schist. Narrow dikes of fine-grained, horn- 

 blende-gabbro and hornblende-norite cut both the massive gabbroic 

 rock and the sulphide ore body. Pegmatite occurs as large irregular 

 bodies and dikes of various thicknesses, which cut not only the massive 

 gabbroic rock but also the dikes of fine-grained basic rock. 



The rocks of the mine. — The main body of schist is a tabular mass, 

 striking northeast and dipping south at an angle of 75°. Along the 

 upper level it has a fairly uniform thickness of 12 feet, but in the 

 lower level its width varies from 1 to 12 feet. This rock differs in 

 no way petrographically from the quartz-mica schist of the main 

 Julian schist body, and is assigned to that formation. The contacts 

 of the schist against the inclosing igneous rock are smooth planes along 

 which a small amount of gouge is developed. That the schist body 

 owes its present position essentially to faulting is, however, doubtful. 

 Some movement along contacts is to be expected where two rocks of 

 such dissimilar physical properties as schist and norite meet. The 

 schist bodies are thought to be fragments torn from some larger body 

 of the same rock by the basic intrusive magma. 



The Cuyamaca Basic Intrusive is represented in the Friday Mine 

 by norites, gabbros, olivine gabbros, olivine norites, brown-hornblende 

 gabbros, peridotites, troctolites, and augite diorite. Norite is by far 

 the most abundant of these types, making up at least one-half of the 

 mass of basic rock exposed here. Peridotite and troctolite are quite 

 rare, as is also augite diorite. With the exception of the augite diorite 

 all the rocks carry pyrrhotite, the amount varying from a trace up to 

 3.0 per cent. Magnetite is rare. These rocks differ in no way from 

 the basic rocks found elsewhere within the mass of the Cuyamaca 

 basic intrusive. 



The massive ore body. — The ore body is an irregular mass the 

 greatest horizontal section of which probably lies between the two 

 levels of the mine. On the upper level its greatest measurement is 

 along a north-south line, a distance of forty feet. The width here 

 varies from five to twenty-five feet. On the lower level the ore body 

 is twelve feet long and only a few feet wide and wedges out before 

 reaching the floor of the working. 



The ore consists of sulphides, of which pyrrhotite and chalcopyrite 

 may be distinguished with the unaided eye, together with various 



