1922] Hudson: Geology of the Cuyamaca Region of California 209 



Basic Dikes 



Dikes of dark, dense, very fine-grained rock cut the various mas- 

 sive basic rocks exposed in the workings of the Friday Mine. Similar 

 dikes cut the massive sulphide ore. All these rocks are brown-horn- 

 blende gabbros and brown-hornblende norites. Their textures differ 

 in no way from that of the coarser, larger masses of the same compo- 

 sition, save in size of grain. All of them carry pyrrhotite in the same 

 manner as the coarser rocks. Dikes of fine-grained norite and norite 

 porphyry were also found at the surface in the neighborhood of the 

 mine. These dikes represent the last activity of the basic intrusion. 

 The fact that they cut the massive sulphide body is evidence of prime 

 importance in the matter of the origin of the ore. 



Pegmatite 



Pegmatite cuts all the rocks of the region. It was not observed 

 in intrusive contact with the massive sulphide body of the Friday Mine, 

 but it does cut the fine-grained dikes which intrude the ore. 



In an outcrop of gneissoid quartz diorite at the junction of the 

 creeks in the northwest quarter of section 12, township 13 south, range 

 3 east, certain narrow pegmatite veins are contorted, though conform- 

 ing to the schistosity of the gneiss. Other veins are perfectly straight 

 and cut across the schistosity. At another locality southeast of the 

 Friday Mine similar facts were observed. There the later veins, the 

 straight ones, carry tourmaline, which is absent from the contorted 

 veins. Sufficient time was not available for further inquiry. The 

 observations suggest, however, that there are two ages of pegmatites, 

 the first related to the quartz diorite intrusion, the second to that of 

 the Rattlesnake granite. 



The pegmatite occurs as large, irregular lenses, and dikes of all 

 sizes cutting all the older rocks. It is especially abundant in certain 

 portions of the Rattlesnake granite. In the schist it occurs in the 

 forms mentioned above, and also impregnates the schist as a lit par lit 

 injection, in the same manner as granitic material. 



Petrography of the pegmatites. — A specimen of very coarse peg- 

 matite from near the center of a large irregular mass exposed in the 

 lower level of the Friday Mine is composed of quartz, abnormal ortho- 

 clase, microcline, albite-oligoclase, oligoclase, and black tourmaline. 



A specimen from a six-inch vein in the large schist body on the 

 lower level of the mine is made up of acidic andesine, labradorite, 

 quartz, and tourmaline. 



