210 University of California Ptiblications in Geology [ VoL - 13 



A suite of specimens was collected to illustrate the intrusion of 

 gabbro by a narrow pegmatite dike at a locality in the end of the west 

 branch of the north cross-cut, lower level of the Friday Mine. The 

 basic rock near the pegmatite is a partially uralitized augite gabbro. 

 Its plagioclase is labradorite, Ab 35 An 6S . The basic rock immediately 

 adjacent to the pegmatite is composed of bytownite, green hornblende, 

 and intrusive quartz. There is no augite whatever. The immediately 

 adjacent pegmatite is composed of quartz and andesine feldspar, 

 Ab e3 An 35 , while the pegmatite a short distance farther from the con- 

 tact has the same constituents with the addition of a small amount of 

 biotite and labradorite. 



It is noteworthy that the feldspars of the pegmatite from the center 

 of the large mass include considerable amounts of potash-bearing varie- 

 ties, while the pegmatites of the narrow dikes have only soda-lime feld- 

 spars. This suggests that through material derived from the basic 

 wall rocks a synthetic soda-lime pegmatite has been produced. 



The tourmaline of the pegmatites examined has a pleochroism of 

 deep steel blue and light blue-gray. The crystals are frequently frac- 

 tured, the cracks being filled by feldspar and quartz. It is clear that 

 the tourmaline was the first constituent to crystallize. 



It is suggested that the alteration of the pyroxene of the gabbro 

 to green hornblende has been brought about by material supplied dur- 

 ing the intrusion of the pegmatite. Water was probably the chief, 

 perhaps the only, chemical agent. It may be that the formation of 

 the green hornblende in all the basic rocks dates from the intrusion 

 of the pegmatites. 



Aplite 



A dike of aplite, varying from six to ten feet in thickness, intrudes 

 the schist body southeast and east of Cuyamaca Reservoir. It is con- 

 tinuous for about two miles, its strike conforming essentially to that 

 of the schist. Locally it is offset, for distances of a few inches to a 

 few feet, and at one place it is offset for several hundred feet. These 

 offsets do not appear to be the result of faulting later than the intru- 

 sion of the dike, but seem to result from the fact that the fracture 

 which the dike filled was not continuous. Another aplite dike, with 

 somewhat sinuous outcrop, cuts the quartz diorite gneiss one mile west- 

 northwest of Julian. 



A microscopic examination of a sample from the first mentioned 

 dike proves it to be a soda aplite, composed of phenocrysts of albite, 

 oligoclase, and quartz in a ground mass of quartz, plagioclase, sericite, 



