204 University of California Publications in Geology [Vol. 13 



The ore minerals, magnetite, pleonaste, and pyrrhotite, are believed 

 to be primary constituents of these rocks. A detailed discussion of the 

 evidence on this point will be given in the chapter on the Friday Mine. 



Rock alteration. — About one-third of the rocks examined contain 

 more or less green hornblende. In many cases this mineral is clearly 

 of secondary origin, formed at the expense of augite or hypersthene. 

 The green hornblende differs in occurrence from the brown variety 

 in that euhedral cross-sections are never seen. The green hornblende 

 is often seen as large, optically continuous, individuals with no rem- 

 nant of the pyroxene from which it is probably derived. Again it 

 occurs in aggregates of smaller individuals, with or without residual 

 masses of pyroxene. In either case the hornblende is charged with 

 magnetite, sometimes as dust, sometimes in irregular masses occupy- 

 ing a coniderable portion of the hornblende body. This magnetite is 

 clearly of secondary origin, a by-product of the alteration of pyroxene 

 to hornblende. Its occurrence, as a dust, is quite different from that 

 of the primary magnetite, such as that described in specimens nos. 

 124 and 202. The alteration of pyroxene to green hornblende prob- 

 ably involves the addition of no material except water. The source 

 of the water in this region may very well have been the pegmatite 

 intrusions. Some evidence bearing on this point is given in the section 

 on pegmatites. 



Two specimens from the Cuyamaca basic intrusive mass were 

 found to contain very small amounts of chlorite. A sample of norite 

 from the area of basic rock east of Descanso contains chlorite and 

 calcite. 



A specimen of olivine gabbro from the north flank of North Peak 

 contains fibrous serpentine developed at the edges of the olivine grains. 

 The only other occurrence of this mineral noted in the district was a 

 minute amount in one specimen from the Friday Mine. 



The greater part of the areas of basic rocks is mantled by a brown 

 to red soil containing boulders of fresh rock. This soil is sometimes 

 underlain by material which represents an intermediate stage of 

 weathering. It is rock decomposed in place with preservation of tex- 

 ture. Green blotches, representing original hornblende or pyroxene, 

 lie in a light gray or yellow, friable matrix which represents the 

 plagioclase. In such material the boulders of fresh rock, noted above as 

 occurring in the red soil, can be seen in every stage of their formation 

 from joint-bounded blocks. The rounding takes place by decompo- 

 sition of the edges of blocks. No attempt has been made to determine 

 the constituent minerals of the weathered products. 



