petro(;raphical notes. 7 



rock indicates that it at one time has been subjected to in- 

 tense pressure; a conclusion interesting in view of the occur- 

 rence of the specimen at the foot of the supposed faultscarp 

 and in close vicinity to the slate contact. 



The detailed description of the rocks from various locali- 

 ties in the granitic range, seems to indicate that the largest 

 part of it is composed of a quartz-mica diorite, containing a 

 varying but small percentage of orthoclase. Nos. 1 and 5 

 both differ from the normal type, the former being more 

 basic, approaching the augite diorites, or even the norites 

 by its percentage of monoclinic and rhombic pyroxene, the 

 latter more acid and a nearly normal granitite. 



It is well known that large masses of coarse granu- 

 lar rock, though they, geologically, may be a unit, do 

 not always remain constant in structure and mineralogi- 

 cal composition. Irregular and ill-defined areas of more or 

 less basic character are often included in the predominant 

 rock, and frequently connected with it by means of transi- 

 tions. 



Thus, although the main mass of the Peninsular Sierra 

 along this section may be, and probably is, made up of 

 quartz-mica diorite, still it does not necessarily follow that 

 this character should be retained beyond this line. 



Southward the great massive continues down the penin- 

 sula for at least two hundred or three hundred miles, where 

 at last it disappears under the mesa sandstones. 



Northward it continues through San Diego and Los An- 

 geles counties, and forms the larger part of the Sierra 

 Nevada until, in th<i northern part of that range, the aurifer- 

 ous slates gradually encroach upon it. 



The rock of this vast area has by previous writers always 

 been referred to as a granite or a hornblende biotite granite. 

 Not unlikely this is petrographically correct, for at least a 

 large part of it, and smaller areas where locally a plagio- 



