1922] Hudson: Geology of the Cuyamaca Region of California 205 



Relation between the different rock types. — The complexity with 

 which the different basic rock types are intermingled has been men- 

 tioned in a previous section. These complex relationships not only 

 obtain throughout the main mass of the basic intrusion but also in 

 many of the outlying masses. The small mass, three-quarters of a 

 mile east of Pine Hills, contains typical gabbro and typical norite, 

 both varying in grain from medium to coarse. The larger mass, to 

 the northeast, contains norite, olivine norite, and quartz norite. None 

 of the natural exposures studied exhibit contacts between the different 

 types of basic rocks. Our last resort to obtain evidence on this point 

 is in the underground workings of the Friday Mine. Here it was 

 impossible to find contacts marking off the norites, olivine norites, 

 gabbros, olivine gabbros, augite diorite, brown-hornblende norites, and 

 troctolites from one another. This statement refers to the massive 

 rocks, and does not apply of course to the very fine-grained, brown- 

 hornblende gabbros and brown-hornblende norites which occur in 

 sharply bounded narrow dikes. 



The hypersthene diorites and augite diorites have a finer grain 

 than that of the general run of the norites and gabbros. Thus the 

 average grain of three rocks from the hypersthene diorite mass is 

 0.3 mm., that of two rocks from the augite diorite mass is 0.4 mm. 

 The granularity of these rocks is thus less than that of the norites 

 and gabbros of the main intrusive mass, whose average grain is about 

 0.58 mm. It should be pointed out in this connection, however, that 

 the average grain of four outlying stocks and dikes of gabbro and 

 norite, all being less than one hundred yards in diameter, is 0.5 mm. 

 The cooling effect of schist and granite walls had little, if any, influence 

 on the grain of these rocks. There is no reason to believe that the 

 norite or gabbro which surrounds the hypersthene diorite would have 

 any greater cooling effect. Therefore, it is concluded, the finer grain 

 of the diorites has no bearing on the question of their age with respect 

 to the norites, gabbros, and other rocks of the basic intrusion. It is 

 more likely that fineness of grain varies with the chemical composition 

 of the rock. 



The main basic intrusive mass, as also the small outlying masses, 

 are intrusive complexes of various basic igneous rocks, the rock types 

 merging one into the other by gradual changes in the proportions of 

 their constituent minerals. Any theory to explain the heterogeneity 

 of the main mass must also account for the variation in the outlying 

 masses. 



