1922] Hudson: Geology of the Cuyamaca Eegion of California 191 



The Stonewall Quartz Diorite 



The most widespread rocks of the mountainous portion of San 

 Diego County are of plutonic type, varying from intermediate to acidic 

 in chemical composition. Three specimens determined by Dr. A. S. 

 Eakle from quarries near the towns of Lakeside, Foster, Santee, and 

 Grossmont, at the western edge of the mountains, proved to be granites, 

 while a fourth was granodiorite. 12 Farther to the northeast quartz 

 diorite is the prevailing rock. It has been described by Calkins, as 

 follows : 



The dominant rock about Bamona, as well as westward to the foot of the 

 mountain range, is one that would commonly be called a biotite granite. Its 

 color is gray with a tinge of olive-green; its texture is moderately coarse. Feld- 

 spar is its most abundant mineral, but quartz is also abundant, and small flakes 

 of black mica occur in moderate quantity. Microscopic study shows that the 

 rock is not a typical granite, inasmuch as the alkali feldspar is very subordinate 

 to the soda-lime feldspar. 13 



Excepting a mass of true granite which will be described in a 

 later chapter as the Rattlesnake granite, the granitic rocks of the 

 Cuyamaca region are low or lacking in alkali feldspar and are to be 

 classed as granodiorites and quartz diorites. In mineral composition 

 they are much like the rock described by Calkins, and might well be 

 correlated with it and termed the Bamona quartz diorite. However, 

 as there may possibly have been more than one period of irruption of 

 magmas of this nature, that of the Cuyamaca region will be termed 

 the Stonewall quartz diorite, after the peak of that name composed of 

 this rock. 



Petrographic description. — These rocks are medium to coarse- 

 grained aggregates of quartz, plagioclase, biotite, and rarely orthoclase. 

 The plagioclase varies from albite to andesine, and in one specimen, 

 from a dark segregation, it is labradorite. Green hornblende occurs 

 in the dark segregations. Orthoclase is present in only three out of 

 fourteen specimens examined, and in these makes up less than 10 

 per cent of the rock. 



In part of the area mapped the quartz diorite is distinctly gneissoid, 

 in other localities, as in the neighborhood of Stonewall Peak, the 

 gneissoid character is barely discernible or completely lacking. This 



12 Mines and mineral resources of Imperial and San Diego counties, Calif. 

 State Miner., Kept., 1913-1914, p. 43. 



is Calkins, F. C, Molybdenite and nickel ore in San Diego County, California, 

 U. S. Geol. Surv., Bull. 640, 191 6, p. 74. 



