1922] Hudson: Geology of the Guyamaca Region of California 201 



gradually coarser till a point about ten feet away is reached, after 

 which there is no change. The light colored rock for an inch or two 

 on the quartz diorite side of the contact is finer grained than is the 

 main mass of the gneissoid rock. 



The diorite is composed of andesine, augite, biotite, and magnetite. 

 The quartz diorite is made up of quartz, oligoclase, and biotite. A 

 thin section, cut across the contact of the two rocks, shows that the 

 quartz diorite has been penetrated by veinlike masses whose greatest 

 width is at the contact and which wedge out within 5 or 10 mm. The 

 minerals of these veinlets are augite, plagioclase, and quartz. The 

 minerals of the veinlets have the same size of grain as those of the 

 diorite. This shows that the fine-grained selvedge of the quartz- 

 diorite gneiss is the result of the permeation of that rock by dioritic 

 material. The biotite of the diorite is most abundant and, in largest 

 individuals, near the contact. 



Ten feet from the contact there is a small amount of biotite in the 

 diorite, as there is also at 100 feet distance. The rock at 100 feet 

 distance is more basic than that at the contact, the two rocks contain- 

 ing 45 and 60 per cent of andesine feldspar respectively. 



Similar relations are found at the contact of augite diorite and 

 gneissoid granodiorite between Cuyamaca and Stonewall peaks. The 

 mineral compositions of three specimens taken from the diorite at 

 various distances from the contact illustrate the case: 



Distance Plagioclase Green 



from , v horn- Magne- 



fcontact Kind Amt. Augite blende Biotite Quartz tite Apatite 



1 in. Albite and oligoclase 55 8 13 24 0.5 



30 ft. Andesine 62 33 X 4 1 



1500 ft. Andesine 67 12 20 X 1 



X minute quantity. 



The granodiorite contains both orthoclase and oligoclase feldspars. 

 The lack of orthoclase in the basic rock near the contact suggests that 

 the potash feldspar has been altered to albite. 



Texture and grain. — Measurements were made, by means of the 

 microscope with micrometer eyepiece, of the average and maximum 

 diameter of grain in all thin sections studied. The method employed 

 in determining the size of grains was to measure the longest diameter 

 of all the grains met with in several traverses across the thin section. 

 The average of these diameters was taken as the "average grain," 

 the maximum as the "maximum grain." Care was taken to exclude 

 cataclastic grains, which as a matter of fact are rarely found. Feld- 

 spar twins were measured as one grain. Where pyroxene was found 



