QUARTZ-MIOA-DIORITE. 103 



IIC. VARIETIES WITH AN EXCESS OF LIGHT-COLORED MINERALS, IN WHICH QUARTZ 



IS ABUNDANT. 



These include very quartzose and very feldspathic varieties of diorite, 

 approaching granite in composition and structure (320 to 323, 275a and 

 286a). They are mostly coarse-grained dikes or veins cutting the diorite 

 mass. They correspond to grades 37 and 40, Table VIII. In this group 

 are also placed certain apophyses (314-319) from the stock that penetrate 

 the sedimentary rocks. 



These rocks are very similar to those of Group IB, but contain more 

 quartz, and the feldspars appear to be still more alkaline. Oligoclase- 

 andesine is the predominant feldspar, but there is some labradorite and 

 some orthoclase. Orthoclase is very abundant in one modification of the 

 rock, which is in fact a fine-grained granite. It forms a large body on 

 the northeast spur of Electric Peak, in the rugged mass of dark-colored 

 diorite needles. Its crystallization is about grade 35, and its microstructure 

 approaches panidiomorphic, as shown in PI. XX, fig. 2. The other varieties 

 are more properly quartz-mica-diorites. In the coarsest-grained forms the 

 ferromagnesian minerals are biotite and hornblende, with no pyroxene, and 

 with biotite in excess of hornblende. The microstructure of one of the 

 coarser-grained varieties (321), grade 37, is shown in PI. XX, fig. 3, and 

 that of the finest-grained one (314), grade 19, in PI. XXI, fig. 1. The 

 latter is a pronounced porphyry. 



lie'. Certain narrow apophyses in the immediate vicinity of the 

 stock are rich in quartz, but contain more calcic feldspar and a variable 

 amount of pyroxene, besides biotite and hornblende. They appear to be 

 quartzose facies of the pyroxene-diorite of the stock, and have not been 

 found cutting the main body or forming dikes at any considerable distance 

 from the stock. Their mineralogical composition is indicated in Table VII 

 by the first six numbers (314 to 319). With increase of quartz the micro- 

 structure becomes more evenly granular. 



III. QUARTZ MICA-DIORITE-PORPHYRY. 



The last magma erupted through the Electric Peak conduit formed 

 quartz-mica-diorite-porphyry. It is a broad wedge-shaped mass cutting 

 the main diorite, narrower toward the north, and sending out dikes into the 

 sedimentary strata to the southwest. The rock is light gray to white, with 



