DIORITE AND DIOEITE-PORPHYRY. 253 



zircon inclosed in hornblende is surrounded by a pleochroic halo. The 

 chemical composition of this rock (analysis 14 on page 261) is that of a 

 quartz-mica-diorite rich in orthoclase — that is, banatite. 



Higher up the spur, at 10,000 feet, the main body of rock (1429) 

 becomes finer grained, the feldspars are more nearly idiomorphic, and the 

 quartz forms large, irregular individuals with micropoikilitic structure, 

 inclosing small feldspars (PL XXXIII, fig. 4). The plagioclase is andes- 

 ine, and contains rectangular inclusions resembling glass. Orthoclase is 

 not so abundant. There is considerable biotite, with augite, hypersthene, 

 and magnetite, but no hornblende. 



A still finer-grained form of this quartzose facies of the core occurs 

 north of the top of the middle spur. It is a very fine-grained gray rock 

 (1414), without phenocrysts. In thin section it exhibits a structure similar 

 to that last mentioned, the idiomorphic labradorite-andesine having a border 

 of allotriomorphic feldspar. There is a moderate amount of augite and 

 hypersthene, which occur in g'roups of small grains rather than in compact 

 individuals, besides magnetite and very little biotite. The chemical analysis 

 (No. 13 on page 261) shows its resemblance to 1427, as well as the 

 chemical differences between them, which correspond to difference in min- 

 eral composition, the latter rock being- richer in biotite, orthoclase, and 

 quartz, with more hornblende than pyroxene. 



In some of the thin sections from the body of the core there are traces 

 of decomposition, producing a little chlorite and serpentine, and in some 

 cases uralite accompanying evidences of slight crushing in the feldspars. 

 But the rocks are to a very great extent perfectly fresh and unaltered. The 

 quartzose facies of the gabbro is younger than the main mass of more 

 basic rock, for it is found cutting it as dikes. Thus, a fine-grained variety 

 (1419), with the same microstructure and composition as the fine-grained 

 form (1414) on top of the core, cuts the aphanitic zone of more pyroxenic 

 rock at the base of the middle spur. It grades directly into more granular 

 rock (1420), which has the structure and composition of the lower part of 

 the main mass of the spur represented by 1 425. 



There is also a 20-foot dike of this quartzose variety which cuts the 

 gabbro on the southwest spur at 9,100 feet and trends toward the southwest, 

 while a body of similar rock, whose form was not observed, occurs on the 

 crest of the same spur, more within the core. The rock from this body 

 (1400) has the same composition as that at the base of the middle spur 



