HOENBLENDE-MICA-ANDESITE-POEPHYBIES. 257 



(1401, 1408) are not quite so rich in quartz as the diorite-porphyries of the 

 core, but they cany considerable quartz in the groundmass, and a few 

 crystals of it occur as phenocrysts. Their crystallization is grade 23 in the 

 first case, and grade 18 in the second. 



Near the base of the west spur there is an 8-foot dike of this rock 

 (1 384), trending west of south, and still lower down the slope there is another 

 (1381). - A small dike of this rock cuts the southwestern edge of the top 

 of the middle spur, and in the cliff west of the lake there is a 15-foot dike of 

 it (1367), trending N. 45° W. Besides these there are indications of other 

 bodies of the same kind, fragments of which are found in the talus in 

 various places. These rocks are fine grained, and resemble the remainder 

 of the dikes of hornblende-mica-andesite-porphyry so closely that a general 

 description will serve for all of them. 



The outlying dikes of this character are found in the immediate neigh- 

 borhood of the core, the longest noted extending about 5 miles to the south- 

 west. A 10 -foot dike of it forms the saddle northwest of the plateau of 

 Hurricane Mesa and trends northwest, being in line with the 15-foot dike 

 west of the lake. 



On the narrow ridge south of Closed Creek there are a number of 

 these dikes, which cut the ridge at a place southwest of the core. Five 

 were noted, three of which are 10, 18, and 20 feet wide and trend toward 

 the south and southwest. They fork and branch out in these directions, 

 and dikes of identical rock are exposed on the southern slope of the ridge, 

 having the same general trends. They may be traced almost continuously, 

 in some instances diagonally, across the steep spurs. 



In thin section the rocks are holocrystalline and fine grained, ranging 

 from grade 18 in the dike (1381) at the base of the southwest spur of the core 

 to grade 10 in the most remote dike (1319), which is 4 or 5 feet wide. The 

 habit of the rocks is andesitic, passing into that of andesite-porphyry at the 

 more crystalline end of the series. The groundmass consists of tabular 

 plagioclase in a matrix of irregular grains of feldspar with a little quartz, 

 besides idiomorphic crystals of magnetite, biotite, hornblende, and pyroxene. 

 Through this are scattered larger crystals of the same minerals. The mega- 

 scopic crystals are abundant, and are andesine-labradorite with zonal struc- 

 ture and variable amounts of inclusions; and idiomorphic hornblende, brown- 

 ish green with a brown border, in places intergrown with augite in the 



MON XXXIT, I'T II 17 



