EARLY BASIC BEECOIA AND FLOWS. 277 



with some intercalated layers of scoria and breccia, forming- a compound 

 sheet 700 to 1,000 feet thick. This massive sheet caps the northeastern 

 spurs of Mirror Plateau and forms the eastern half of its top, passing west- 

 ward under rhyolite. The basalts of this sheet differ somewhat in outward 

 appearance. Some are dark and dense, with small phenocrysts. Others 

 have a semiwaxy luster and belong to the orthoclase-bearing varieties 

 already mentioned. 



West of the mouth of Cold Creek irregularly bedded basic breccia 

 forms the lower thousand feet of the ridge between this creek and Willow 

 Creek. Immediately over it is a sheet of porphyritic basalt, with pheno- 

 crysts of feldspar and pyroxene. This sheet is 200 feet thick, and consti- 

 tutes the base of the broad shoulder which sets back from the steep face of 

 the ridge. 



East of the mouth of Cold Creek basic breccia forms the lower 1,500 

 feet of the northern end of the flat-topped mountain east of Pyramid Peak, 

 and rises still higher in the next peak east, which is just beyond the one 

 hundred and tenth meridian. Here, at 9,850 feet elevation, it forms the 

 northern summit of the peak, the breccia being rough and imbedded and 

 dark gray in color (1472). The northern face of the peak is precipitous, 

 and consists of rudely bedded breccia carrying masses 3 feet in diameter. 

 The rock bears abundant phenocrysts of pyroxene and feldspar, and is 

 mostly dark colored, in places red. On the southern slope of this peak it 

 is composed of very small fragments (1473) in a purplish-red matrix. 



Horizontal basalt flows cap the next peak south, whose summit is 

 about 10,025 feet in altitude, and also form the top of the ridge to the east 

 and the smooth table-topped mountain lying southeast, The basalt on the 

 peak is dense, with few phenocrysts of olivine (1475, 1476). It proves to 

 be a leucite-bearing shoshonite, described in Chapter IX. Other flows of 

 basalt in this neighborhood are more porphyritic. 



The precipitous exposures on the ridge east, and on the south face of 

 the twin peaks Castor and Pollux, show the lower parts of these mountain 

 masses to be made of similar dark, rough, and rudely bedded or imbedded 

 breccia. But the upper thousand feet, above the 10,000-foot line, consists 

 of nearly horizontal sheets of basalt of various thicknesses. The slope of 

 the top of the table mountain south is at a slight inclination westward. 



The basalt sheets extend westward across the saddle at the southeastern 



