1922] 



Vaughan: Geology of San Bernardino Mountains 



381 



On the south side of Mission Creek near its mouth basalt outcrops 

 from beneath the fanglomerate. The weathered surface appears at 

 first sight to be a fine agglomerate, but on breaking off the outer crust 

 the material is found to be a somewhat decomposed basalt. One 

 peculiar thing about this outcrop is that at its eastern end it exhibits 

 the fluted forms so typical of badland erosion. East of Hog Ranch 

 another strip more than half a mile long is exposed in the side of the 

 canon. As in other eases it also is overlain by fanglomerate. 



A small body of olivine basalt is found in one of the little gullies 

 on the north side of Little Morongo Creek, about a mile and a half 

 from its mouth. This is evidently a neck, for it is cut by erosion and 

 extends to a considerable depth despite the fact that it is not more 

 than seventy feet across. Immediately surrounding the basalt proper 

 is a shell of altered basalt mixed with fragments of granite torn from 

 the walls of the pipe. 



The flat-topped hill two miles east of The Pipes (pi. 17b) is almost 

 wholly of basalt, the underlying fanglomerate forming only a compar- 

 atively thin bed. On the north side of the hill this fanglomerate 

 nearly pinches out and is seen to rest on granite. The basalt mass 

 consists of several flows from ten to thirty feet thick, the total aggre- 

 gating a little over 200 feet. The vesicular character of the upper 

 portion of the individual flows usually serves as a distinct line of 

 demarcation. 



The lowermost flow is about ten feet thick and is a dark brownish, 

 nearly black, rock. It contains many rounded olivine phenocrysts, 

 some of which are as much as a millimeter in diameter. These are 

 light greenish in color with light brownish limonite stains around the 

 borders. Tabular crystals of plagioclase up to three millimeters in 

 length are present and under the microscope were determined as 

 labradorite near andesine. This seems to form a large part of the 

 rock with all gradations in size from the smallest in the ground mass 

 to the largest phenocrysts. The ground mass is dark and compact 

 and, in addition to the needle-like crystals of labradorite, consists of 

 numerous grains of augite and magnetite. The augite is partially 

 altered to chlorite and distinct reddish rims of hematite can be recog- 

 nized around some of the magnetite. 



A specimen fifty feet from the base differs somewhat from the 

 above. Large euhedral olivine phenocrysts and laths of basic bytown- 

 ite are imbedded in a ground mass of small laths of basic labradorite 

 and grains of colorless augite. Many blades of hematite and some 



