1922] Yaughan: Geology of San Bernardino Mountains 369 



effect. Lower down in this gully another granite intrudes a mass of 

 sediments: limestone, schist, and quartzite. Quartz and orthoclase are 

 in equal amounts, some in vermicular intergrowth. Oligoclase is 

 present and a little microcline. Green biotite is in moderate amount. 

 The few grains of magnetite are red around the edges, evidently due 

 to alteration to hematite. 



To the south and east of this mass of granites stretches the great 

 area of undifferentiated schists. Throughout these there are small 

 patches of granites and granite-gneisses; but not until we reach Big 

 Morongo Creek do we find the characteristic granitic rocks predomi- 

 nating. A specimen from the north side of Little Morongo Creek just 

 above Morongo Valley is medium coarse-grained, with the feldspars 

 often more than five millimeters across. The biotite, however, is rather 

 fine, the individual flakes seldom exceeding a millimeter. Orthoclase 

 predominates over both quartz and the plagioclase, oligoclase, but the 

 rock is still to be classed as a quartz monzonite. Apatite is exceed- 

 ingly rare and present only as the finest needles. Magnetite grains 

 are also small and scattered. Farther to the northeast on the west 

 side of the valley just before reaching the divide the granite is rather 

 varied as to texture, but the mineralogical content seems to be fairly 

 constant. One variety of fine-grained, yellowish granite has a "salt 

 and pepper" appearance due to the fine flakes of biotite. Under the 

 microscope this biotite is found to be of a greenish color. Quartz and 

 orthoclase showing Carlsbad twinning make up the mass of the rock, 

 with the latter in excess. Small areas of micrographic intergrowths 

 are common and a little oligoclase is present. Apatite is unusually 

 abundant both as small needles and stout prisms, but magnetite is 

 scattered rather sparingly. Another specimen from a little farther 

 north presents quite a different appearance in the hand specimen, as 

 it is somewhat coarser, but in thin section it is almost identically the 

 same, even to the amount of apatite both as needles and stout prisms. 



On the opposite side of Morongo Valley is a medium coarse-grained 

 granite with equal quantities of quartz and orthoclase and a small 

 amount of albite. The most prominent accessory is biotite, and even 

 that is rather scattered. Prisms of zircon and more slender ones of 

 apatite are present ; also a few grains of magnetite and titanite. The 

 occurrence of the latter is peculiar in that it forms some rather large 

 crystals. 



The rocks around Chaparrosa Spring have been described as greatly 

 resembling those intruding the sediments farther west, but beyond 



