142 H. W. TURNER 



rocks or acid granolites, those containing free silica or quartz as 

 an essential constituent, that is to say, in considerable amount. 

 Seven types of quartz-granolites have thus far been recognized. 

 The relative age of all of them is not definitely ascertained, but 

 so far as known it is expressed in the order in which the different 

 types are enumerated, as follows : biotite-granite, granodiorite, 

 quartz-monzonite, porphyritic quartz-monzonite, Bridal Veil 

 granite, soda-granite and aplite, potash-aplite and pegmatite. 



BIOTITE-GRANITE 



Biotite-granite forms very large areas in the central portion 

 of the Sierra Nevada. It is particularly abundant in the Big 

 Trees and Yosemite quadrangles, 1 where it has been examined 

 most closely. The coarse biotite-granite is a rock susceptible of 

 easy recognition in the field. Potash-feldspar is an abundant 

 constituent, and by its conspicuous development in relatively 

 large crystals tends to give the rock a porphyritic look. Other 

 minerals less readily seen with the naked eye are quartz and 

 biotite ; the former in distinct grains of irregular shape, and 

 the latter so arranged as to give a suggestion of gneissic or 

 banded texture, even in hand specimens. Perfectly fresh speci- 

 mens are secured with difficulty as the rock weathers to a con- 

 siderable depth and becomes somewhat friable. Under the 

 microscope the porphyritic texture is generally inconspicuous. 

 The component minerals are soda-lime-feldspar (oligoclase) > 

 quartz > potash-feldspar> biotite> titanite> apatite> zircon. 

 The relative proportions of these minerals are deduced from a 

 calculation as noted later. Chlorite is usually present as a decom- 

 position product of the biotite, and secondary epidote may often 

 be noted. Rutile-like needles were observed in some quartzes. 



Biotite-granite usually weathers in yellowish tones, and in 

 forms suggestive of bedding, due to a more or less well-devel- 

 oped gneissic structure. Indeed, at many points the biotite- 

 granite has been greatly compressed and sheared, so that much 



1 As used by the U. S. Geological Survey a quadrangle is the area of country 

 covered by a topographic sheet of the Atlas of the United States. 



