PETROGRAPHY OF PLUTONIC ROCKS 191 



in abundance to the orthoclase, and were determined to be oligoclase in 

 most instances. Quartz is abundant as small grains. Some brown 

 biotite is present, but is often partially weathered into chlorite, epidote, 

 or muscovite. Green hornblende in spindles and a small number of 

 magnetite grains complete the composition of the rock. 



Quartz-monzonite. — Among the specimens of plutonic rocks collected 

 in the Sierra Madre range are three which appear to be quartz-mon- 

 zonites. Although these specimens are considerably weathered and the 

 determination of their feldspars is not as satisfactory as one would wish 

 it to be, still the bulk of the evidence favors their classification as above. 



One specimen (A. M. S. number 7) comes from the Henniger flats — 

 Eaton canyon trail, about half a mile above the canyon. It is fine 

 grained, dark gray in color, and shows feldspar, quartz, hornblende, and 

 biotite. In thin sections the structure is seen to be hypidiomorphic 

 granular, and the orthoclase appears to be slightly in excess of the plagi- 

 oclase. Zonal structure and zones of minute inclusions occur in some 

 of the orthoclase crystals. A few of the plagioclases show pericline twin- 

 ning. Brown biotite, a little green hornblende, and numerous small 

 particles of magnetite are the other constituents. This rock is charac- 

 terized by the nearly equal amounts of orthoclase and plagioclase and 

 by the xenomorphic occurrence of its minerals, very few crystal faces 

 being developed except in the plagioclase feldspars. 



A. M. S. number 30, from the flanks of mount Lowe, below Alpine 

 tavern, is another of the quartz-monzonites. It is coarser grained than 

 A. M. S. number 7, and contains some apatite. The plagioclase, which 

 is slightly less abundant than the orthoclase, appears to be about equally 

 divided between andesine and oligoclase. Specimens of this rock were 

 compared with a granodiorite from lake Tahoe, Sierra Nevada mountains, 

 and found to be similar in general appearance, although the Tahoe rock 

 was somewhat coarser grained. A microscopic comparison of the two 

 showed the Tahoe specimen to contain proportionately less biotite than 

 A. M. S. number 30, and also showed the former to contain much more 

 plagioclase than orthoclase. 



A quartz-monzonite from the south side of mount Lowe near the sum- 

 mit (A. M. S. number 31) is similar to number 30, except that it contains 

 no hornblende, less quartz, and a little microcline and muscovite. The 

 orthoclases in number 31 are much jointed and the cracks filled with a 

 dark colored opaque substance. 



Granodiorite. — Granodiorite is by far the commonest rock in the San 

 Gabriel mountains, at least in the region north of Pasadena. It and its 

 associated gneisses constitute most of the Sierra Madre and are also found 



XXVI— Bull. Geol. Soc. Am., Vol. 16, 1904 



