PETROGRAPHY OF DIKE ROCKS 199 



dikes of micropegmatite, but the fractured condition of these dikes and 

 of the adjacent rock area made it impossible to study their relations 

 satisfactorily in the field. Hand specimens of the rock (A. M. S. number 

 13) are light colored, very fine grained, and appear quite fresh. With a 

 pocket lens quartz, feldspar, and muscovite are distinguishable. The 

 section shows a decided micropegmatitic structure, the intercrystalliza- 

 tion of quartz and orthoclase in parallel positions producing the typical 

 graphic appearance. These pegmatitic crystals are imbedded in aggre- 

 gates of xenomorphic grains of quartz, orthoclase, and a small amount 

 of plagioclase, none of which show the pegmatitic structure. Small 

 amounts of muscovite and biotite, with occasional grains of magnetite, 

 are also found in the groundmass. The biotite is generally altered to a 

 brown, non-pleochroic mineral, but in some instances is changed to mus- 

 covite. Some of the latter mineral, however, may be primary in origin. 



Quartz-honiblende-porphyrite. — A dike of quartz-hornblende-porphyrite 

 is found in the Big Santa Anita canyon, about 300 yards below the main 

 falls. It (A. M. S. number 2) is a grayish rock showing small feldspar 

 crystals imbedded in a groundmass of feldspar and hornblende. Micro- 

 scopically plagioclase is seen to be the dominant mineral. It occurs 

 principally as rather broad, lath-shaped crystals, considerably weath 

 ered, but appearing to have numerous small inclusions. Alteration 

 begins in the center of each crystal, and kaolin appears to be the princi- 

 pal resulting product, although muscovite is occasionally present. Next 

 in importance to the plagioclase is orthoclase, which is generally much 

 altered to kaolin. It occurs both in automorphic and xenomorphic crys- 

 tals and usually contains numerous small inclusions. The principal 

 basic mineral is a green, weakly pleochroic pyroxene, which has under- 

 gone a process of uralitization. It occurs in rather small crystals be- 

 tween the larger feldspars. Hypersthene is common in small grains. 

 The quartz occurs in small xenomorphic grains scattered throughout 

 the groundmass. Small, slender, lath-shaped crystals of a bluish min- 

 eral are also present. Magnetite is common, and pyrite and ilmenite 

 are also present in lesser quantities. Apatite occurs sparingly in long, 

 narrow, colorless crystals showing parallel extinction, while epidote is 

 found in small grains. 



Quartz-hornblende-porphyrite also occurs as a dike which crosses the 

 old Sturtevant trail southeast of mount Wilson. Microscopically this 

 rock (R. A. number 32) is gray in color and rather fine-grained in text- 

 ure. It is much jointed, and fresh surfaces are hard to obtain, but 

 where they are examined carefully they usually disclose feldspar crys- 

 tals large enough to be seen by the unaided eye. In thin sections pla- 

 gioclase is seen to be the most abundant mineral. It occurs in small, 



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



