200 ARNOLD AND STRONG — CRYSTALLINE ROCKS OF SAN GABRIEL 



elongated prisms, between which are xenomorphic grains of orthoclase, 

 hornblende, and, rarely, quartz. Inclusions are common in the plagio- 

 clase crystals, and twinning according to the Carlsbad law was often 

 noticeable. The orthoclase grains show zonal structure quite commonly. 

 Hornblende is abundant in small aggregates, showing yellowish green 

 to light green pleochroism. Chlorite is a prominent constituent, being 

 a secondary product of the hornblende. A little secondary muscovite 

 is also present. 



Diabase porphyry. — A dike of diabase porphyry 4 feet wide breaks 

 through the light colored granite rock which forms the cascades in Big- 

 Santa Anita canyon near Sturtevants camp. The porphyry is more re- 

 sistant to weathering than the intruded granitic mass, and, as a conse- 

 quence, protrudes slightly from the bed of the stream and from the sides 

 of the canyon. The rock appears to be affected by a series of invisible 

 joint planes which become apparent when the rock is subjected to shock. 

 Fracturing along these cracks takes place easily, but otherwise the rock 

 is very hard and resistant. Hand specimens of the porphyry show it to 

 be very dark colored, micro-crystalline, and with a few phenocrysts of 

 feldspar scattered about in it. Fresh fractured surfaces exhibit the lus- 

 trous faces of the phenocrysts. Sections viewed microscopically show a 

 noncrystalline groundmass of small lath-shaped crystals and irregular 

 grains or pseudomorphs of chlorite, the latter being the alteration pro- 

 duct of pyroxenes. Phenocrysts of orthoclase, plagioclase, and rarely of 

 pyroxenes are scattered throughout the groundmass. Inclusions are 

 common in the feldspar phenocrysts, one case in particular being noted 

 where a single orthoclase contained a C -shaped mass of chlorite and an 

 isolated mass of unaltered enstatite. 



METAMORPHIC ROCKS 



In general. — As previously mentioned, metamorphics play an impor- 

 tant part in the composition of the Sierra Madre or border range of the 

 San Gabriel mountains. The gneisses are by far the commonest type in 

 this class of rocks, but schists are also present, though in less abundance 

 than the gneisses. The following metamorphics were obtained in the 

 territory : Hornblende-diorite-gneiss, biotite-granite-gneiss, -hornblende- 

 schist, and garnetiferous schist. 



Hornblende-diorite-gneiss. — Gneisses of this type are probably the com- 

 monest rock in the southern range, with the exception of the granodiorite. 

 They are intimately associated with the granitic facies and are probably 

 derived from the latter by shearing stresses. They are generally fine 

 grained, distinctly banded in light and dark, and with the banding irregu- 

 larly and intricately contorted. A typical example of hornblende-diorite- 



