PETROGRAPHY OF METAMORPHIC ROCKS 201 



gneiss is found in the west wall at the mouth of the Big Santa Anita 

 canyon. Megascopically the rock (A. M. S. numher 4) is fine grained, 

 distinctly colored in light and dark bands, and shows quartz, feldspar 

 and hornblende. The bands are not continuous, but are in strips vary- 

 ing from one-fourth to one-half inch (5 millimeters to 10 millimeters) 

 in width and from 2 to 12 inches (5 centimeters to 30 centimeters) 

 in length. The dark coloration is caused by hornblende. Under the 

 microscope the gneiss is seen to consist of fine, xenomorphic crystals of 

 plagioclase, orthoclase, and quartz in relative abundance in the order 

 named. The feldspars are more or less fractured and weathered, calcite 

 resulting from the alteration of the plagioclase and traversing the rock 

 in small veins. The hornblende occurs in varying proportions, being 

 quite abundant in the dark bands. It is the green variety, and is present 

 both as grains and in spindles having their long axes parallel to the 

 banding planes. The fracturing so apparent in the feldspars is not present 

 in the hornblende. Chlorite results from the alteration of the hornblende. 

 Magnetite in grains is also abundant, being arranged in rows in more or 

 less regular fashion parallel to the banding. A chemical analysis of this 

 hornblende-diorite-gneiss (A. M. S. number 4) from Big Santa Anita 

 canyon, by A. M. Strong, gives the following results : 



Si0 2 62.41 



A1 2 3 13.91 



Fe 2 3 9. 87 * 



CaO 3.15 



MgO 2.22 



K a O 3.19 



Na^O 3.34 



H 2 2.57 



100.66 



A transition from hornblende-diorite-gneiss to quartz-monzonite takes 

 place along the Henniger Flats-Eaton Canyon trail. A gneiss very simi- 

 lar to A. M. S. number 4 occurs in the bed of the canyon at the foot of 

 the trail, while 209 yards up the trail, and covering the top of a small 

 ridge over which the trail passes, is another gneiss (A. M. S. number 8). 

 This latter differs from that in the canyon by having much larger crys- 

 tals of hornblende, all of which are oriented with their longest axes in 

 a certain direction, but which show no regularity of arrangement (or 

 banding) in the direction perpendicular to the long axes. This irregu- 

 lar distribution of the oriented crystals gives the rock a very peculiar 



♦The iron was all determined as Fe 2 3 , no separation of FeO being made. This makes the total 

 iron given a little higher than the true amount. 



