198 ARNOLD AND STRONG— CRYSTALLINE ROCKS OF SAN GABRIEL 



rock is seen to be made up of large pseudomorphs of brown hornblende, 

 each being composed ofi numerous very small columnar crystals, all of 

 which are oriented with their long axes parallel to the long axis of the 

 pseudomorph. Between the more or less continuous rows of these small 

 crystals are strings of quartz grains. Calcite occurs abundantly through- 

 out the rock, and the quartz and calcite are doubtless the alteration 

 products of the mineral of which the pseudomorph is but the skeleton. 

 Magnetite occurs quite abundantly scattered throughout the mass, and 

 an occasional zoisite is seen. Originally there were probably some feld- 

 spars in this rock, but if so all traces of them have been lost in the pro- 

 cess of alteration. 



DIKE ROCKS 



Occurrence limited. — With the exception of aplite, dike rocks are not 

 of very common occurrence in the San Gabriel mountains. The basic 

 dikes, though not the most abundant, are still the most conspicuous, for, 

 as a rule, their dark color contrasts strongly with the lighter colored 

 granodiorites, gneisses, etcetera, which are intruded by the dikes. Aplite, 

 micropegmatite, quartz-porphyrite, and diabase porphyry are the dike 

 rocks recognized by the writers in the territory examined. 



Aplite. — Aplite occurs abundantly, usually in small dikes cutting the 

 granodiorites and other plutonics. It is easily recognizable by its light 

 color, and is generally finer grained and less decomposed than the 

 intruded rocks. " 



A typical aplite dike cuts the granodiorite about a quarter of a mile 

 below Devils gate, northwest of Pasadena. It varies from 6 inches to 2 

 feet in thickness, is light gray in color, and microcrystalline in structure. 

 In thin sections the rock (R. A. number 26) is seen to be made up largely 

 of quartz, which occurs in small, clear, xenomorphic chrystals, without 

 inclusions. Next in importance is the orthoclase, which is generally 

 found in clouded xenomorphic grains, but which occasionally occurs in 

 crystals showing one or more well developed faces. Inclusions of quartz 

 and orthoclase particles were noted in some of the larger orthoclase crys- 

 tals. . A small amount of plagioclase, much weathered, but showing 

 albite twinning, is also present in the rock. This constituent appears 

 to have crystallized earlier than the orthoclase or quartz, as its crystal 

 faces are much better developed than those of the two latter minerals. 

 A little biotite, showing fine cleavage and green to brown pleochroism, 

 together with some secondary epidote and chlorite, complete the com- 

 position of the rock. 



Micropegmatite. — Along the borders of the Pine Flats granodiorite area, 

 at the head of the main branch of Pine Flats canyon, are several small 



