BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 16. pp. 183-204 April 13, 1905 



SOME CRYSTALLINE ROCKS OF THE SAN GABRIEL MOUN- 

 TAINS, CALIFORNIA* 



BY RALPH ARNOLD AND A. M. STRONG 



(Presented before the Society December SO, 1904) 



CONTENTS 



PftKe 



Introduction 183 



Location of the San Gabriel mountains 184 



Topography 185 



Previous literature 187 



Age of the San Gabriel mountains 188 



General character of the rocks 188 



Detailed petrography 189 



Plutonic rocks 189 



General characteristics 189 



Biotite-granite 189 



Quartz-monzonite 191 



Granodiorite 191 



Hornblendite 197 



Dike rocks 198 



Occurrence limited 198 



Aplite 198 



Micropegmatite 198 



Quartz-hornblende-porphyrite . 199 



Diabase porphyry 200 



Metamorphic rocks •. 200 



In general 200 



Hornblende-diorite-gneiss 200 



Biotite-granite-gneiss ... 202 



Hornblende-schist 203 



Garnetiferous schist 203 



Summary 203 



Introduction 



The following paper embodies the results of some studies which were 

 begun by the writers in 1897, while students in the department of geology 



♦Published by permission of the Director of the U. S. Geological Survey. 

 XXV— Bull. Geol. Soc. Am., Vol. 16, 1904 (183) 



