320 University of California Publications in Geology [Vol.13 



page 



Alluvium .' 393 



Structure 393 



Folds modified by intrusions and faults 393 



Quaternary faults 396 



Summary 405 



Geological history 406 



Ore deposits 409 



INTRODUCTION 



Stretching eastward from the San Fernando Valley is the highest 

 and most rugged mountain range in Southern California. Ahout 

 sixty miles from its western extremity there is a break known as the 

 Cajon Pass. The portion west of this is called the San Gabriel Range. 

 Eastward from the pass the San Bernardino Mountains extend for 

 eighty miles to an intersection with a small chain of hills, known as 

 the Cottonwood Mountains, which have a northeast trend. To the 

 north of the range is the Mojave Desert. On the south the mountains 

 are limited by the western part of the Colorado Desert, the San Gor- 

 gonio Pass, and the San Bernardino Valley. 



The area described in this paper embraces that portion of the San 

 Bernardino Mountains north of the San Gorgonio Pass and a small 

 part of the Mojave Desert. It includes the entire San Gorgonio 

 Quadrangle and a strip across the northern end of the San Jacinto 

 Quadrangle. The total length is about 41 miles and the width 29 

 miles, which makes the area a little less than 1200 square miles. 



During the summers of 1915, 1916, and 1917 the writer made ex- 

 cursions into this part of the San Bernardino Mountains and the 

 Mojave Desert immediately to the north for the purpose of studying 

 the geology of the region. The field is mostly a wild country and the 

 writer is much indebted to Mr. R. H. Charlton, the forest supervisor, 

 for information concerning trails, water, etc., and to the Swartout & 

 Blair Cattle Company, the Banning Water Company, Mr. "William 

 McCormick, and Mr. C. L. Metzgar for their hospitality. In present- 

 ing the results the author has been greatly aided by criticisms and 

 suggestions from Professor A. C. Lawson. 



