1922] Hudson: Geology of the Cuyamaca Region of California 211 



and perhaps a little orthoclase. The quartz and feldspars are set to- 

 gether as a mosaic of mutually interfering grains. The feldspars 

 show no tendency to elongation except to a slight degree in the pheno- 

 crysts. 



Geologic Structure 



The dominant agencies that have determined the structure of this 

 region are (1) compression and (2) igneous intrusion. Bodies of 

 schistose rocks lie within the great mass of quartz diorite in positions 

 determined while the quartz diorite magma was still molten. Without 

 doubt the sediments now represented by the schist series were folded 

 prior to the development of the batholith. The quartzite layers of the 

 schist series, if mapped in detail, would probably furnish the key to 

 this folded structure. 



Cutting across the fabric of the schist-quartz diorite complex are 

 the two great igneous masses of the Cuyamaca and Rattlesnake intru- 

 sions and smaller masses related to the Cuyamaca Basic Intrusive. 



The detailed mapping of this region has not shown the existence 

 of any important faults. The "Preliminary geologic map of San 

 Diego County, California," 20 by A. J. Ellis, shows one major fault 

 zone and two ' ' lines of topographic expression which suggest the pres- 

 ence of faults" in the Cuyamaca region. The major fault runs along 

 the southwest flank of Agua Tibia Mountain, determines the south- 

 west boundary of the flat intermountain valley on the Valle de San 

 Jose grant, determines a line of topographic depression through Ban- 

 ner Canon, passes directly through Banner and thence southeastward. 

 The physiographic evidence for this fault seems good. The writer 

 has nothing to add from his study of the Cuyamaca region. In the 

 area studied this supposed fault is entirely within granite. 



There is good reason to doubt the existence of the other two faults 

 indicated by Ellis. One of these is supposed to follow the courses of 

 Green Valley and Chariot Canon, leaving that canon so as to pass three- 

 quarters of a mile east of Banner, and continuing on to the north along 

 the western edge of San Felipe Valley. While this fault may exist 

 along the edge of San Felipe Valley, it is surely not present to the 

 south for the reason that a prominent dike in the upper end of Green 

 Valley crosses the supposed fault without offset. The physiographic 

 features that probably led to the mapping of the third fault, in the 

 region west of the Cuyamaca Mountains, can be explained as due to 

 differential erosion. 



20 U. S. G. S., Water Supply Paper 446, pi. 3. 



