1922] Hudson: Geology of the Cuyamaca Region of California 183 



Quartzite. — Beds of bluish, dense, fine-grained quartzite, often with 

 some biotite, are rather common within the schist series. Their thick- 

 ness varies generally from a few inches to ten feet, but several beds 

 of much greater thickness were noted. The north-south ridge east 

 and northeast of Cuyamaca Reservoir, which divides the drainage to 

 the Pacific Ocean from that through Oriflamme Canon to the Salton 

 Sea, is determined by the presence of a layer of quartzite varying from 

 50 to 100 feet in thickness. This bed can be traced without interrup- 

 tion for almost two miles. The quartzite beds, if mapped, would 

 probably furnish the key to the original structure of the schist body. 



Amphibolite schist. — Layers of amphibolite schist were found with- 

 in the schist series in the southwest quarter of section 9, township 13 

 south, range 4 east, and in the northwest quarter of section 8, of the 

 same township. The rocks are distinctly schistose and are composed 

 of a fine-grained aggregate of basic plagioclase and green hornblende. 



Actinolite schist was found on the dump of the Helvetia Mine, in 

 the northwest quarter of section 4, near Julian, and a considerable 

 body of similar rock lies within the schist two miles southeast of 

 Cuyamaca Reservoir, on the road to Rattlesnake Valley. 



Paragneiss and coarse schist. — Along the western edge of the main 

 schist body, extending from the vicinity of Julian to at least as far 

 south as the latitude of Stonewall Peak, there is a zone of paragneisses 

 and coarse schists varying in width from less than one-quarter to over 

 three-quarters of a mile. Similar rocks were found at certain locali- 

 ties along the northeast margin of the main body of schist, and also 

 along the contacts of many of the smaller schist masses which are 

 inclosed in quartz diorite. The rocks of these border zones are inva- 

 riably much coarser in texture than is the fissile schist described above. 

 The micaceous varieties generally have poor or irregular fissility, being 

 gnarled or contorted. Included within the gnarly schists and gneisses 

 are short lenses, with blunt ends, of gray and gray-blue dense quartzose 

 rock. These are generally but a few inches thick and less than one 

 foot in length. 



Of the more interesting types of paragneiss and schist a few are 

 selected for brief description. Sillimanite-quartz-mica-gneiss was col- 

 lected a short distance east of the quartz diorite, one mile northeast 

 of the Cuyamaca Reservoir. It contains quartz, muscovite, oligoclase, 

 biotite, tourmaline, and sillimanite. Medium grained layers, an inch 

 or more in thickness and composed of the two micas, quartz, feldspar, 

 and tourmaline, alternate with coarser grained layers, up to one inch 



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