1922] Hudson: Geology of the Cuyamaca Region of California 187 



1500 feet 4a. Fissile, evenly banded, quartz-muscovite schist. Weathered 

 surfaces resemble slate. Contains no quartzite. 

 40 feet 4b. Quartzite. 

 360 feet 4c. Fissile, fine-grained schist. 

 150 feet 4d. Quartzite, in part thin-bedded. 

 200 feet 4e. Fissile, fine-grained schist. 

 20 feet 4/. Quartzite, thin-bedded. 

 3000+ feet 5. Evenly banded, coarse quartz-mica schist, containing a few 

 quartzite layers and some injected material. In part carries 

 andalusite. 

 Injection gneiss. 

 Gneissoid quartz diorite. 



About one-half mile to the southeast practically the same sequence 

 of rocks is present, save that here member 5 of the above section is 

 represented by an equal or greater thickness of coarse, generally 

 gnarly, quartz-mica-sillimanite gneiss and other paragneisses, with 

 lenses of dense quartz-pyroxene rock. 



Origin of the schist series. — The fissile schists composed chiefly of 

 quartz and two micas and banded with numerous quartzite layers can 

 hardly have had any other than a sedimentary origin. The layers of 

 quartzite are essentially parallel to the schistosity, showing that the 

 schistosity in general conforms to the original bedding. Whether there 

 is isoclinal folding or not cannot be said. These rocks are thought to 

 have resulted from the metamorphism of a series of shales and fine- 

 grained, clayey sandstones, with beds of nearly pure quartz sandstone. 



The amphibolite and actinolite schists, on the other hand, if we 

 may judge from their mineral composition and mode of occurrence, 

 are probably derived from andesitic or basaltic lavas. 



The sillimanite gneiss, andalusite gneiss, and other paragneisses as 

 deduced from their mineral composition differ in no way chemically 

 from the fissile schists, leaving out of account, of course, the igneous 

 or pegmatitic injected matter. The presence of quartzitic lenses and 

 alternating layers of varying mineral composition proves their sedi- 

 mentary origin, and they are therefore thought to have been derived 

 from the same types of sediments as were the fissile schists. 



The contorted or gnarly character of many of these rocks is prob- 

 ably due to a second schistosity imposed on an earlier. In support 

 of this interpretation is an example from the coarse quartz-mica schist, 

 number 5 of the section given on this page. The rock is an evenly 

 banded, medium grained quartz-mica schist in which the schistosity 

 is parallel to the original bedding. The banding strikes E-W and 

 dips vertically. There is another set of planes in the rock which curve 



