REVIEWS 909 



various places, but have not been thoroughly studied. The 

 massive granites are exposed in a limited tract surrounding 

 Prescott, including Granite Mountain, and farther southwest in 

 the Grand Canyon of the Colorado, in the valley of the Colorado 

 in Yuma county, and in the Little Dragoon Mountains, Cochise 

 county. These massive granites form a basement for the other 

 rocks of the region. 



In a number of places are found fissile granites which appear 

 to lie between the massive granites and the schistose strata 

 which form the floor of much of the area of western Arizona. 



Schists are well exposed in northwestern Arizona, in Mohave 

 county, with E.-W. strike, tilted at high angles. Very similar 

 exposures occur in other districts, as near Oracle and Mammoth, 

 in Pinal county ; in tracts in Pima county, and in portions of 

 Yuma county. In Graham and Cochise counties, and, to a less 

 extent, in Gila county, with occasional outcrops near the adjoin- 

 ing boundary of Maricopa and Yavapai counties, the same trend 

 is prominent. Erosion has exposed portions of the same 

 terrane, thrown into the N.E.-S.W. trend, in limited areas in 

 Graham and Yavapai counties, and, possibly, in the northeast 

 portion of Yuma county. 



Blake 1 describes the salient features of the geology of Arizona. 

 The Santa Catalina, Rincon, and Rillito mountains consist largely 

 of granitic gneisses and schistose rocks of pre-Cambrian age with 

 a highly complex folded structure, and exhibiting a high degree 

 of metamorphism. Taken together, these mountains may be 

 regarded as the main axis of ancient uplift, and of insular land 

 areas in the pre-Cambrian and Paleozoic periods, the beginning 

 of the "Arizona Land." 



The gneiss of the southern side of the Santa Catalina near 

 Tucson is regarded as Archean. It is remarkable for its regu- 

 larity of stratification and its great thickness, probably over 10,000 

 feet. It occurs in great tabular masses made up of thin layers 

 which, when seen laterally, give the appearance of evenly strati- 



1 "Some Salient Features in the Geology of Arizona with Evidences of Shallow 

 Seas in Paleozoic Time," by William P. Blake, American Geologist, Vol. XXVII, 

 iqoi, pp. 160-67. 



