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University of California Publications in Geology [Vol. 13 



to be a very fine-grained granitic rock consisting of equal amounts of 

 quartz and orthoclase and a little plagioclase, probably basic oligo- 

 clase. Other minerals in small amounts are biotite, muscovite, mag- 

 netite, and titanite. Another rock of similar nature differed in having 

 a few prisms of zircon and in containing no plagioclase. The general 

 attitude of the schistosity is horizontal, but undulatory rather than 

 flat. Both the quartzite and the granites are cut vertically by numer- 

 ous lamprophyre dikes. A specimen of coarse texture, with individual 

 crystals more than six millimeters across, was found under the micro- 

 scope to be largely hornblende with a much less amount of plagioclase, 

 probably oligoclase, showing pericline and Manebach twinning. A 

 few grains of magnetite are also present. 



Farther down the canon these dikes cut light granitic rocks in all 

 directions and in some cases have been mixed with them and so drawn 

 out by regional movements as to resemble dark strata between lighter. 

 A specimen of the darker rock near the branch to Viscera Spring was 

 found to be essentially the same as those described above. These 

 basic rocks are so numerous as to constitute a very significant part of 

 the country rock, but no less remarkable is the occurrence of the acid 

 rocks into which they are intruded. A thin section of light yellow 

 granitic rock from about two miles below the turn to Viscera Spring 

 contains equal amounts of quartz and orthoclase with a little biotite 

 having a tendency toward parallel arrangement. No plagioclase was 

 seen in the section, although it is exceptionally large. Several grains 

 of titanite are present. The magnetite is partly altered to hematite 

 and limonite, as evidenced by the red color on the thin edges by trans- 

 mitted light and yellowish brown stains. This rock is an aplite such 

 as is usually found as rather limited dikes traversing granite ; but here 

 the mass is of considerable area, extending more than three miles along 

 Kattlesnake Canon. 



Four miles below the branch to Viscera Spring the aplite and 

 lamprophyre complex is intruded by a large mass of dark rather 

 coarse-grained granite high in biotite. Orthoclase predominates over 

 the quartz, but only a little plagioclase, oligoclase, is present. Minor 

 accessories are magnetite, titanite, and apatite. The intrusive nature 

 of this rock is clear, for it contains many inclusions of the others. 

 This mass extends nearly to the mouth of the canon, but does not 

 remain constant in its mineralogical composition. A specimen from 

 just south of Twohole Springs, megascopically resembling that 

 described above, in thin section was found to contain micrographic 



