722 DESCRIPTIVE GEOLOGY. 



more crystalline base, with the feldspars much better developed, irregularly 

 scattered tlirough the base, and giving the rocks more the character of true 

 feldspar-porphyries. Most of the feldspars would appear to be triclinic. 

 At Indian Canon, where there is a great development of these lower rocks, 

 they show still greater variety in superficial habit, but much the same min- 

 eral composition, in general being. lighter-colored, with prevailing reddish- 

 gray tints. Rocks occur here in which the feldspars are poorly developed, 

 but instead translucent quartz-grains appear scattered through the base 

 quite like the porphyroids described from the region of Wright's Canon, 

 and resembling in hand-specimens true quartz-porphyries. In many of the 

 rocks, the iron seems to be segregated in spots, and to occur as an ochreous 

 substance, filling cavities and fissures. Near the summit of Indian Canon 

 occurs a porphyroid quite unlike those already described, as it carries con- 

 siderable reddish-colored calcite. Under the microscope, the red color is 

 seen to be due to the presence of oxide of iron. The calcite also encloses 

 grains of quartz. 



A specimen of one of the more coarsely-crystalline porphyroids from 

 Cottonwood Canon, and not far below the main limestone belt, has been 

 subjected to chemical analysis by Mr. B. E. Brewster. The rock has a 

 brownish-gray color, and carries both feldspar and quartz, the analysis 

 yielding : 



Silica 74.74 74.72 



Alumina U.14 14.10 



Ferric oxide ^. 0.79 0.81 



Lime 1.51 1.51 



Magnesia 0.39 0.44 



Soda 0.92 0.75 



Potassa 5.29 5-42 



Ignition ].8h 1.83 



99.66 99.58 



It is noticeable that in this analysis the low percentage of soda and 

 the comparatively high percentage of potassa remain nearly the same as in 

 the chert and unaltered argillite. The absence of hornblende and augite, 



