514 Noble — Geology of the Grand Canyon, Arizona. 



It was noted in the summary of the features of the second 

 member that the shales became successively slates and jaspers 

 above, while their color changed from red to blue. In the 

 third member the shales are represented entirely by jaspers 

 and quartzites. Just below the contact the induration is very 

 great, and the jaspers are tough and vitreous ; the prevailing 

 color is blue or black. Above the contact the overlying rocks 

 are hard blue slates for 20 feet, succeeded by 100 feet of less 

 indurated slate, grading upward into the original red shale. 



The metamorphic effects above and below the contact differ 

 in degree of intensity as well as in kind : above the contact 

 the induration and decoloration characterize only about 100 

 feet of strata ; below the contact this action extends through 

 300 feet. Above the contact the strata are characterized by 

 baking and decoloration only, changing the red shale into a 

 blue slate ; below the contact there has been a considerable 

 addition of silica, transforming the red shales into blue and 

 black jaspers ; added to this are the effects of baking and 

 decoloration. 



It may be said in summary that the third member is charac- 

 terized by argillaceous shales in the lower portion, which grade 

 upward into arenaceous shales and sandstones through the 

 interesting series of alternations described in division a jP 

 There is hardly a stratum in the entire member that does not 

 bear marks of shallow water origin, manifested by either sun- 

 cracks, ripple-marks, or cross -bedding. 



Fourth Member.— Sandstone and Quartzite. 

 Section on the Shinumo. 



a. Purple-brown sandstone of fine grain containing locally 



an occasional lense of fine conglomerate and sometimes 

 a thin local bed of red or purple shale. The sand- 

 stone is cross-bedded throughout 406' 



b. Compact, white quartzite of fine and uniform grain, 



displaying a faint cross-bedded structure. This quartz- 

 ite is the most resistant rock in the fourth member. It 

 is exposed everywhere in one massive perpendicular 

 cliff face, which does not display the slightest break 

 except where it is cut by faults. Wherever its base 

 rests upon a shaly lense the under surface displays 

 well preserved mud-cracks. The face of the cliff is 

 stained magenta by the ferruginous cement of the 

 shale lenses in the overlying sandstones, which washes 

 down from above _. 119' 



c. Purple-brown sandstone of the same character as a 353' 



d. Banded, white quartzite, stained magenta on the expo- 



sures, and forming a cliff — 120' 



