502 Noble — Geology of the Grand Canyon, Arizona. 



against the fault line in a graceful arc. Far up the canyon 

 at its head under the Muav Saddle, the more recent fault has 

 reversed its throw and restored the throw of the monoclinal 

 flexure. There is not a detail of this structure that may not 

 be seen at a glance. 



Sediments oe the Unkar Group. — Ltthology. 



Preliminary Outline.— -The pre-Cambrian sedimentary rocks 

 of the Shinumo area represent the greater part of the Unkar 

 group of the Grand Canyon series of Algonkian age. The upper, 

 or Chuar, group is not represented in the area. Although 

 these sediments present no more evidence of alteration or meta- 

 morphism, aside from local igneous contact phenomena, than 

 the overlying beds of the Paleozoic, they are destitute of fos- 

 sils or decisive evidence of life. In the absence of fossils the 

 natural basis for dividing the group into component mem- 

 bers is the lithology. On this basis the Unkar group in the 

 Shinumo area is divisible into iive members, succeeding each 

 other in conformable stratigraphic succession. It is not in- 

 tended that the importance of this division should be greatly 

 emphasized. Its chief value lies in the fact that it furnishes a 

 basis for comparing the lithological succession of the Unkar 

 group in this area with that in the type locality described by 

 Walcott 30 miles to the east, as well as a basis for distinguish- 

 ing in a broad way the main changes in the physical conditions 

 under which these sediments were laid down. 



At the base, resting upon the profoundly eroded and base- 

 leveled surface of the metamorphic rocks of the Vishnu series, 



Tonto Group 

 Unconformitv. 



Unkar Group. -< 



Synopsis. 



5. Micaceous shaly sandstone (exclusive 



of 36 feet of intrusive rock) 2297 



4. Sandstone and quartzite 1564 



3. Argillaceous and arenaceous shale 

 (exclusive of 950 feet of intrusive 

 rock) _ . 580 



2. Calcareous shale and limestone 335 



1 . Basal conglomerate 6 



Total,, --- 47S2ft. 



Unconformity 

 Vishnu series. 



