182 abstracts: geology 



and injected by veins of pegmatite and aplite. Farther down the 

 river, gneisses are the prevaihng rocks. The schists are beheved to 

 be of sedimentary origin, the gneisses, igneous. 



About 5000 feet of strata of the Unkar group, Algonkian, Grand 

 Canyon series, are present. They He in a wedge-shaped mass that is 

 inset in the Vishnu schist by block-faulting along the line of the West 

 Kaibab fault, and are intruded by a sill of diabase 1000 feet thick. 

 Five formations are differentiated: Hotauta conglomerate, overlain by 

 Bass limestone, Hakatai shale, Shinumo quartzite, and Dox sand-stone. 



The Cambrian is divided into three formations: Tapeats sandstone, 

 overlain by Bright Angel shale and Muav limestone. The Carbonif- 

 erous comprises Redwall limestone, Supai formation, Coconino sand- 

 stone, and Kaibab limestone. These beds have a gentle southwesterly 

 dip of 100 to 200 feet to the mile. Plateau surfaces on both sides of 

 the canyon are developed on same hard formation, the Kaibab limestone. 

 Course of canyon is at right angles to dip of strata. Therefore the 

 land north of canyon slopes toward the brink, and the land south slopes 

 away; therefore the attitude of the canyon is like that of a trench dug 

 along a hillside. Drainage of plateaus on both sides of the canyon 

 runs southwesterly with the initial slope of the land; on south side of 

 canyon the heads of many plateau valleys are truncated by the southern 

 wall of the canyon. 



The factors chiefly responsible in determining topographic form in 

 the canyon country are: the horizontal attitude of the beds; hori- 

 zontal continuity and vertical variation (alternation of hard and soft) ; 

 faults traversing them; great height of land above the sea; and aridity 

 of the climate. There is, however, another factor whose importance 

 has not been realized: this is the influence of minor joints and fractures 

 not associated with notable displacement. The courses of most of the 

 smaller side-gorges have been guided by these fractures and most of 

 the buttes and temples have been blocked out by them. 



The Esplanade, a wide terrace which runs through the Kanab divi- 

 sion of the canyon, is not, as has been thought, a local base-level due to 

 a pause in the uplift of the region, but is a structural bench developed, 

 like the Tonto Platform of the Kaibab division, on one of the hard sets 

 of beds in the Paleozoic. 



Examples of pre-Cambrian faults of which the West Kaibab fault 

 is the most notable, additional to those already known along which 

 movement has recurred in post-Paleozoic time, are described and the 

 wide prevalance of this phenomenon in the Grand Canyon region is 

 shown. L. F. N. 



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