The Shinumo Area. 515 



e. Fine-grained, purple sandstone with a white band in the 



middle. The white band is constant and presents a 

 conspicuous feature by which this purple sandstone 

 can be distinguished at a distance of several miles. 

 The rock is cross-bedded and sometimes displays a 

 " twisted and gnarled " structure _ __ 150' 



f. Compact, cliff-forming, white quartzite of the same 



character as b, though not so massive in structure 250' 



g. Banded white quartzite 20' 



h. Irregularly bedded sandstone . 



1. "Curiously twisted and gnarled layers," of fine- 

 grained white sandstone containing large red spots of 

 a circular and elliptical form. The upper part of the 

 bed is more massive. The twisted and gnarled struc- 

 ture seems to have been a phenomenon of the original 

 deposition. It gives the rock an appearance which 

 suggests that the original sand was moist and plastic 

 and once flowed by rolling over and over in the form 

 of a quicksand. 105' 



(It may be noted that a bed of this character is 

 described by Walcott in his section in Unkar Val- 

 ley. (Walcott b, page 511.) It occurs at the same 

 horizon as the bed described above, and contains the 

 same red spots.) 



2. Banded, purple sandstones 20' 



3. Green, cross-bedded sandstone 21' 



146' 



The total thickness of the fourth member of the Unkar is 

 1564 feet. 



Slides were cut from several specimens of the sandstones 

 and quartzites. All were found to consist of small rounded 

 quartz grains, the size of which seldom exceeds # 7 mm . This 

 extreme fineness and roundness of the grains, as well as the 

 cleanness of the sorting, is remarkable. The cement is usually 

 siliceous, sometimes slightly ferruginous. A slide was made 

 from a specimen taken from one of the small conglomerate 

 lenses in division a. The conglomerate was found to consist 

 of small rounded quartz pebbles, lying in a fine arkose matrix. 

 Occasional large angular fragments of orthoclase and micro - 

 cline were revealed. 



In summary it may be said that the fourth member is char- 

 acterized by great thicknesses of pure, fine-grained sandstone 

 of a uniform character. All divisions of the member are 

 resistant to the weather and form cliffs. Hippie marks and 

 cross-bedding occur throughout. 



Ftfth Member. — Micaceous Shaly Sandstone. 



Section measured in Fault Wash, west side of Shinumo 

 Canyon. 



