GEOLOGICAL IMPORTANCE OF SEDIMENTATION 561 



found to occur, but one holding a similar stratigraphic position, 

 being embraced between the metamorphic formations of the Vishnu 

 and Archean below and the unconformable Middle- Cambrian above. 

 Walcott gives the series a total thickness of 11,950 feet, divided into 

 two terranes, the upper or Chuar containing 5,120 feet of strata of 

 which 285 feet are limestones and 4,835 feet brown to black or varie- 

 gated shales and some reddish-brown sandstones. The lower or 

 Unkar terrane attains a total thickness of 6,830 feet of which from 

 no to 210 feet are limestones and the balance brown to vermilion 

 sandstones and shaly sandstones with a basal conglomerate 30 feet 

 in thickness. The Unkar is characterized by a great thickness of 

 reddish-brown sandstones. The detailed section of the Unkar as 

 given by Walcott follows, the italics being introduced : ^ 



SECTION FROM THE SUMMIT DOWNWARD FEET 



1. a) Massive bed of gray to reddish magnesian limestone, passing below 



into a calciferous sand rock 50-150 



h) Light gray, shaly sandstone 25 



c) Irregular, massive beds of yellowish-brown sandstone . .... 50 



d) Partially crossbedded, fine grained, purplish -brown sandstone. ... 50 



e) Reddish-brown sandstone and sandy shales, ripple-marked 200 



475 



2. Lava beds: 



a) Nine lava flows aggregating 770 



b) Interbedded sandstones 30 



At Chuar lava hill the lava beds are 1,000 feet thick. 



800 



3. Sandstones (upper) 



a) Shaly, vermilion, rather fine-grained sandstones, with intercalated 

 bands of greenish-gray, followed below by 700 feet of vermilion beds of a 

 uniform character, and massive beds with arenaceous, shaly partings, the 

 massive beds breaking up into shale and sandstone on the talus slopes. 

 Ripple-marks and shrinkage cracks characterize the upper, shaly beds . . 1730 



b) The vermilion sandstones of a) pass into chocolate colored sandstones, 

 that for 125 feet down unite in the general slope of the beds above. 

 Below, a cliff is formed of five massive bands of chocolate-colored, 

 slightly micaceous sandstone, separated by shaly sandstone partings of 



a greenish color below and a chocolate color above 925 



c) Reddish-brown to chocolate, more or less shaly sandstone, 125 feet, 

 underlain by 300 feet of friable sandstone and arenaceous and mica- 

 ceous shale 425 



d) Irregularly bedded, compact sandstone: 



Curiously twdsted and gnarled layers 15 



Massive, grayish layer 10 



Light gray layer with reddish spots, friable, shaly in places 125 



3230 

 I " Pre-Cambrian Igneous Rocks of the Unkar Terrane," Fourteenth Annual 

 Report (1894) U. S. Geological Survey, Part II, pp. 510-12. 



