TRIASSIC PORTION OF SHINARUMP GROUP 115 



eight hundred feet of strata, on the Little Colorado, one may question 

 the correctness of Button's identification of this datum horizon. This 

 doubt is strengthened by the statement that for 650 feet above the 

 "conglomerate" the "strongly-colored sandy shales abounding in 

 selenite and silicified wood .... resemble so exactly the Permian 

 below that it is quite impossible to distinguish them lithologically " 

 (8, p. 135). Between these shales of Permian aspect and the Wingate 

 sandstone which Button correlates with the Vermilion Cliff occur 

 eight hundred to nine hundred feet of strata which are rarely so well 

 exposed that their character can be ascertained. They are referred 

 to as "lighter colored, pale, dull-red shales." It seems inherently 

 probable that the base of the Triassic series is in this ill exposed part 

 of the section, not far below the Vermilion Cliff or Wingate sandstone. 



The original sweeping assertions of Powell and Button that the 

 Shinarump Group, including all beds between the Aubrey and the 

 Vermilion Cliff, extends, in almost unmodified development, north 

 through the Plateau country to the Uinta Mountains were based on 

 insufficient knowledge. There is much evidence to show that the 

 strata of the section in question do not preserve that wonderful 

 constancy of character, nor the uniform thickness to be inferred from 

 Button's words, which have been quoted. This holds true for both 

 Triassic and Paleozoic parts of the section. 



Let us first review the knowledge concerning these deposits in the 

 upper reaches of the canon of the Colorado, and of its branches, 

 the Grand and Green Rivers. 



Vicinity of the Henry Mountains. — Going up the Colorado less 

 than seventy-five miles above the mouth of Paria River, where Powell, 

 Button, Gilbert, and others have examined the section, we come to 

 the area included in Gilbert's study of the Henry Mountains. For 

 that area he has given the following general section of the Shinarump 

 Group (12, p. 6). 



-l-Op FEET 



a. Variegated clay shale ; purple and white above and chocolate below, with 



siHcified wood . . . . . . • . . . . 300 



h. Gray conglomerate, with silicified wood; the Shinarump conglomerate . 30 



c. Chocolate-colored shale, in part sandy ...... 400 



730 



