lOO WHITMAN CROSS 



The foregoing brief statement will, it is hoped, be sufficient to 

 show the desirability of tracing the horizon represented by the basal 

 beds of the Dolores formation through the Plateau country, not only 

 to establish the base of the Trias but to discover the most favorable 

 localities for the study of the older, and probably Paleozoic red beds, 

 the full section of which is perhaps not yet known. According to 

 present knowledge the horizon in question is not far below the Ver- 

 milion Cliff sandstone in the central and northern parts of the Plateau. 

 In southern Utah and Arizona there is evidence of a decided increase 

 in the thickness of the Triassic beds below the Vermilion Cliff sand- 

 stone, sufficient to warrant the suspicion that the true Shinarump 

 conglomerate of the Shinarump Cliffs, while a thousand feet below 

 that sandstone, may still be at the same horizon as the "saurian 

 conglomerate" of the Dolores formation. 



We will turn now to a scrutiny of the literature bearing upon the 

 subject. 



The Shinarump conglomerate described. — The term "Shinarump 

 conglomerate " appears to have been used for the first time by Powell, 

 in 1873, while describing the geologic structure and naming the topo- 

 graphic features of the district lying north of the Grand Canyon of 

 the Colorado (21, p. 457, 458). Powell says that from the Grand 

 Canyon northward one travels over Carboniferous beds to the first 

 line of cliffs, one hundred to four hundred feet high. 



This escarpment is capped by a firmly cemented conglomerate containing 

 many fragments of silicified wood, and over its surface are scattered many like 

 fragments, and sometimes huge tree trunks, which are the remnants of rocks at 

 one time overlying the conglomerate, but now carried away by erosion. Under- 

 lying this cap are variegated sandstones and marls. The whole group is probably 

 of lower Triassic age. 



In 1875, Gilbert, Marvine, and Howell, of the Wheeler Survey, 

 published the results of their work in the country adjacent to the 

 Colorado River, and refer to the Shinarump conglomerate as some- 

 thing defined by Powell. Their observations covered a wide territory, 

 including that of the Shinarump Cliff, and there seems to be no doubt 

 but that their references are to a single well-defined conglomerate 

 stratum or bed. 



Gilbert assigned to the Trias 2,500 to 3,500 feet of beds between the 



