436 H. E. Gregory — Shinarump Conglomerate. 



cliff of Permian shales buried by conglomerate indicates a long 

 erosion interval. It is interesting to note that westward from 

 House Rock Springs across the Kaibab fold, not only the 

 upper sandstones of the Permian, but also the conglomerate is 

 absent, thus bringing the Triassic shales in immediate contact 

 with Permian shales, and removing those features which most 

 clearly indicate an erosion interval. Davis* figures the uncon- 

 formity at Yellowstone Springs, previously noted by Dutton, f 

 and I have seen it at several localities including those already 

 mentioned. 



An unconformity below the Shinarump conglomerate was 

 not reported by Ward nor by Darton along the line of the 

 Santa Fe, and my rapid reconnaissance of parts of the same 

 region led to no satisfactory results. Likewise no conclusive 

 evidence of unconformity was obtained in the San Juan Oil 

 Field either by Woodruff or by me. It may be that the uncon- 

 formity does not exist on these borders of the Plateau Province ; 

 but it is also possible that its presence may have been over- 

 looked. An unconformity without change of dip, especially in 

 places where the conglomeratic phases are absent and where 

 the rocks approach uniformity in color, may escape notice 

 during the rapid traverses incident to reconnaissance. In any 

 case the abrupt transition from the characteristic Permian beds 

 to the unique series lying immediately above indicates a 

 marked change in physical conditions, and even in the absence 

 of direct proof the existence of an unconformity might be 

 postulated. 



In this connection I must dissent from any general accept- 

 ance of the statement of Ward that the Shinarump con- 

 glomerate " contains somewhat large but always well-worn 

 pebbles and cobbles derived from the underlying formations."* 

 At no place in Utah, Arizona, or New Mexico, have I observed 

 in the Moencopie material from which the quartz pebbles of 

 the Shinarump conglomerate may have been derived. In fact, 

 the origin of such quantities of siliceous material at this horizon 

 is so far an unsolved problem. 



Correlation. 



The relation of the Shinarump conglomerate to the Triassic 

 strata of southwestern Colorado has been discussed by Cross,;}: 

 whose detailed studies of the mountains of southern Colorado 

 have furnished a series of stratigraphic units which have greatly 

 facilitated held work in the Navajo country. The equivalents 

 of the La Plata, McElmo, and Dolores formations of Colorado 



* Bull. Mus. Comp. Zool., Harv. Coll., vol. xlii, p. 15, fig. 5. 

 + TJ. S. Geol. Surv., Monograph II, pp. 44-80. 

 i Jour. Geol., vol. xvi, 1908. 



