H. E. Gregory — Shinarump Conglomerate. 437 



are widely exposed in southern Utah, northeastern Arizona, 

 and northwestern New Mexico. The typical Shinarump con- 

 glomerate, however, does not appear in the sections so far pub- 

 lished for the San Juan country. In discussing its absence Cross* 

 suggested that the Shinarump conglomerate may be represented 

 in Colorado by the " Saurian conglomerate" found in the 

 midst of the Dolores (Leroux, in part, of Ward) (Triassic) for- 

 mation, — a view which has served the useful purpose of a 

 working hypothesis. 



The " Saurian conglomerate," a rock consisting typically of 

 elongated pebbles of limestone frequently accompanied by chert 

 (the "mortar beds" of Ward), occurs at each of the many local- 

 ities where the Dolores is exposed. Several beds are present, 

 usually grouped in the middle or lower half of this formation. 

 Identity of position, composition, structure, and fossil content, 

 leave little room for doubt that these limestones extend from 

 the La Plata mountains to the Little Colorado, and from Lee's 

 Ferry to Fort Wingate. Wherever observed, the " Saurian 

 conglomerate" occurs in the midst of variegated shales and is 

 above, sometimes several hundred feet above, the Shinarump 

 conglomerate. 



The evidence so far collected from the entire area under dis- 

 cussion has led to complete agreement on the part of Cross f 

 and the writer, that the Shinarump conglomerate is not repre- 

 sented in the Dolores of southwestern Colorado, and that the 

 " Saurian conglomerate " layers of the Dolores lie above the 

 Shinarump in the San Juan valley. The Shinarump may not 

 have extended beyond the Plateau Province, but may have 

 been removed during the pre-Dolores erosion interval, demon- 

 strated by Cross, the extent and significance of which for the 

 Plateau Province I propose to discuss in another connection. 



The stratigraphic location of the Shinarump conglomerate 

 involves the position, extent, and age of the underlying shales 

 and sandstone (Moencopie of Ward). In regard to these beds 

 some confusion exists in the literature because no attempt was 

 made by Ward to harmonize his studies with the results ob- 

 tained by Powell, Dutton, Gilbert, and Walcott, and because 

 the preliminary conclusions of Cross:): were substantially modi- 

 fied as the result of later study. § 



My observations on this point are in accord with the results 

 of Ward's study ; that upwards of 700 feet of chocolate shales 

 and sandstone (Moencopie) rest unconformably upon the "Upper 

 Aubrey" (Kaibab limestone of Darton) Carboniferous, and are 



*Idem, pp. 121-122. 



f Private communication, February 10, 1913. 

 {Bull. Geol. Soc. Amer., vol. vi, p. 482, 1905. 

 § Jour. Geol., vol. xvi, p. Ill, 1908. 



