Mesozoic and CcBnozoic Geology and Paleeontology. 189 



G. K. Gilbert^- fouDcl the Cretaceous strata well displaj-ecl upon the 

 flanks of the Henry Mountains, in Southern Utah, where they consist 

 of four principal sandstones, with intervening shales, and have a thick- 

 ness of 3,500 feet. They also contain thin beds of coal, one of which 

 was observed at the foot of Mount Ellen, four feet in thickness. The 

 lower 500 feet he referred to the Henry's Fork Group. 



Dr. A. C. Peale,f geologist of the Grand river division, said that 

 the massive, j^ellow silicious sandstone, in some places quartzite, at the 

 base of the Cretaceous, is so well defined lithologically, that there has 

 never been any difficulty in separating it from the overlying shales. 

 Along the edge of the plains in Colorado, it is underlaid by greenish 

 shal}" beds, sometimes lignitic near the top, generally in part or wholly 

 covered, which have always been referred to the upper part of the 

 Jurassic. In the West these shalv beds still persist, and the massive 

 sandstone, although still recognizable without difficulty, is much 

 thinner, being on Ij^ from 50 to 100 feet, and as we descend, in the sec- 

 tions carried below, we find other beds of silicious sandstone separated 

 by shaly beds that are arenaceous, calcareous and argillaceous. In 

 these beds, in 1874, he found a sassafras-leaf, which led him to refer 

 them to the Lower Cretaceous. He drew an arbitrary line separating 

 the Cretaceous and Jurassic. The beds below have the same lithologi- 

 cal characters to the top of the red beds, with this exception, that 

 limestones occur more frequently toward the base. In Arizona, G. K. 

 Gilbert found Jurassic and Cretaceous fossils, associated in beds, re- 

 sembling those usually referred to the Jurassic. He is of the opinion 

 that we can not draw au}^ line between the two formations, palieontolo- 

 gically, or lithologically, but for convenience in description it is best 

 to draw an arbitrary line, which may be changed as we obtain more 

 facts in relation to the formation. 



There is a narrow outcrop of the Dakota Group on the south side of 

 the Gunnisoi), above the Grand Canon, between the breccia and the 

 granite. It appears, and is faulted, at the head of the Uncompahgre 

 river and on Dallas Forii, the latter stream flowing on the line of the 

 fault. Between this creek and the San Juan Mountains it rises until 

 it reaches the summit of the foot hills, appearing from beneath the 

 shales. On the Uncompahgre plateau, it dips gently to the eastward, 

 and is the surface formation until we approach Escalante creek. Be- 

 tween the latter and Roubideau's creek, there are some isolated 



■■• Kep. on the Geo. Henry Mountains. 



f 9th Rep. Hayden'sU. S. Geo. Siir. Terr. 



