5(5 SEDIMENTARY GROUPS OF THE PLATEAU PROVINCE. 



thousand feet is a massive, homogeneous, saccharoid limestone ; the lower 

 one thousand feet is composed chiefly of thin beds of indurated limestones 

 of very irregular stratification surfaces. These beds are somewhat argilla- 

 ceous. The group is not well exposed in Cataract Caiion, as the river has not 

 yet cut through the beds at that point, and some very curious displacements 

 along the river serve to obscure the characteristics of the beds that are 

 exposed. In the Uinta Mountains the two members seen in the Grand 

 Canon are represented by two massive, indurated limestones, often con- 

 taining chert and separated by arenaceous shales. 



TON TO GKOUP. 



I have elsewhere called these the rust colored beds, but Mr. Gilbert 

 has called them the Tonto Group, and I accept his name. These beds are 

 seen to overlie unconformably the beds of the Grand Canon Group and of 

 the Grand Caiion Schists. They are seen well exposed along or near the 

 bottom of the Grand Canon, where the river makes its double detour 

 around tlie Kaibab Plateau, and again farther westward, where the river 

 makes another detour around the Shi-wits Plateau. 



A group of sandstones and arenaceous shales is found below the Red 

 Wall horizon in Lodore and Whirlpool Canons, where- the Green River 

 cuts through the Uinta Mountains. In the beds of this latter place I have 

 discovered Carboniferous fossils, and suppose them to be of the same age as 

 the Tonto Group ; yet, as Mr. Gilbert has considered the Tonto beds to be 

 of Silurian Age, I have called the latter Lodore Group provisionally. 

 From geological considerations, I am inclined to consider the Tonto Group 

 as forming the base of the Carboniferous series. The supposed Cnmami 

 and metamorphosed corals discovered by Mr. Gilbert are not deemed by me 

 to furnish sufficient evidence of their greater age. Their geological rela- 

 tions being apparently the same as the Lodore series, I am inclined to 

 refer them to the same horizon ; the latter have been demonstrated to 

 be Carboniferous. My opinion is strengthened by the fact that I find in 

 the Grand Canon 10,000 feet of sandstones, shales and limestones, under- 

 lying them unconformably, and hence separated by a long period of ero- 

 sion, and 'at the base of this latter series I have found Silurian fossils. I 



