220 THE CAMBRIAN. [buli 91 



tiou, and which contain the Potsdam fauna, as identified by Dr. 

 Hay den. 1 



When describing the section of the Grand Canon of the Colorado 

 where it passes through the Kaibab plateau, Maj. J. W. Powell states 

 that the summit of the section was formed by Carboniferous limestone. 

 This extends from the edge of the plateau down 3,500 feet, and beneath 

 it occurs 1,000 feet of conformable rocks of undetermined age. These 

 in turn extend down to the unconformity between them and the subja- 

 cent 10,000 feet of strata that were subsequent^ referred to the Grand 

 Canon group. At the close of the same paragraph, in speaking of the 

 latter series of rocks, he says : 



The beds themselves are not conformable to the overlying Carboniferous rocks ; 

 that is, the Carboniferous rocks are spread over their upturned edges. 2 



It thus appears that all of the strata down to the unconformity were 

 referred to the Carboniferous. This includes the series of sandstones 

 referred to the Potsdam by Dr. Newberry. 



As the result of his studies of the lower strata of the Colorado Canon 

 region at Diamond Creek and at the mouth of the canon, Mr. G. K. Gil- 

 bert in 1874 designated the series of sandstones referred to the Potsdam 

 by Newberry, as the Tonto group, and considered it to belong to the 

 Primordial division of the Silurian. 3 The data for this conclusion were 

 published in the following year. 4 



The section at the mouth of the Colorado has 80 feet of vitreous 

 sandstone resting unconformably upon the granite, and 605 feet of 

 shales alternating with limestones between the sandstone and a super- 

 jacent massive limestoue. 5 At the mouth of the Kanab Creek 100 feet 

 of green arenaceous and micaceous shales occur between the superja- 

 cent massive gray limestone and the level of the river. 6 From the 

 stratigraphic position of the Tonto sandstone and shales and the pres- 

 ence of a species of the genus Cruziana he was led to conclude that the 

 Tonto group is certainly Lower Silurian in age, and probably Primor- 

 dial. 7 



The section of the Tonto series at the mouth of the Grand Canon was 

 measured more in detail by Mr. A. R. Marvine in the same year. He 

 traced 8 the formation to the south and southeast along the margin of 



1 Op. cit., p. 57. 



2 Exploration of the Colorado River of the West and its tributaries, explored in 1869, 1870, 1871, and 

 1 872. Washington, 1875, p. 212. 



3 On the age of the Tonto sandstone. (Abstract.) Washington Phil. Soc. Bull., vol. 1, 1874, p. 109. 



4 Report on the geology of portions of Nevada, Utah, California, and Arizona examined in the years 

 1871 and 1872. Report on Geog. and Geol. Expl. and Survey west of the 100th merid.,in charge of 

 Lieut. Geo. M. Wheeler, voJ. 3, Geology, 1875, pp. 184-186. Report on the geology of portions of New- 

 Mexico and Arizona, examined in 1873. Ibid., vol. 3, 1875, pp. 521, 522. 



5 Report on the geology of portions of Nevada, Utah, California, and Arizona examined in the years 

 1871 and 1872. Report on Geog. and Geol. Expl. and Survey west of the loOth merid.,in charge of 

 Lieut. Geo. M. Wheeler, vol. 3, Geology, 1875, p. 163. 



6 Op. cit., p. 162. 



7 Op. cit., pp. 185, 186. 



8 Report on the geology of route from St. George, Utah, to Gila River, Arizona, examined in 1871. 

 Rept. on Geog. and Geol. Expl. and Surv. west of 100th merid., in charge of Lieut. Geo. M. Wheeler, 

 vol. 3, Geology, 1875, p. 199. 



