98 YEARBOOK, PUBLIC MUSEUM, MILWAUKEE [Vol. II. 



what is known as the "Cathedral Stairs," the most difficult part of the 

 trail. 



All the rocks forming the canyon wall above the base of this Red- 

 wall Limestone, belong to the Carboniferous Period — the age in which 

 the great coal beds of the world were laid down. The rocks below, 

 however, are much older, and are called the Tonto group. This group 



Fig. 54. — Hermit Camp at the foot of Lookout Point, 

 Grand Canyon. 



consists largely of eight hundred feet of shale of a greenish color, and 

 at the bottom, a sandstone about one hundred and fifty feet in thick- 

 ness. This great thickness of shale makes a broad shelf in the canyon 

 wall and it forms such a pronounced feature throughout a great part 

 of the canyon, that it has been especially named Tonto Platform. Upon 

 this platform it is possible to travel from the eastern end of the Grand 



