422 POPULAR SCIENCE MONTHLY 



its six hundred feet of deep red cliffs, presents one of the most striking 

 members of the landscape. The Eed Wall constitutes the Lower 

 Carboniferous of the region. 



Then there is a terrace — ' The Esplanade ' — often several miles in 

 width, composed of green shales and sandstones. It is the ' Upper 

 Tonto ' formation. It leads by gentle slopes to the brink of another 

 precipice, which is formed by the quartzites and hard sandstones of the 

 ' Lower Tonto.' The Upper and Lower Tonto are the local names of the 

 formation composing the system known the world over as the Cambrian. 

 Together they are about seven hundred feet thick. The Lower Tonto 

 forms the capping of the inner canon in the Bright Angel region. 



Fig. 5. Hurling Rocks over the Rim. Upper Aubrey Limestones, SaDdstone and Lower 

 Aubrey Sandstone seen on Cliff two miles distant. 



Below it are jagged, twisted, dark-colored metamorphic rocks, which 

 represent the Archaean — the oldest system of rocks in the world. 

 Tourists call these rocks ' granite,' but they 'are schists and gneisses. 

 They are in layers and have been subjected to extensive folding. The 

 river has cut through the Archaean rocks to a depth of over a thousand 

 feet and produced walls so precipitous that in most places a sure-footed 

 prospector could not cling to them. Between them the river runs with 

 the greatest speed and fury. 



The visitor may see all these formations from the upper rim, but 

 as he descends to the river and passes them in succession, he obtains 

 a clearer idea of their relations and characteristics. They are clearly 

 differentiated by constitution, structure and color. A study of these 

 various strata makes it evident that the topography of the country is 



