WATER AS A MECHANICAL AGENT. 



187 



The effects of alternation in hard and soft layers, distantly spaced or 

 grouped, appear throughout the scene. Besides, there are columnar lines 

 due to vertical joints in the harder beds, or to rill-work down the vertical 

 and sloping surfaces. 



Marble Canon. From a photograph. 



The rock of the level region either side of the canon, and of the upper 

 part of the walls, is Carboniferous limestone. Below are Paleozoic sand- 

 stones and other limestones, descending to the Cambrian ; at bottom, in 

 some parts, and for a height of 500 to 1000 feet above, the rocks are granitic. 



Many views of the Colorado Canon also show ranges of flat-topped 

 mountain heights to the north, all of which have similar architectural 

 features in their declivities, yet with peculiarities belonging severally to 

 the rocks of the different periods represented. As described by Dutton, 

 first, in the ascent to the summit, there are the Triassic "Vermilion Cliffs"; 

 above these the white and red Jurassic ; then the pale yellow, gray, and 

 brown Cretaceous strata ; and at the top great plains, the High Plateaus of 

 Utah, the highest nearly 12,000 feet above sea level, which, unlike the 

 slopes, are covered in some parts with forests. The vegetation at the sum- 

 mit is accounted for, says Dutton, by the fact that the rainfall there is 30 

 inches a year, while only four to eight inches in the lower country. These 

 mountain plateaus are remnants of formations that once covered the canon 

 region and extended far away into Arizona. 



