150 



GEOLOGY OP THE HIGH PLATEAUS. 



and cleaves off by its vertical joints. Take now a series of these alternat- 

 ing massive layers and softer shales, the long process of erosion gives a 

 series of perpendicular walls, alternating with sloping taluses. This com- 

 posite architecture is one of the most persistent features of the formation. 

 Something like it is seen in the Carboniferous strata forming the walls of 

 the Marble Canon of the Colorado, but there are also many wide differences 

 both of detail and ensemble. 



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Pig. 3. — Generalized profile of Vermilion Cliff. 



The thickness of the Upper Trias is from. 1,100 to 1,800 feet, being 

 greatest in the vicinity of the old shore line, and very slowly attenuating 

 to the eastward. 



THE JUEA8SIC. 



The Jurassic series consists of two members, the lower being a massive 

 sandstone of great thickness, the upper a series of calcareous and gypsifer- 

 ous shales from 200 to 400 feet thick. Underneath the sandstone is a small 

 group of shaly beds, which are presumed to be of Jurassic age, but no deter- 

 minable fossils have been taken from them. It has been a long-standing 

 and difficult question whether the Jurassic sandstone is not, after all, a 

 mere upward continuation of the Vermilion Cliff beneath. Much color 

 was given to this supposition by the fact that no unconformity between 

 them has been detected in this vicinity, and still more by the fact that as 

 we travel eastward and southeastward from the High Plateaus the distinc- 



