MINING DISTEIOTS. 3 



rocks, and, notably, syenitic granite, with occasional occurrences of granulite 

 and greisen, accompany the ejections of granite. 



The date of this orographical period is assigned to the late Jurassic, on 

 grounds which will be found fully discussed in the first volume of the present 

 series. It is but just, however, to state here that the foundation of this 

 important discovery in geological dynamics was laid by Professor J. D. Whit- 

 ney, who early pointed out that the Sierra Nevada was folded after the Lias 

 and prior to the Cretaceous, whose strata of sand and clay rocks repose 

 unconformably upon the upturned and metamorphosed mass of Jurassic 

 slates. 



This Exploration has demonstrated that all the parallel ranges of the 

 Grreat Basin, including the chain of the Wahsatch, its eastern wall, belong to 

 the same system of upheaval, and that, while the Pacific built upon the west- 

 ern base of the Sierras those fringing deposits of sand and clay w^hich thickened 

 through the undisturbed period of the Cretaceous and a wide range of the 

 Tertiary, the Atlantic, or, more exactly, that ocean which covered the Missis- 

 sippi Basin, beat upon the east flank of the Wahsatch and laid down a series of 

 Cretaceous and Tertiary strata, exactly corresponding with the coast deposits 

 of the Pacific. 



At length, after accumulating to an extraordinary thickness, these outly- 

 ing and later shore-beds, subsequently to the Miocene, were themselves folded 

 into mountains parallel and outside of the earlier system. 



As granites accompanied the upheaval of the earlier stratified group, so 

 volcanic rocks have poured out from ruptures of the second mountain uplift. 

 From the crests of the ranges, from the fissured bottom of the Pacific Ocean, 

 from innumerable vents over the whole area of the western mountain system, 

 there burst forth a series of volcanic eruptions which in many instances have 

 overflowed and completely masked the earlier ranges, and in others have filled 

 old depressions, building everywhere immense piles of lava mountains, and 

 lifting here and there volcanic cones of the most impressive order. 



Long prior to the deposition of the great Palaeozoic beds, a limited 

 group of chains, composed of granites with crystalline schists and interstrati- 

 fied layers of specular iron, was lifted in Arizona and probably over a con- 

 siderable area of North Central Mexico. Later than the main Tertiary moun- 



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