b 



04 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



r 



J^ot only were the grounds on which the Jurassic age was assigned 

 to the granite not subsequently discussed, but in point of fact an entire 

 change of view took place previous to the publication of Volume L, the 

 reason for this absolute reversal of opinion in regard to a fundamental 

 point in the geology of the Cordilleras being nowhere given, and 

 what is still more inexplicable — no allusion being made to it. 



Mr. Emmons, in describing part of the same range (the Wahsatch), 

 states (II., pp. 355, 360-363, 365) : 



"A body of Archaean slatea and granite is surrounded, and partly covered, 

 on all sides except the west, by a conformable series of sedimentary rocks, of 

 an aggregate thickness of over 30,000 feet, extending in age from the Cambrian 

 to the Jurassic inclusive. The granite mass^ though eruptive, has not been 

 protruded through this immense thickness of overlying rocks, but their beds 

 were deposited around and over a submerged mountain -range of granite sur- 

 rounded by Archsean rocks ; and subsequent elevation, flexure, di.slocatioB, 

 and erosion have produced the conditions represented on the map, where it 

 will be soen that of this conformable series, now bent and twisted, difTcrent 

 horizons from the Cambrian up to the Middle Coal Measures are at different 

 points in contact with the granite body. Of the immense arch which once 

 covered this body, the western half has been faulted down, while the top of 

 the arch, with its thickness of 30,000 feet of rock masses, has been broken up 

 and worn away by atmospheric agencies. .... In Big Cottonwood Caiion, 



. . . . is a small exposure of granite It is difficult to say whether this 



rock should be considered as part of the main granite body, which it does not 

 resemble very closely, or with the later outbursts of granite-porphyries and 

 diorites, which are found intersecting the sedimentary rocks of this region. 

 These dikes of porphyry and diorite are very frequent, especially around the 

 Clayton Peak mass and in the region where the mineralization of the beds hag 

 been most developed. .... Near this granite or diorite body of Big Cotton- 

 wood Canon, in one of the beds of the upper part of the Wahsatch limestone, 

 is a dike about 20 feet wide, of syenitic granite-porphyry, so classed by Zirkel, 



which resembles the granite in general appearance 



*' The Palseozoic beds of this region, which fold around and partly cover the 

 granite body, have been subjected to intense compression and local metamor- 

 phism, twisted and contorted in every direction, faulted and dislocated, and 

 penetrated by intrusive. dikes and mineral veins ... The great belt of 

 Wahsatch limestone, which forms the main stratigraphioal landmark in these 

 formations, .... forms massive cliffs at the southern head of Little Cotton- 

 wood, and here already, in the mantling of white, through its general blue 

 color, shows the commencement of the metamorphism which has marbleized 

 its beds in great degree from here to the mouth of Mill Creek Canon, where 

 they disappear beneath the Sal^; Lake plain. This belt includes, as has 

 already been stated, the three groups of Lower Coal-Measures, Sub-Carbon- 



