S. F. Emmons — Little Cottonwood Granite. 147 



which not only Mesozoic but a great thickness of Eocene 

 Tertiary beds must have been denuded, hence a long period 

 must have elapsed between the older eruptions and this one, 

 which, by analogy with the regions to the westward, may be 

 assumed to have occurred during the Miocene period. 



It has been assumed as a result of the 40th Parallel work — 

 an assumption that has not yet been seriously modified by later 

 investigations — that while there was in the Cordilleran region 

 toward the close of the Carboniferous a general elevation of 

 the Great Basin region accompanied by greater or less erosion, 

 the main mountain-building movement occurred at the close of 

 the Jurassic. In the Wasatch some evidence of a transgression 

 at this period was found by the 40th Parallel geologists, but it 

 was not insisted on in their reports because without a reexamina- 

 tion of the Held it could not be considered absolutely conclusive. 

 It would now appear probable that the eruption of the Clayton 

 Peak mass must have been contemporaneous with or closely 

 followed this Jurassic movement. If future investigations 

 prove that the Cottonwood body is of the same age as the 

 Clayton Peak mass, there will be found exposed here in unusual 

 detail the contact phenomena of an enormous granitic batholith, 

 extending in horizon from the Archean across a vertical column 

 of 25 to 30,000 feet of sedimentary beds of varying composi- 

 tion, and an opportunity will be afforded to study what influence, 

 if any, has been exerted on the intrusive magma by the rocks 

 through which it passed and which, presumably, must have been 

 absorbed by it. 



U. S. Geological Survey, Washington, D, C. 



