130 ANNALS NEW YORK ACADEMY OF SCIENCES 



are surely connected witli them. If such a contact could be seen, it would 

 shed much light upon the relative ages, l)ut in tlie absence of actual 

 proof, we may only I'eason abont them. 



If we assume that the Little Cottonwood granite, the Alta granodiorite 

 and the Clayton Peak ([uartz diorite are connected below, as is commonly 

 done, the}' are probably not of very different ages and may be taken as 

 being as young as the most recent sediments cut. This would nuike them 

 post-Triassic. If the fracturing of the beds and intrusion of the dikes 

 came after the folding, which is thought to be late Cretaceous, and if this 

 occurred contemporaneously with the intrusion of the larger bodies, as 

 might be the case, then the Little Cottonwood granite, as well as most of 

 the other igneous masses, are post-Cretaceous. 



The extrusive andesites of the Kamas prairie to the east are in contact 

 with the Vermillion Creek beds of the Eocene as reported by the Fortieth 

 Parallel geologists.^® They are thus later than these early Eocene beds 

 and represent the latest igneous activity of the region. Their relation to 

 the porphyritic dikes and granitoid intrusives of the Cottonwoods is not 

 known, but they are probably much later. The Little Cottonwood granite 

 was no doubt uncovered during the period of erosion which followed the 

 post-Cretaceous folding. The extrusions came after the upturned Paleo- 

 zoic and Mesozoic beds had been strongly truncated, covering the old 

 surface in the depression between the Wasatch and Uinta mountains. 



The date of the intrusion of the granite will presently be further dis- 

 cussed when the problem of overthru sting and faulting near Alta is taken 

 up. From the above, it appears that the granite probably came in imme- 

 diately preceding or possibly accompanying the folding in post-Cretaceous 

 time. The eruptive andesites are post- Vermillion Creek and belong un- 

 doubtedly to the Tertiary period. 



STRUCTURE NEAR ALTA 



In the Alta region, the most obvious structnre is an eastward dipping 

 monocline, which to the north and south slowly curves westward, in ac- 

 cordance with the general dome structure for this part of the range. 

 The strata stand at a considerable inclination, averaging between 35 and 

 45 degrees, but locally the dip may be mnch moi'e and in some parts 

 notably less. This simple stnicture is much complicated in places by 

 folding and faulting. The folds are confined to a zone within the sedi- 

 mentary series, the formations above and below having the ordinary 

 monoclinal attitude. This condition has been brought about by over- 



•"'«S. F. Emmons: V. S. (Jeol. Expl. 40tli Par., Vol. I. pp. r)8f)-r.S7. IST-S. 



