HINTZE, GEOLOGY OF WA.-^ATC/f MOl XTAiyS, UTAH 129 



trusive and pre-Jurassic in age and the chief folding of the sediments 

 as Jurassic. 



The Little Cottonwood granite has commonly been regarded as lac- 

 colithic in structure, since its intrusive character has been known. While 

 the inclosing quartzites do dip away in all directions from the central 

 igneous mass, suggesting that they may have been arched up by the in- 

 trusion, the essentials of laccolithic structure are nowhere shown. The 

 far-reaching metamorphic effects of the granite upon the contiguous 

 sediments, its uneven ragged contact on all sides and its thorough crys- 

 talline coarse texture all indicate a mass of irregular shape and great 

 size. It would seem advisable, therefore, to speak of the Little Cotton- 

 wood mass as a stock and reserve the term laccolith for the more special 

 type of intrusive. 



As to the geologic data of the intrusion, there is also much uncertainty. 

 The latest sediments cut are Algonkian, and possibly Lower Huronian, 

 in age. If the mass were known to be laccolithic, then the latest sedi- 

 ments affected by the arching would give the desired information; or, 

 if the doming of the strata is due to the intrusion of the granitic stock, 

 then the age might quite easily be stated as later than the youngest beds 

 that are involved. But it is difficult in this region of strong folding to 

 distinguish between the flexing due to regional folding and that due to 

 a special cause such as intrusion, where the two come so close together. 



A few general considerations may lead to a closer approximation of 

 the date of the intrusion than can be made from the sediments cut by it. 

 The Little Cottonwood granite mass lies in an east-west zone of eruption 

 which has been active in some parts in post-Tria^sic, probably Tertiary 

 time. At Bingham, it is marked by a large body of post-Carboniferous 

 monzonite and trachytic extrusion. Still farther west, the sheets and 

 dikes of the Mercur and Ophir districts are in the westward continuation 

 of this belt. Just east of Alta is a large irregular stock of granodiorite 

 which cuts Carboniferous limestones and adjoining it to the east is the 

 Clayton Peak mass of quartz diorite which cuts Triassic strata. The 

 interrelations of these three main intrusive bodies have not been discov- 

 ered in the field. They are not in surface connection with each other, so 

 far as known, but a northeast-southwest system of dikes and veins is 

 characteristic of the whole region; and closely associated with the ore 

 bodies. These dikes are clearly later than the folding, since they do not 

 show deformation and from their similarity to the larger intrusive masses 

 they may be assumed to have come from them, though none has actually 

 been traced to the junction point. They are seen to disappear beneath 

 rock debris within a few hundred feet of the larger bodies, however, and 



