jS. F. Emmons — Little Cottonwood Granite. 145 



same general mineralogical composition as the diorite. These 

 general relations are shown in the accompanying prohle of the 

 range on a general east and west line running through Twin 

 Peak, the head of Little Cottonwood Canvon, and Clayton 

 Peak. 



1 



'to>vUv)r>j 



A broad depression between the eastern foothills of the 

 Wasatch Mountains and the western flanks of the Uinta Moun- 

 tains is largely covered by andesitic lavas, overlying bedded 

 tuffs of an undoubted extrusive nature, which are of Tertiary 

 (probably post-Eocene) age. These rocks have a striking 

 resemblance in their mineralogical composition to the diorites 

 and porphyries of the Clayton Peak mass. 



While, therefore, it must be admitted that the geological 

 history of the region, as traced by the geologists of the 40tli 

 Parallel, is not entirely correct, it is yet too early to state 

 exactly how far and in what way it should be modified. This 

 must await a thorough resurvey of the region based on large 

 scale maps, a work which has already been inaugurated by the 

 geologists of the Survey ; inasmuch, however, as it will pro- 

 bably be several years before this work will be sufficiently far 

 advanced to afford final solutions of some of the most critical 

 problems, it may be permitted to state what these problems are 

 and to present some alternative hypothesis as to their probable 

 solution. It is certainly a most striking fact that these several 

 eruptive bodies, which apparently are of sufficiently similar 

 mineralogical and chemical composition to have originated in 

 the same general magma, occur along the line running a little 

 north of east, which, if extended westward, would pass through 

 the mining district of Bingham Canyon where occur the most 

 important intrusions of porphyritic eruptive rocks in the 

 Oquirrh Mountains, and where there have also been outpour- 

 ings of later extrusives of consanguineous character along the 

 flanks of the range. The same line, extended still farther west- 

 ward to the parallel range of the Aqui Mountains, passes 

 through a small body of extrusive, andesitic lavas and tuffs on 

 the east flanks of that range, which, so far as known, is the only 

 point where eruptive rocks are found in it. Attention was 

 called to this fact in my original description of the geology of 

 this region,^ and also to the fact that along this line in the 

 *40t]i Parallel Report, vol. ii, Descriptive Geology, p. 459. 



