220 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



lifted in early Mesozoic time: no more accurate determination of the 

 period is available. The uplift was accompanied by marked folding, 

 but little faulting. There is no evidence of a second period of general 

 dynamic action. 



The present topography of the range is shown in the sections pub- 

 lished* to be entirely due to 'erosion. 



Oquirrh mountains. — The Oquirrh mountains lie immediately north of 

 the Tintic mountains, being separated from them only by 20 miles. 

 These mountains have been described by Mr Gilbert, Mr Emmons, and 

 by the writer. The general interpretation of the structure by the differ- 

 ent observers is the same. The range is marked b}^ a series of compar- 

 atively close folds, from which the present topographic features have 

 been derived by erosion. Mr Emmons f observes that in the southern 

 portion of the range faulting has been an extremely subordinate phe- 

 nomenon as compared with plication. 



The relation of the topographic forms to the folds skows that the 

 former are due almost entirely to differential erosion. In the Mercur 

 district intrusive sheets of rhyolite (quartz ])orphyry) have been laid 

 bare, and now, by reason of their greater resistance, constitute hills l3'ing 

 above the more easily eroded limestone. 



Promontory range. — The Promontory range lies north of Salt lake and 

 west of the Wasatch mountains, and may be considered as an extension 

 of the Oquirrh range. It has been described by Messrs King and Hague,J 

 according to whom the range consists of a central anticlinal fold, with a 

 syncline on each side. The description given indicates deej) erosion of 

 the folds to form the present topographic features. 



Aqu/l range. — The Aqui range, as described b^'' King,§ is formed l)y a 

 great anticlinal fold. In the southern portion of the range this fold is 

 deformed by a powerful fault, which brings the Cambrian rocks against 

 the lower Carboniferous. This fault is depicted upon a section in the 

 atlas accompanying the Fortieth Parallel rei)ort. It shows an enormous 

 displacement and a relatively insignificant fault scarp, which has a 

 height of only about one-tenth the throw. 



The relation of the amount of displacement to the height of the scarp 

 shows the effects of powerful erosion since the faulting ; indeed, the fact 

 that the higher side is made up of quartzite, which is more resistant than 

 the limestone on the lower side, suggests that erosion may be solely re- 

 sponsible for the cliff. 



*0b. cit., plate 77. 



t Economic Geology of the Mercur Mining District, Sixteenth Ann. Kept., part ii, p. .361. 



X Explorations of the Fortieth Parallel, vol. i, p. 730, and vol. ii, p. 420, 



I Op. cit., p. 735. 



