116 BULLETIN OF THE ESSEX INSTITUTE. 



very obliquely the entire Front Range and the beds 

 lying upon either flank of the range and sloping away 

 from its crest, may therefore be appropriately designated 

 the Ute Fault. Erosion has cut deeply enough over the 

 top of the arch to remove the sedimentary rocks from 

 the downthrow as well as the upthrow side of the fault. 

 The Ute Fault cuts everj^ formation of the region from 

 the fundamental granite and the Potsdam to the Laramie, 

 and in its maximum throw must exceed the aggregate 

 thickness of the Paleozoic and Mesozoic terranes ; and its 

 completion, at least, must date from relatively late geo- 

 logical times. 



SANDSTONE DIKES NORTHWRST OF MANITOU. 



The contact of the granite and sedimentary rocks is 

 obscurely exposed in the southeast bank of Ruxton 

 Creek, beneath the bridge of the Colorado Midland Rail- 

 road. It is exposed again and more satisfactorily in the 

 cut on the railroad at Iron Spring Station (Map, 5). The 

 cut and the hillside just above it show the Potsdam beds in 

 normal succession — white sandstone, red sandstone with 

 glauconitic layers, and red calcareous strata passing into 

 the normal Manitou limestone. Near the granite the 

 beds are tilted l)y the drag of the great fault to a dip of 

 90° Avhich rapidly suljsides to a northeast dip of a1>out 

 45° degrees at the northeast end of the section. The 

 actual contact can be located within a foot or two ; and 

 along this line both the granite and sedimentary rock 

 are much crushed, the bedding of the sandstone is almost 

 obliterated, and all the indications suggest a fault. 



Within two hundred feet southwest of the fault the 

 granite incloses several sandstone dikes. One of these 

 is exposed in the railroad cutting a])Out forty feet from 

 the fault and the l)ase of the Potsdam. It is fifteen feet 



