MAKVLNE.] GEOLOGY FEATURES OF THE MOUNTAIN BOEDER. 135 



uent butte just west of the junction of the two Baint Vrain Creeks is 

 a southward continuation of the ridge of red-beds to the north, and, at 

 its western base, its lower beds are found faulted- downward about 300 

 feet, showing about 50 feet of granite at the base of the hill, section 8. 

 This forms the minor echelon fault e. Ui)on the south side of the river 

 caiion it may be seen as an exceedingly gentle fold, and on still farther, 

 where the red-beds lie at a very low angle reaching far up upon the gentle 

 mountain slope, it is still barely indicated, (section 9.) From here south- 

 ward the pronounced multiple character of the fold ceases with the 

 dying away of the fold e, though the influence of the various flexures 

 may still be felt here and there in the plains in slight changes of dips. 

 The gentle plications found at the Baker and Marshall mines may be the 

 southern continuation of some of the folds at the north. 



From a few miles south of Saint Vrain Creek the flatness of the dip 

 gives the outcropping beds a wide zone of exposure upon the surface, 

 the lower measures reaching farther west upon the long mountain-slope 

 than at any other point. The dip, however, rapidly steepens in going 

 south, causing the zone of outcrops to contract within narrow limits, 

 which is accomplished by the approach toward the mountains of the 

 outer lines of ridges. At Left-Hand Creek they are gathered within 

 quite a narrow space, the lower beds dipping eastward at an angle of 

 20° or 30°. 



From here southward to South Platte, this one line of upthrow is per- 

 fectly continuous, and, though peculiar in its character, being something 

 more than a mere simple upward fold, it is nowhere prominently 

 offset en echelon by another fold at the north, though the tendency is 

 exhibited at a few points, as at Bear Caiion and Golden City. The gen- 

 eral direction of this line is at first south, gradually swinging to about 

 south 20° east, or about parallel with the echelon folds, of which, in 

 reality, it may be considered as a very long one. Taken as a whole, the 

 general course of the line of outcropping hog-backs along the mountain 

 front, as indicated by a line drawn tangent to the southern ends of the 

 echelon folds at the north, and following the more connected line of out- 

 crops at the south, presents a great cresent or arch-like form, with its 

 flat concavity to the east ; the span north and south being sixty miles, 

 the ver-sine, or depression westward, at the middle, being about ten 

 miles. The southern portion of the fold yet remains to be examined. 



North from Boulder City the lower red-beds appear in a low ridge, the 

 western i)ortions of which are thrown over a few degrees beyond the 

 vertical, as in section 12, to be described later, the remaining beds being 

 pretty effectually obscured by the gravel terraces. They probably quite 

 abruptly assume a very gentle eastern dip, however. 



At Bear Caiion, as frequently noticed before, from beneath the high 

 gravel terraces the full series, 'from Cretaceous No. 3 downward, are all 

 fully exposed, dipping very regularly 50° to 55° eastward. But beyond 

 the high ridge of harcT Triassic sandstone, and the granite behind it, a 

 second and higher ridge appears. It is of the same sandstone, a fault 

 separating the two, the down-throw being on the west side, as shown in 

 section 10. The western ridge probably never stood so high above its 

 I)resent position as to be in the same plane with the eastern sandstone 

 as it now stands, being subsequently faulted down into its present posi- 

 tion, but both were probably simultaneously tilted from the horizontal 

 into their present positions by the one operation. 



Scarcely a mile south of this section, in South Boulder Peak, a portion 

 of the western ridge has been caught in the faulting and bent abruptly 

 upward, forming a sharp synclinal, as shown in section 11. Though 



