104 GEOLOGY OF THE DENVER BASIN. 



occupies the western of the three rents and forms the prominent north-and- 

 south dike of this Ideality. The block of strata inclosed by the fractures 

 extends north and south a little over a mile, east and west about 2 miles. 

 Its component strata include rocks of the Pierre, Fox Hills, Laramie, and 

 Arapahoe formations, completely shattered and in a most chaotic state, 

 except in the extreme eastern part of the area, where their dip becomes 

 more regular, and gradually shallows to the horizontal beyond the disturbed 

 region. The amount of displacement which the interlaid t block has suf- 

 fered is variable for the east-and-west fractures, while an actual estimate 

 along the north-and-south fracture can not be made, on account of the 

 shattered condition of the beds and the impossibility of recognizing definite 

 horizons. 



The north-and-south fracture approximately coincides with a stratifica- 

 tion plane, the planes of the east-and-west faults intersecting this. The 

 north-and-south plane is now nearly vertical, though at the time of erup- 

 tion it was probably more or less inclined, the strata having suffered some 

 folding since. The east-and-west fractures are apparently vertical. 



The development of the three fractures is probably the combined 

 result of the general folding that took place along the Colorado Range and 

 of the presence of a mass of eruptive material seeking an outlet at the 

 surface through the channel of least resistance. The north-and-south 

 fracture, occupied by the main dike, probably antedates the others and was 

 primarily of slight displacement, the more extensive throw of its southern 

 half being due to a local enlargement of the eruptive body. The east-and- 

 wesl displacements were concomitant features. The presence of the locally 

 enlarged mass of eruptive material, the increasing pressure as the folding- 

 advanced, the yielding nature of the overlying (days, which also dipped 

 toward the east, all united to at last compel a rupture and dislocation of the 

 overlying beds in the location and directions platted. The resistance to 

 the strain developed by this condition of affairs is well shown at several 

 points in the disturbed area, but especially along the northern fault line, 

 where there is abundant evidence in the normally flexed ends of the 

 opposing beds, of distortion and bending before the strata finally yielded 

 to the force of compression brought to bear upon them. 



