280 GEOLOGY OF THE DENVEB BASIN. 



Till': YAl.MONT DIKE. 



Thi'ee miles east of Boulder, on the smith hank of Boulder ('nek, 

 just above the little town of Valraont, there begins a sharp ridge which 

 runs due cast, almost continuously, for 2 miles. 'This ridge is caused by a 

 \ ertical dike of doleritic basall cutting at this horizon the < Iretaceous shales 

 and class of the Fox Hills formation, 'The upper part of the ridge, toward 



its western end. is formed by a wall of basalt 20 to 40 feet wide which 



projects a few feet above the debris-covered slopes, beneath which lie 

 horizontal shales. Near the abrupl western end the dike reaches its greatest 



height, which is not more than 200 feet above the creek bed. For more 



than 1 mile its course is nearly straight, with an undulating, gradually 



descending crest. The eastern extension is represented by a succession of 

 knolls and small cones lying somewhat irregularly ill the strike of the main 

 dike, with smooth spaces between them, indicating that the superficial 

 Continuity of the basalt is broken. 'These isolated basalt masses are less 

 and less prominent eastward, and the last observed outcrop, almost exactly 



2 miles from Valmont, is simply a low mound lying in a pasture on the 

 north side of the road. On the northern side the slopes of the ridge are 

 abrupt; on the south, toward the western end, a terrace abuts against it, 

 rising nearly to the level of the dike. 



The rock forming the dike is compact, dark-gray, macrocrystalline 

 basalt, properly designated a dolerite from its structure. It is fully described 



in the second section of this chapter. In the main part of the dike the 



rock is divided into quadrangular Mocks of varying size, by three systems 

 of lissures which run in horizontal and vertical directions, the vertical 

 fissures being respectively parallel and normal to the side walls of the 

 mass. The cracks parallel to the walls of the dike are nearest together, 

 and in places the structure is practically tabular. No transverse columnar 

 structure is de\ eloped. 



From the above facts we may derive support for the supposition that 



the present exposures of the Valmont dike represent portions of an 

 eruptive channel far below the horizon which constituted the surface at 

 the time of eruption, lor the structure found indicates that contraction 

 resulting from the cooling of the mass did not progress so predominantly 



