302 GEOLOGY OF TIIE DENVER BASIN. 



last substance to crystallize should be rich in potash. It seems probable 

 that the abundance of biotite in these basalts is also connected with this 

 higher percentage of potash. 



The augite is seen to have the normal composition for basaltic augite. 

 Titanic acid was unfortunately not tested tor, as its presence in the rock 

 was not ascertained until after the augite had been analyzed. As no 

 titanium minerals are visible in the rock, it is probable that the augite 

 contains the greater part of the amount of titanic acid found. 



BASALT OF THE RALSTON DIKE AND THE ADJACENT MASSES. 



occurrence. — Referring to the first section of this chapter for any details 

 not germane to the present discussion, the reader is requested to note the 

 form, number, and relative positions of these basaltic masses as shown 

 upon the map. The main dike runs north and south approximately on the 

 line of the Fox Hills and Pierre formations, which are here folded to nearly 

 vertical position. Adjacent to the south end of the dike are a number of 

 large and small masses with irregular rounded outlines, which appear at 

 various horizons of the Fox Hills. One mile to the southward is the 

 northern edge of Table Mountain, capped by a sheet of basalt whose 

 average elevation is about equal to that of the higher portions of the 

 Ralston dike. 



conditions of consolidation. — From the evidence already given in this chapter, 

 the age of the Table Mountain basalt sheets, is referred to the Denver 

 epoch, and they are supposed to have come from the vents indicated by 

 the Ralston dike and adjacent masses. But this assumption, together 

 with the age of eruption, requires the supposition that the relative horizons 

 of the present outcrops have changed materially since the eruption, in a 

 manner which has resulted in bringing the surface flows down to a level 

 with a portion of the eruptive channel that was once far below the surface. 

 The folding of the Arapahoe and Denver beds displayed to the southward 

 supplies the easy and natural explanation of this change of level, and also 

 explains the otherwise obscure relations of the basaltic masses to the 

 peculiar fault in the area north of Table Mountain. This connection of 

 the basall with the folding movement and displacement is here reviewed 



