298 GEOLOGY OF THE DENVER BASIN. 



basalt of surface flows, each more than 100 feet in thickness and but 1 or 

 2 miles from the vent. The smaller streams of Table Mountain seem to 

 represent the product of the first eruption of the magma, which was on a 

 much smaller scale than the latter ones. With this relationship in mind 

 the following descriptions will, to say the least, be of greater interest than 

 they otherwise would be. 



BASALT OF THE VALMONT DIKE. 



occurrence. — A description of the ] ihvsiographic features of the Valmont 

 dike has been given in Section 1 of this chapter, and it is here necessary 

 only to repeat a few particulars. This dike is situated 3 miles east of Boulder 

 and is about 2 miles long, with an average width of 20 to 4(1 feet. The 

 rock is clearly shown in vertical position as it cuts the horizontal, friable, 

 sandy shales of the Fox Hills Cretaceous formation. 



Conditions attending the consolidation. ( )n tllC natural assumption that tile Vei'V 



great similarity between the Valmont and Table Mountain basalts implies 

 a practically contemporaneous origin of the two rocks, we must suppose 

 the Valmont dike to have been formed during the early part of the 

 Denver epoch. Through analogy we may infer that this dike represents a 

 channel through which lavas corresponding to the Table Mountain Hows 

 were poured out upon the surface of that time, though these particular 

 flows may more naturally be supposed to come from the nearer dikes on 

 Ralston Creek. Between the horizon in the Valmont dike represented by 

 the present outcrops and the land surface of the time of eruption there was 

 a space tilled by the upper strata of the Fox Hills, the entire thickness of 

 the Laramie, and probably the Arapahoe formation, with perhaps a part 

 of the Denver beds. Erosion has now entirely removed all but the first 

 formation, but we have no evidence to show that the other deposits did not 

 extend northward beyond the point in question. 



The rock of the Valmont dike which is to be described was probably 

 formed under the following conditions: The magma was in an eruptive 

 channel •_'() to 40 feet wide, at a distance from the surface of several 

 hundred, or perhaps 2,000, feet. Its walls were of loose, sandy shales. 

 When the magma became stationary the walls were presumably already 



