284 GEOLOGY OF THE DENVEB BASIN. 



having an easterly dip, while the sandy shales dip 75 to 90 westerly. 

 The contact surface is visible in several places, particularly on the west 

 side, ami the line is found to be quite irregular in detail. It runs higher up 

 on the west slope than on the east. The sandy shales in contact with the 

 eruptive rock are baked and hard, but no marked changes have been 

 produced in them. 



Eas1 and south of this cone, at distances of a few hundred feet, arc 

 two small masses of dike form. 



A nuich larger bodv than any of the foregoing is that whose southern 

 end is almost in the bed of the small arm of Van Bibber Creek, heading 



near Schooler's quarry, to the northwest and which extends northward in 



irregular dike form for 1,500 feet, with an average width of 150 feet. Few 



distincl contacts are visible about this mass, and its outline as drawn upon 

 the map i- fixed in accordance with the rule used for all of these bodies in 



Cretaceous shales, viz. vdiere the surface configuration i> determined by the 

 erosion of such soft material from about an undecomposed massive rock — 



as basalt- the contact lines must lie quite closely indicated by the actual 

 outcrops of the latter. 



The remaining basalt body of note here included is the largest of the 

 group. It lies directly east of the preceding, and tonus a low and rather 



rounded hill. Between the two i-. a grass) depression. From east to west 

 this mass measure- about 1,500 feet, and from north to south 2,000 feet. 



In all these masses the rock type is the same as in the Ralston dike, 



though a little more compact, especially in the smaller ones. Spherical 

 sundering is very well shown in the largest body. 



As already mentioned, it is rare that the contact surface is well shown 

 in any of these masses, hut it is perfectly (dear from their relation to one 

 another and to the known position of the strata that all represent eruptive 

 channels which are approximately vertical and that the eruption occurred 

 after the strata were folded about as at present. All consist of the type of 

 rock shown by the two sheets of Table Mountain, and it is natural to 

 consider the Ralston dike and the larger irregular masses near hv as the 

 channel through which the material of the surface Hows came up. The 



smaller bodies are undoubtedly offshoots from one or the other of the larger 



