144 GEOLOGY OF THE DENVER BASIN. 



entering Dry Creek from the west, where, at a contour considerably below 

 that marking the base of the Arapahoe on either side of the creek, there is 

 an isolated remnant of the gritty sandstone belonging in the lower portion 

 of the later formation. 



The difference in level between the lower limit of the Arapahoe in the 

 outlier to the west of Dry Creek and the main body of the formation to 

 ihe east is attributable in part to difference in level of the lake floor and in 

 part to the general southeasterly dip of 1° to 3°, which the strata west 

 of T lie creek have. 



The base of the Arapahoe formation in the region east of the Platte 

 River probably nowhere attains a depth of over 150 feet. West of the 

 Platte, however, and particularly as distance south of the Davidson section 

 increases, there is a constant and rapid gain in thickness to the maximum. 



The depth attained by the base of the Laramie along the line of 

 Section II is estimated at a minimum of between 900 and 1,000 feet — a 

 slight increase over that of Section I. This is based upon a boring on the 

 line of tin- section in the bluffs on the west side of the Platte directly west 

 of Brighton; here a depth of a little over 600 feet was reached, in which 

 nothing but clays and ironstones of the upper Laramie were encountered. 

 No water was struck and nothing having the appearance of the basal sand- 

 stones dt' the Laramie was observed. This, with the same allowances as were 

 made for Section I, would place the base of the formation in this locality 

 at a, depth of at least 970 feet. Another hole, about 5 miles south of 

 Section II, at a depth of 1,200 feet, showed clays, with a few interbedded 

 sandstones and several narrow seams of coal at about 800 feet; the records 

 do not definitely indicate, however, whether or not the base of the Laramie 

 was jmssed. 



THE EALSTON SECTION. 



The important structural features are the Ralston dike and the 

 accompanying disturbed conditions of the strata near the western end of 

 the section; the unconformity between the Arapahoe and Denver forma- 

 tions, especially well shown in the hills west of the Platte; and the 

 occurrence of the large area of Laramie in the eastern part of the field 

 and the relations of this formation to those surrounding it. Besides these, 



