THE GENERAL PLAINS STRUCTURE. 145 



there are the usual considerations regarding the depth of the strata, bearing 

 upon the economic questions involved. 



The Ralston dike and immediate vicinity have been described in 



connection with the foothills, and will not be discussed further. 



The Arapahoe-Denver nonconformity, like that between the Laramie 

 and Arapahoe, is one of erosion. It is conspicuously shown in the many 

 gulches entering the Platte Valley from the west, north of Clear ('reek. 

 Over this region the Denver either forms the merest coating upon the 

 Arapahoe or occurs in isolated outcrops which lie at various heights upon 

 the gentle slopes formed of the older formation. The northern limit of 

 the main body of the Denver formation lies to the south of the line of the 

 present section and has an irregular trend in a west-southwest direction. 



SECTIONS III. IV, ami \ . 



To more clearly understand the geological structure of the area of 

 Laramie strata within the map limits east of Denver and the relations that 

 exist between this formation and the several of younger age surrounding it, 

 it is necessary to consider jointly the eastern portions of Sections III, IV, 

 and V. 



The Laramie here, as elsewhere, suffered considerable erosion prior to 

 the deposition upon it of the younger sediments; this is evident at many 

 points on the periphery of its outcrop, in the wavy line of union between 

 it and the younger formations. Moreover, the area of exposed Laramie is 

 part of the locus of an early hill of which the northern, western, and 

 southern slopes are clearly shown, first, by the occurrence of the Denver and 

 Arapahoe formations on these sides, practically at the same level with the 

 Laramie, the edges of the overlying formations being Leveled upon that 

 beneath; secondly, by the perceptible increase in depth at which the lower 

 beds of the adjacent formations are found in bored wells as distance is 

 gained from the periphery of the Laramie exposure; and thirdly, by the 

 fact that to the east the Monument ( 'reek beds, at a still higher topographic 

 level than any of the other formations, overlie the Laramie, a wavy line of 

 separation also occurring between the two. 



Within the area, of Laramie its clays ami associated ironstones are 

 frequently encountered in creek bluffs, wells, and borings, and south "1' a 



MON XXVII 10 



