DKXVEi; OF THE PLAINS. 181 



in both Denver and Arapahoe epochs. Fine sediments wereoften disturbed 

 and locally removed at the beginning of periods of rapid deposition of coarser 

 materials. Such alternations of sediments were especially mentioned while 

 discussing the exposures of South Table Mountain 



While the sediments of the plains are usually finer grained than those 

 lit' the western border, and hence are distinguishable with less readiness, 

 there is another element at first contributing to the difficulty in the appear- 

 ance of quartz and feldspathic grains plainly not of eruptive origin. This 

 feature will be discussed more fully in a following section, and it is only 

 necessary here to mention the conclusion reached that such materials came 

 from the Arapahoe shores on the north and south. 



Hints from topography. — While the area of the map east of the line of Green 

 and Table mountains is essentially plain, there are many diversities found 

 to lieai- more or less direct relation to the underlying solid rock formation 

 in spite of the numerous Pleistocene divisions which play the chief roles. 

 A thorough acquaintance with the region immediately underlain by the 

 Denver beds shows several peculiarities to be named. 



The strata have been described as prevailingly loose and friable, vet 

 they resist erosion in a way of their own. A clay matrix for many beds 

 causes the disintegration to proceed slowly and only on the very surface. 

 Water may remove the outside layer easily, but the wet clay below holds 

 the loose grains together tor some little time. In many strata, too, a still 

 more powerful agentexists in the secondary cementing substance of zeolitic 

 character which has formed sometimes very abundantly. The dark, friable 

 sandstone exposed in a surface quarry on the south side of Bear Creek 

 contains 54.59 per cent of substance soluble in hydrochloric acid, and a 

 fine-grained, brownish bed shown in the ravine near the old St. Luke's 

 Hospital, Highlands, was found to contain 3s. 22 per cent of soluble matter, 

 a large amount of gelatinous silica being formed in each case. 



The result of this resistance to erosion is to produce rounded bluffs on 

 all considerable streams, as Bear, (dear. Van Bibber, and Coal creeks, and 

 along the Platte. Such lines of bluffs generally represent practically con- 

 tinuous outcrops, while solid rock appears only in gullies or on actual 

 stream banks. The Arapahoe and Laramie do not produce such forms, 



