ENDLICH ON EROSION IN COLORADO. 853 



one hundred feet thick, can be achieved. Wherever they were found, 

 they were observed to occupy prominent points, mostly on short, nar- 

 row ridges with very steep slopes. Series of what appear to be " walls" 

 are formed of the same sandstone, and will be discussed under architect- 

 ural forms. 



Gradual denudation, the widening of fissures and seams, in the course 

 of time breaks up the wall, and isolated columns are left to mark its 

 former course and extent. Nowhere were the walls observed to have 

 been formed to such perfection within Colorado as in the region of the 

 White Eiver. Undoubtedly the sandstones there are unusually well 

 adapted to illustrate the various results of erosion. Their peculiar com- 

 position and the position they occupy have alike been favorable to sub- 

 ject them to the most intense and varied erosive action. During the 

 first visit to this locality, the impression made upon the explorer is a 

 very lasting one. On all sides the most curiously wrought and some- 

 times almost mystifying forms and figures beset the traveller. Day after 

 day he may ride along the hills, and at every turn a surprise is awaiting 

 him. Though that which may be seen here of such objects is not by 

 any means unique, the enormous variety and the rich stores from which 

 to select cannot but elicit admiration. Other i)roducts of erosion, too, 

 are plentifully represented, some of which will be alluded to below. 

 Erosion on a grand scale may be favorably studied in this region, and 

 the evidences of the large masses that formerly have existed there create 

 a profound feeling of surprise regarding the vast power that must have 

 been utilized in transporting them. 



B. — Second Group. 



Dikes. 



In quoting dikes as "products of erosion", it becomes necessary to 

 define the basis upon which this is done. Dikes, strictly speaking, are 

 certainly not products of erosion. They are essentially the casts of 

 moulds formed by sedimentary or other rocks. Injected into these 

 moulds — fissures in this instance — they either remain hidden from sight 

 at first, or the injected material flows over and forms hills of greater or 

 less extent. It is by the means of erosion, however, that dikes, resem- 

 bling walls in all their essential exterior features, are brought to light, 

 and become natural walls. Until this is accomplished, they remain for- 

 eign matter placed into most intimate relations with the general country- 

 rock. Owing to the physical character of this rock, the dikes may 

 either remain hidden, or they may eventually acquire positions entirely 

 isolating them for a certain distance. In this case, they appear as 

 mural forms, and enter into consideration in connection with erosive 

 13roducts. They occur very numerously, and apart from their relations 

 to erosion are subjects of absorbing interest. 



