ENDLICH ON EEOSION IS COLORADO. 855 



determining the quantity of erosion as the existence of these dike-walls. 

 It may here incidentally be mentioned, that not unfrequently the casts, 

 of edges of strata may still be found upon the sides of such walls, and 

 they certainly furnish an applicable indication as to what relative height 

 the sedimentary beds must at one time have extended. 



AECHITECTUEAL FORMS. 



As iu the preceding class, so here, too, we have essentially such forms 

 which are produced directly by erosive action and such that are merely 

 made more prominent thereby. In case of stratigraphical disturbances, 

 hard strata may acquire positions which render them of great importance 

 in the landscape. By the removal of certain portions, displaying more 

 striking features, perhaps, than otherwise would have appeared, erosion 

 certainly does its share toward increasing their characteristics. It is 

 necessary only to quote Cathedral Rocks near Monument Park as an 

 instance of this kind. There the strata stand on edge, rising in vertical 

 columns for more than 400 feet. Erosive action has modified and deter- 

 mined detail-features, but its effect had nothing to do with the present 

 position of the rocks. In discussing architectural forms, we can appro- 

 priately distinguish two groups : such representing either complete or 

 ruined structures, and such simulating architectural ornamentation. 

 Both of these are well developed in Colorado, more particularly the latter. 

 At numerous localities are they found, and the number of varieties they 

 present is very great. 



A. — First Group. 



WHITE RIVER REGION. 



In this region it is again the Upper Green River sandstone that enters 

 into consideration. The formation of eroded walls has been discussed 

 above. Architectural forms are but a series of walls in this instance. 

 Mainly the prevalence of joint-fissures and undermining by fluviatile 

 erosion caused the occurrence of the remarkable forms here observed. 

 On the north side of the river, the bluffs rise to a relative elevation of 

 more than 2,000 feet. For a considerable distance, the highest portions 

 of these hills are covered by products of erosion closely resembling ruins 

 of houses and castles. Erosion here has been carried on on a grand scale. 

 Enormous masses of sandstones have become detached by undermining 

 and frost, and have rolled down far below their original positions. Ver- 

 tical faces, often regular as though cut by hand, mark the places whence 

 these masses came. For the purpose of indicating the effect produced 

 by these curious conditions I quote from a letter: — " On the north side 

 of the river a perpendicular wall rose to the height of 500 feet, and 

 innumerable walls and turret-shaped rocks ornamented the steeply slop- 

 ing summit. Seen thus by the slanting rays of a setting sun, the effect 

 was that of a ruined city. A mighty citadel occupied the highest point, 



