ENDLICH ON EROSION IN COLORADO. 857 



admirably for imitating forms that can readily be compared to those of 

 Gothic architecture. The form of the spires is similar, and the numerous 

 inclosed bowlders of varying size produce effects comparable with the 

 ornamentation of Gothic structures. At a number of points, such 

 conditions were noticed, often producing singularly beautiful pictures. 

 Perhaps one of the most striking views may be obtained from the sum- 

 mit of Uncompahgre Peak (14,235 feet above sea-level). Looking down 

 from there upon a vast mass of rugged mountains, we find that to the 

 north and west the trachytic conglomerates occupy a definite horizon. 

 Thousands of spires are clustered along the sides of mountains, rivalling, 

 as it were, the densely studded spires of that gem of Gothic archi- 

 tecture, the cathredal of Milan. Situated as they are, they stand out 

 in bold relief when viewed from below, but seen from above they pro- 

 duce a profound impression by their great numbers. 



Primarily erosion by flowing water cut deep, narrow channels into the 

 yielding material, forming sharp ridges, which soon were separated into 

 detached portions. Subsequent erosion, every agent available being 

 employed, wrought the curious and rare forms we now observe. Ee- 

 raoval of the harder beds overlying the conglomerates affords free access 

 to water, and though many of the spires and towers may disappear in a 

 comparatively short space of time, the supply of fresh material is prac- 

 tically inexhaustible. 



Other products of erosion might appropriately be placed into this 

 group. Differences of density in rocks, more particularly parallel to 

 the planes of bedding, will cause fluviatile as well as pluvial erosion to 

 carve them into unique forms. Shelved and scolloped edges of i)lateaus 

 and bluffs, segregation into regular and highly ornamented columns, and 

 minute decorations thus produced, might well be employed as models 

 for the Land of the artisan. 



CAYES. 



Caves that owe their formation to erosion may be formed in two dif- 

 ferent ways. They may be due to either chemical or mechanical action. 

 By means of decomposition and by subsequent removal of the material, 

 either mechanically or in solution, the first effect is accomplished. Many 

 of the smaller caves in limestone, for instance, were formed by a solu- 

 tion of the carbonate of lime in water charged with carbonic acid gas. 

 The second group, the one which we shall here discuss, is formed by 

 erosive agents, which are usually recognized specifically as such. As 

 the initiatory step toward the formation of a cave, or as the most primi- 

 tive form thereof, we may regard the results produced by fluviatile ac- 

 tion in undermining certain portions of rocks or strata. Dependent upon 

 the local force of the water and the cohesive qualities of the overhang- 

 ing material, " shelves " of considerable extent may frequently be pro- 

 duced. In tough shales, such as are found in someof the Tertiary groups, 

 we may often find excavations of this kind of appreciable size. Sand- 



