ENDLICH ON EEOSION IN COLORADO. 843 



haps, lies the charm of attraction that the groups of South River i^osse&s, 

 besides that imparted to the*m by their wild surroundings. 



Mr. Ehoda has described the monuments from this region in the Annual 

 Eeport of 1875. He aptly expresses the feeling impressed upon the ob- 

 server of those enormous masses in the following words: — "These are 

 sentinels in more senses than one — sentinels guarding from profane eyes 

 the holy secrets of nature — for the stones which they bear upon their 

 shoulders, far over the traveler's head, carry a menace not to remain 

 unheeded." The seclusion of the spot and its location away from the 

 general route of travel or mining exploration have permitted this wonder- 

 ful occurrence on South River to remain hidden tljus far from the sight of 

 the white man. Indians, in former days, attracted probably more by the 

 presence of game and grass than by the beauty here exhibited, made 

 frequent visits to the valley, as their trails and old remains of camps 

 testify. To them the animate portion of this world appeals more directly 

 than the mute witnesses of nature's skillful power. 



Structurally^ the monuments of South River differ widely from those 

 heretofore described. In giving the definition of such as may be classi- 

 fied '^accidental", mention has indirectly been made thereof. The 

 material out of which the forms of this locality were carved is a heavy 

 deposit of trachy tic conglomerate. Its thickness may be regarded, at this 

 locality, as exceeding 600 feet. Almost every variety of conglomerate 

 is here represented. Taking it as a whole, it is composed of bowlders 

 of varying size, cemented by a mixture of sand and clay. Wherever, 

 during the process of deposition, these latter constituents have become 

 predominating, the rock assumes the character of a typical sandstone. 

 At such places, too, stratification may sometimes be observed. The 

 main mass of the conglomerate, however, shows no stratification, and 

 regular deposition of the bowlders is a very subordinate feature. Tra- 

 chytic material makes up the entire mass, clay, sand, and bowlders. It 

 is evident that ultimately the height or size of the monument must be 

 determined by the dimensions and weight of the cap-stone. We find! 

 single blocks sometimes weighing several tons. A secondary j)roduct, 

 acting as cement, may be noticed in the form of quartz, intimately 

 associated with argillitic matter. Were this to occur throughout large 

 masses of the conglomerate, then it would far more persistently repel 

 the action of erosive agents; but its appearance is very limited. In 

 color, the monuments and walls are brown, showing numerous shadings 

 into red, yellow, gray, and white. In part, such changes are due to the 

 physical constitution of the conglomerate. Wherever it more nearly 

 resembles sandstone, the shades become lighter. An admixture of mag- 

 netite, which is contained in the trachytes, upon decomposition produces 

 bright red or brownish-red colors. Owing to the character of the mass 

 containing it, however, this mineral cannot be decomposed, excepting at 

 such i)laces where the rock is compf^ratively protected from erosion. At 

 other points, the removal of material progresses so rapidly that not 

 Bull. iv. Ko. 4 7 



