ENDLICH ON EROSION IN COLORADO. 861 



What lias been said of the formation of caves within the conglomer- 

 ates applies perfectly to the arches. They are, in reality, nothing but 

 caves which extend entirely through the walls. It is probable that both 

 sides were simultaneously attacked. This would account in a measure 

 for the regularity of outline. Whichever portion of the wall was 

 exposed to the " weather side " was cut away more rapidly than the other. 



In connection with this subject it maybe stated that not unfrequently 

 compact trachytes contain inclusions, of greater or less extent, composed 

 of soft " ashy " material. These, in the course of time, will weather out, 

 forming caves and sometimes arches of varying extent. Wherever we 

 observed occurrences of such character in Colorado, they were so situated 

 as to aftbrd ample opportunity for the removal and ultimate transporta- 

 tion of detritus, excepting a very few cases. These latter were such 

 where the fall of loosened material had evidently been a sudden one, and 

 of considerable quantity. 



On the eastern slope of the Sangre de Cristo Eangle, near Indian Pass, 

 a small "gateway" was observed occurring in red Carboniferous sand- 

 stone.* Standing perfectly isolated near the crest of a small ridge was 

 a block of sandstone about 10 to 12 feet high. Near the centre were two 

 openings; the upper one small, the lower one large enough to admit 

 the passage of a man. At first sight, this peculiar position for an arch 

 seemed rather inexplicable. Upon examination, however, it was found 

 that five different strata composed the block. Counting from above, the 

 first, third, and fifth strata were hard, the second and fourth soft. A 

 vertical crack runs through the entire distance of the block. Water 

 collecting in the crack saturated the soft strata and eventually accom- 

 plished their disintegration. Aided by frost and sand-blasts, the 

 crumbling sandstone soon fell away, producing the openings we 

 observed. This instance is one where only atmospheric agents could 

 reasonably be supposed to have exerted any influence. Although this 

 is an unusually clear case, it points out a method whereby excavations 

 of some extent may be produced at places where they can by no me'ans 

 be so readily explained. 



At various localities along.the Front Range, arches occur in the sand- 

 stones. Besides the causes above mentioned as facilitating the formation 

 of such i)roducts of erosion, still another may be mentioned. In case 

 flowing water undermines a certain stratum or series of strata, and 

 continues this process for a long time without the overhanging portions 

 caving in, the entire width of the dividiug portion may be cut. Thus a 

 subterranean passage of varying dimensions will be formed. This form 

 of arch is usually designated as a " tunnel ", limiting the former term to 

 such occurrences where but a narrow wall of rock is perforated. The 

 ingenuity of man has likened these products to various other objects 

 dependent upon their form and dimensions. Many of them are compared 

 directly to the works of man. In Europe, wide and deep arches are 

 * Compare Annual Report for 1875, Plate XX. 



