850 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



s:on of textural irregularities within the masses, they, on the other hand, 

 greatly aided the rapid accomplishment of disintegration and transpor- 

 tation. After valleys, mostly narrow, had been cut into the yielding 

 rock, the space afforded to the water was sufBciently great to remove it 

 from the summits of hills and ridges. Thus the remnants Vv'e now find 

 there were preserved, surrounded by a talus formed from their own detri- 

 tus. Pluvial erosion and chemical changes within the rock itself wrought 

 many changes, lessening and modifying the remaining rock-masses. 

 Frost prepared the softer portions for removal, and sand-blasts carved, 

 most skilfully, the intricate forms we often observe. Sandstones can 

 be found in that region, as in others also, that show very remarkable 

 reticulation upon their surfaces. It is not so evident, at first sight, 

 whereby and why this curious effect of erosion is produced. This 

 species of reticulation manifests itself in a manner as if the material 

 composing the net were laid upon the surface of the rock. The meshes 

 are excavated proportionately to the size of the reticles, and often show 

 a remarkably regular arrangement. Such occurrences can be observed 

 both parallel with the stratification of the sandstones and at varying 

 angles to it. Primarily, this result may be derived from the existence 

 of argillitic inclusions within the sandstone. They are less able to re- 

 sist eroding influences, and by gradually disappearing from the exposed 

 surface may produce the effect of reticulation if somewhat regularly dis- 

 tributed. This, however, appears to be the less frequent mode of forma- 

 tion. It may be assumed that minute joints, now closed, traversed in 

 various directions the sandstones. Infiltration of water containing cer- 

 tain minerals, either in solution or in suspension, will tend to render those 

 portions immediately adjoining the joints harder, more compact. Com- 

 plete evidence is extant, proving that very many of the sandstones are 

 laminated as to texture, while structurally they may appear perfectly 

 homogeneous. Such lamination is one that can readily be detected by 

 testing the hardness at right angles to the stratification. We have, 

 then, the result: a block of sandstone traversed in various directions by 

 alternately soft and hard zones. Upon exposure, frost will rapidly take 

 advantage of this feature, and other erosive agents will soon remove the 

 more easily yielding portions, leaving the harder ones in the form of 

 reticulated hasreliefs. Within certain formations, more particularly the 

 Upper Cretaceous atid Lower Tertiary sandstones of Southern Colorado, 

 this occurrence may be regarded as characteristic. Erosion by sand-blast 

 is probably one of the most effective in producing the result described. 

 Dependent upon the amount of erosive influence to which the statu- 

 esque rocks are exposed will be the maintenance of their forms. It is 

 scarcely possible to give any general rule for the shape and continuity 

 of the harder, permeating portions, unless they reach the extreme form 

 of concretionary inclusions. Although these are by no means wanting 

 in the sandstones of the White Eiver region, the results we have above 

 mentioned are due to irregular changes of texture within the sandstones. 



