398 



THE POPULAR SCIENCE MONTHLY. 



the sea ; not by an extravagant and violent use of power, but by tlie 

 slow agencies which may be observed generally throughout the world, 

 still acting in the same slow, patient manner. 



There are yet some interesting facts to be observed concerning 

 these inter-hog-back valleys. Their floors are usually lower than the 

 general surface farther away from the mountains. There seem to be 

 two causes for this : The great fold having been lifted and truncated 

 prior to the exj^osure of the rocks farther away from the mountains, 

 its strata present their edges, instead of their upper surfaces, to the 

 down-falling rain, and the softer beds are not so well shielded by the 

 harder. Erosion hence progresses more rapidly than where the beds 

 are approximately horizontal. 



Again, the mountains, with peaks among the clouds, condense 

 their moisture, and a greater quantity of rain falls on them, or in 

 their vicinity. The region of country adjacent to the mountains re- 



FiG. 8. MoNOCLixAL Vallet. 



ceives a portion of this extra rainfall, so that this dynamic agency in- 

 creases from the plains to the summits of the mountains, probably in 

 some direct ratio. This increase of the eroding agency, and the greater 

 exposure of the soft beds, probably account for the fact that the low- 

 est country is at the foot of the mountains. 



There is a limit to the effect of these conditions, for it should be 

 observed that no valley can be eroded below the level of the jirincipal 

 stream, which carries away the products of its surface degradation ; 

 and where the floor of such a valley has been cut dowm nearly to the 

 level of such a stream, it receives the debris of the adjacent cliffs and 

 mountains, and in this way the rocks composing the floor are usually 

 masked, to a greater or lesser extent. The same tojjographic facts 

 under like conditions, are found on the eastern slope of the Rocky 

 Mountains, in Colorado Territory, and tlie valleys which run into the 

 South Platte from the south, between the hog-backs, are lower than 



