STRUCTURE OF THE VALLEY DEPOSITS 275 



wells concerning which information can be had, the subjacent beds com- 

 monly made up of fragmental materials in general as appears at the sur- 

 face. The pebbles in these beds are usually not much water -worn, except 

 where they have evidently been transported from a considerable dis- 

 tance by the mountain torrents and accumulated on their detrital areas. 

 In such positions they have the character common to such pebbles. In 

 all the instances where I have been able to identify the source of these 

 fragments they have come from the rocks of the neighboring mountains. 

 Although the observations I have made are limited, they go to show that 

 the proportion of materials coarse enough to be termed pebbly is lajge, 

 commonly more than three-fourths of the mass, and that these pebbles 

 are much decayed. 



As regards the size and distribution of the fragments, they are clearly 

 larger the nearer they are to the bases of the mountain walls. There is^ 

 indeed, a tolerably uniform decrease in their size as they lie farther away 

 from the highest part of the slopes ; yet it is not uncommon to find 

 fragments as much as a foot in diameter at a distance of 2 or 3 miles 

 from the source whence they came. The extent to which they are de- 

 cayed usually increases as they lie farther out toward the center of the 

 valley. Where the rocks are granitic and of a rather perishable char- 

 acter, as about the Silver Bow river, near Butte, Montana, the coarse 

 element in the deposit does not commonly extend for any considerable 

 distance from the foot of the mountains. In these conditions the ma- 

 terial of the slopes ordinarily consists of the disintegrated rock of which 

 the crystals are sufficiently well preserved to warrant the term arkose 

 being applied to the mass. The formation of these peculiar beds is 

 greatly favored, if, indeed, it is not mainly brought about, b}^ certain 

 species of ants which abound in nearly all parts of the Cordilleras. 

 These creatures accumulate upon their hills quantities of the fragments 

 which have become loosened by the decay of the crystalline rocks. It is 

 not uncommon to find in one of their heaps as much as 2 or 3 cubic feet 

 of such debris. At several places, particularly in Arizona, I have noted, 

 over large areas, as many as 50 of these hills to the acre of ground, with 

 the result that in a single square mile there may be several hundred cubic 

 yards of material ready to be swept away by a torrential rain, to be laid 

 doAvn in the lower parts of the valley. 



It may be incidentally noted that one of these species of ants which 

 particularly abounds in the southern part of the Cordilleras has another 

 habit which facilitates soil erosion both by rain and wind. This insect 

 completely removes the grass and other lowly vegetation over a circle 

 from 10 to 20 feet in diameter, in the center of which lies the nest or hill. 

 As the roots of the plants are extirpated the ground is deprived of its 



