458 The American Naturalist. [May, 
causes to be built around it a bank of sand many feet high, often 
fifteen or twenty feet. The mesquite, and indeed almost all arid 
land-plants, have great root extension, by far the greater part of 
the plant being beneath ground, and the disintergrating offect of 
this peculiarity must be considered. 
Among animals there are at least three types which are doing 
much work of erosion. Formerly the bison, and now their 
successors,—cattle,—have done an appreciable geological work. 
In the formation of trails and the general tramping of the 
ground they have aided in protecting the soil against the action 
of wind and water. Recently in southern New Mexico, on the 
Pecos, I have had my attention forcibly called to the importance 
of cattle in this'direction. Several small streams had their course 
considerably lengthened by the puddling of the stream-bed by 
cattle. 
Ants are very abundant on the plateau, and they are continually 
at work tunnelling the soil and bringing fresh earth to the sur- 
face. On the upper Rio Grand, near Embudo, there is a clayey 
gravel containing many small garnets. The ant-hills in the 
vicinity are composed chiefly of garnets. The clay is apparently 
washed out, and the coarser particles have remained. Ant-hills 
are frequently washed away during a rain, but they are quickly 
rebuilt. A peculiar ant, the “agricultural ant,” so called, is com- 
mon in the south-west. These creatures have a clear space 
around their homes, thus exposing the bare ground to the action 
of the elements. 
The work done by prairie dogs, while in individual cases ap- 
parently insignificant, must amount to a grand sum total. Over 
large tracts their burrows open to the air at intervals averaging 
not more than twenty-five feet apart. I have no means of telling 
how far they extend into the ground, but from the appearance of 
their mounds I should judge it to be only a few feet. This 
mound rarely exceeds a height of two feet, and generally less. 
Being in the form of a truncated cone, with a very gradual slope ; 
they are sometimes three feet in diameter. Their burrow is sel- 
dom more than six inches wide, so that its extreme length can- 
not be many feet. That the creatures are constantly at work is 
