
Parr IL. Sucr. iii. §1.] ACTION OF ANIMALS. 455 
_ up to the surface, the fine particles are exposed to meteoric influences ; 
_ notably to wind and rain. Even a grass-covered surface may, from 
this cause, suffer a slow denudation. 
Burrowing animals, by throwing up the soil and subsoil, expose 
these to be dried and blown away by the wind. At the same time 
their subterranean passages serve to drain off the superficial water 
and to injure the stability of the surface of the ground above them. In 
_ Britain the mole and rabbit are familiar examples. In North America 
the prairie dog and gopher have undermined extensive tracts of 
pasture land in the west. In Cape Colony wide areas of open 
country seem to be in a constant state of eruption from the burrow- 
ing operations of multitudes of Bathyergi and Chrysochloris—small 
-mole-like animals which bring up the soil and bury the grassy 
vegetation under it. The decomposition of animal remains gives 
rise to some of the same chemical changes as are produced by that 
of plants. ; 
oy The flow of streams is sometimes interfered with, or even 
diverted, by the operations of animals. Thus the beaver, by cutting 
down trees (sometimes one foot or more in diameter) and construct- 
_ ing dams with the stems and branches, checks the flow of water- 
- courses, intercepts floating materials, and sometimes even diverts 
the water into new channels. ‘This action is typically displayed in 
Canada and in the Rocky Mountain regions of the United States, 
Thousands of acres in many valleys have been converted into lakes, 
which, intercepting the sediment carried down by the streams, and 
being likewise invaded by marshy vegetation, have subsequently 
become morass and finally meadow-land. The extent to which, in 
these regions, the alluvial formations of valleys have been modified 
and extended by the operations of the beaver is almost incredible. 
The embankments of the Mississippi are sometimes weakened to such 
an extent by the burrowings of the cray-fish as to give way and 

Fig. 170.—SHELL-BORINGS IN LIMESTONE. 
allow the river to inundate the surrounding country. Similar results 
have happened in Europe from the subterranean operations of 
rats. . 
3. Some Mollusca (Pholas, Saxicava, Teredo, &c., Fig. 170) bore 
into stone or wood, and by the number of contiguous perforations 
greatly weaken the material. Pieces of drift-wood are soon riddled 
