116 ELEMENTARY GEOLOGY 
happens, it drags masses of soil and rock to the air, 
thus exposing them directly to the destructive action 
of the weather. This ever-acting work which plants 
are engaged in, is one of the most important of the 
| agents of rock de- 
struction. 
Action of Burrow- 
ing Animals. Ani- 
mals that bore into 
the ground are help- 
ing to pulverize the 
rock fragments. 
This is true of the 
prairie dog, the 
mole, and__ partic- 
ularly of the earth- 
worms, which live in 
such great numbers 
Fia. 50. 
in the soil. Perhaps 
Trees growing in a gorge near Ithaca, N. Y. 
These are extending their roots into the one of the most 
shale, and prying off fragments. ; 
important groups of 
animals, in this respect, is that of the ants. Cer- 
tainly this is so in some tropical countries, such as 
Brazil, where the earth is almost everywhere tun- 
nelled by these busy and interesting creatures. Thus 
both animals and plants are constantly engaged in 
this work of grinding the rock fragments. Not only 
