WEATHERING 115 
up one of the lichens, we find it firmly attached to the 
stone, which as far as we can see is solid. 
When we succeed in tearing off the lichen, we bring 
with it tiny fragments of the rock; and if we look 
where the plant grew, we shall see numerous root-like 
extensions entering minute crevices in the rock. As 
the plant grows, these “roots” increase in size, and as 
they do so, they 
pry off tiny frag- 
ments. One may 
see this same _ pro- 
cess upon nearly 
every ledge or 
bowlder, no matter 
of what kind. It 
may be that on 
some ledge we find 
a bush, or even 
a tree (Fig. 50), | 
thrusting its roots along the cracks; and then, on a 
Fic. 49. 
Rock with lichens upon its surface. 
larger scale, we are enabled to see the action of grow- 
ing plants in prying rocks apart. 
Myriads of roots of trees, shrubs, and grass, are 
engaged in this same task of pulverizing the rocky 
particles that form the soil. These roots may reach 
down to the solid rock, and entering this, help to break 
it up. When a tree is blown over (Fig. 51), as often 
