and edges, throwing them up into sand beds, and carrying off the finer
particles to deposit elsewhere. Now visit a quiet cove or inlet and
see how the quiet water is laying down the fine particles, making a
clay bed. Notice also how the water plants along the border are
helping. They act as an immense strainer, collecting the suspended
particles from the water, and with them and their bodies building beds
of soil rich in organic matter or humus.

The sun, besides expanding and cracking the rocks by its heat, helps
in another way to make soils. It warms the water that has been
grinding soil on the beach or along the river banks and causes some of
it to evaporate. This vapor rises, forms a cloud and floats away in
the air. By and by the vapor forms into rain drops which may fall on
the top of some mountain. These rain drops may wash loosened particles
from the surface or crevices of exposed rocks. These drops are joined
by others until, by and by, they form a little stream which carries
its small burden of rock dust down the slope, now dropping some
particles, now taking up others. Other little streams join this one
until they form a brook which increases in size and power as it
descends the mountain side. As it grows by the addition of other
streams it picks up larger pieces, grinds them together, grinds at its
banks and loads itself with rocks, pebbles, sand and clay. As the
stream reaches the lower part of the mountain where the slope is less
steep, it is checked in its course and the larger stones and pebbles
are dropped while the sand and finer particles are carried on and
deposited on the bottom of some broad quiet river farther down, and
when the river overflows its banks, are distributed over the
neighboring meadows, giving them a new coating of soil and often
adding to their fertility. What a river does not leave along its
course it carries out to sea to help build the sand bars and mud flats