trainer, 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
there. The rain drops have now gotten back to the beach where they
take up again the work of grinding the soil.

The work of moving water can be seen in almost any road or cultivated
field during or just after a rain, and particularly on the hillsides,
where often the soil is loosened and carried from higher to lower
parts, making barren sand and clay banks of fertile hillsides and
destroying the fertility of the bottom lands below.

We have already noticed the work of freezing water in splitting small
and large fragments from the rocks. Water moving over the surface of
the earth in a solid form, or ice, was at an earlier period in the
history of the earth one of the most powerful agencies in soil
formation. Away up in Greenland and on the northern border of this
continent the temperature is so low that most if not all