rocks in their path.

Wind blows finer rock fragments along, and they lodge in cracks. Fine
dust and the seeds of plants are lodged there. The rocky slopes of the
Yosemite Valley are all more or less covered with trees and shrubs that
have come from wind-sown seeds. These plants thrust their roots deeper
each year into the rock crevices. The feeding tips of roots secrete
acids that eat away lime and other substances that occur in rocks. Dead
leaves and other discarded portions of the trees rot about their roots,
and form soil of increasing depth. The largest trees grow on the rocky
soil deposited at the base of the slope. The tree's roots prevent the
river from carrying it off.

When granite crumbles, its different mineral elements are separated.
Clear, glassy particles of quartz we call sand. Dark particles of
feldspar become clay, and may harden into slate. Sand may become
sandstone. Exposed slate and sandstone are crumbled by exposure to wind
and frost and moving water, and are deposited again as sand-bars and
beds of clay.

The most interesting phase of soil study is the discovery of what a work
the humble earthworm does in mellowing and enriching the soil.

THE WORK OF EARTHWORMS

The farmer and the gardener should expect very poor crops if they
planted seed without first ploughing or spading the soil. Next, its fine
particles must be separated by the breaking of the hard clods. A wise
man ploughs heavy soil in the fall. It is caked into great clods which
crumble before planting time. The water in the clods freezes in winter.
The expansion due to freezing makes this soil water a force that
separates the fine particles. So the frost works for the farmer.

Just under the surface of the soil lives a host of workers which are our
patient friends. They work for their living, and are perhaps unconscious
of the fact that they are constantly increasing the fertility of the
soil. They are the earthworms, also called fishworms, which are
distributed all over the world. They are not generally known to farmers
and gardeners as friendly, useful creatures, and their services are
rarely noticed. We see robins pulling them out of the ground, and we are
likely to think the birds are ridding us of a garden pest. What we need
is to use our eyes, and to read the wonderful discoveries recorded in a
book called "Vegetable Mould and Earthworms," written by Charles
Darwin.

The benefits of ploughing and spading are the l