heat was produced, pieces of rock were pressed together
to form new rock masses, some portions becoming dissolved in water. Why,
I myself, almost feel the stress and strain of it all. Can you?

"Then, too, there were great changes in temperature. First everything
was heated to a high temperature, then gradually became cool. Just think
of the cracking, the crumbling, the upheavals, that such changes must
have caused! You know some of the effects in winter of sudden freezes
and thaws. But the little examples of bursting water pipes and broken
pitchers are as nothing to what was happening in the world during those
days. The water and the gases in the atmosphere helped along this
crumbling work.

"From all this action of rubbing, which action we call mechanical, it is
easy enough to understand how sand was formed. This represents one of
the great divisions of soil--sandy soil. The sea shores are great masses
of pure sand. If soil were nothing but broken rock masses then indeed it
would be very poor and unproductive. But the early forms of animal and
vegetable life decaying became a part of the rock mass and a better soil
resulted. So the soils we speak of as sandy soils have mixed with the
sand other matter, sometimes clay, sometimes vegetable matter or humus,
and often animal waste.

[Illustration: Constant Cultivation of the Soil Saved George's Cabbages

Photograph by Karl W. Helmer]

"Clay brings us right to another class of soils--clayey soils. It
happens that certain portions of rock masses became dissolved when water
trickled over them and heat was plenty and abundant. This dissolution
took place largely because there is in the air a certain gas called
carbon dioxide or carbonic acid gas. This gas attacks and changes
certain substances in rocks. Sometimes you see great rocks with portions
sticking up looking as if they had been eaten away. Carbonic acid did
this. It changed this eaten part into something else which we call
clay. A change like this is not mechanical but chemical. The difference
in the two kinds of change is just this: in the one case of sand, where
a mechanical change went on, you still have just what you started with,
save that the size of the mass is smaller. You started with a big rock,
and ended with little particles of sand. But you had no different kind
of rock in the end. Mechanical action might be illustrated with a piece
of lump sugar. Let the sugar represent a big mass of rock. Break up