At almost any rock outcrop the result of {116} the breaking-up process
may be seen; the outer portion is softer, more easily broken, and of
different color from the fresh rock, as shown by breaking open a large
piece. The wearing away of the land surface is well shown in rain
gullies, and the carrying along and depositing of sand and gravel may
be seen in almost any stream. In the Northern states and Canada, which
at one time were covered by a great sheet of ice, moving southward and
grinding off the surface over which it passed, most of the rock
outcrops are smoothly rounded and many show scratches made by pebbles
dragged along by the ice. The hills too have {117} smoother and rounder
outlines, as compared with those farther south where the land has been
carved only by rain and streams. Along the coast the wearing away of
the land by waves is shown at cliffs, found where the coast is high,
and by the abundant pebbles on the beaches, which are built of
material torn from the land by the waves. Sand bars and tidal flats
show the deposition of material brought by streams and spread out by
currents. Sand dunes and barrens illustrate the carrying and spreading
out of fine material by the wind.

[Illustration: Slab containing fossil shells]

[Illustration: Conglomerate or pudding-stone]

In many regions the beds of sedimentary rocks, which must have been
nearly horizontal when formed, are now found sloping at various angles
or standing on edge, the result of slow deforming of these beds at an
earlier time. As some beds are more easily worn away than others, the
hills and valleys in such regions owe their form and position largely
to the different extent to which the harder and softer beds have been
worn down by weather and by streams. The irregular line of many coasts
is likewise due to the different hardness of the rocks along the
shore.

It is by the study of the rocks and of the remains of life found in
them, by observing the way in which the surface of the earth is being
changed and examining the results of those changes and by concluding
that similar results were produced in former times in the same way,
that geologists are able to read much of the past history of the
earth, uncounted years before there were men upon it.

Plants, Ferns, and Grasses

_By Dr. L. C. Corbett, Horticulturist,
United States Bureau of Plant Industry_

The appearance of the blossoms and fruits of the fields and forests in