of the stream they slowly rasp and wear it deeper. The valley will
be deeper in the future; it has been less deep in the past.

In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the
rocks, and the land forms to which they have been sculptured, the
record of a long succession of past changes involving the origin
of sand grains and their gathering and deposit upon the bottom of
some ancient sea, the cementation of their layers into solid rock,
the uplift of the rocks to form a land surface, and, last of all,
the carving of a valley in the upland. Everywhere, in the fields,
along the river, among the mountains, by the seashore, and in the
desert, we may discover slow changes now in progress and the
record of similar changes in the past. Everywhere we may catch
glimpses of a process of gradual change, which stretches backward
into the past and forward into the future, by which the forms and
structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long
process is geology. Geology treats of the natural changes now
taking place upon the earth and within it, the agencies which
produce them, and the land forms and rock structures which result.
It studies the changes of the present in order to be able to read
the history of the earth's changes in the past.

The various agencies which have fashioned the face of the earth
may. be divided into two general classes. In Part I we shall
consider those which work upon the earth from without, such as the
weather, running water, glaciers, the wind, and the sea. In Part
II we shall treat of those agencies whose sources are within the
earth, and among whose manifestations are volcanoes and