earthquakes and the various movements of the earth's crust. As we
study each agency we shall notice not only how it does its work,
but also the records which it leaves in the rock structures and
the land forms which it produces. With this preparation we shall
be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.





PART I

EXTERNAL GEOLOGICAL AGENCIES

CHAPTER I

THE WORK OF THE WEATHER


In our excursion to the valley with sandstone ledges we witnessed
a process which is going forward in all lands. Everywhere the
rocks are crumbling away; their fragments are creeping down
hillsides to the stream ways and are carried by the streams to the
sea, where they are rebuilt into rocky layers. When again the
rocks are lifted to form land the process will begin anew; again
they will crumble and creep down slopes and be washed by streams
to the sea. Let us begin our study of this long cycle of change at
the point where rocks disintegrate and decay under the action of