quietly changing; but our lives are so short, and the history of
even the oldest cities is so brief in comparison with the rate at
which most of the changes take place, that we as a rule are aware
of only the uncommon and sudden ones.

The occurrence of earthquakes establishes the unmistakable fact
that there are forces at work from within disturbing the surface,
while land-slides, and even little gullies washed out by the rain,
show that other forces are working from without.

The vibration or trembling of the earth which we call an "earthquake"
always arouses alarm, and frequently occasions great destruction
and loss of life. Only a few of the various causes that may bring
about earthquakes are as yet fully understood. Earthquakes are
very interesting, however, because they are often associated with
the birth and growth of lofty mountain ranges.

Volcanic eruptions, hot springs, and the high temperature which
exists toward the bottom of deep mines show us that the interior
of the earth is very hot. It is thought that at one time the whole
earth glowed with heat, but as ages passed it became cold upon
the outside and a solid crust was formed.

Every one has observed that fruit becomes wrinkled as the pulp
within dries and contracts. The materials of the earth occupy more
space when they are hot than when cold, and as the interior portion
is still cooling, the outer layer or crust continues to shrink down
upon it, forming folds or wrinkles, as in the case of the skin of
an apple.

There is probably no portion of the surface that is fixed in its
present position. The land is either rising or sinking continually.
If the area that is pushed upward is large, it becomes a plateau;
but if long and narrow like a wrinkle, it forms a mountain range.
We should not be aware of these movements in many cases if it were
not for the horizontal shelf cut upon the borders of the land by the
ocean waves. Along some coasts old wave-cut cliffs stand hundreds
of feet above the present ocean level. Other coasts have sunk,
so that the water has flooded the adjoining land and made a new
shore line.

When the movements of the land are sudden, they manifest themselves
to us through earthquakes. The crust of the earth is not so flexible
as the skin of an apple, and when the strain upon it becomes too
great it suddenly breaks. The rock walls usually slide past one
another along such a fracture. If the rising wall becomes high
enough it