the particles this way or
that, or, as we say, change their place, and those which merely
produce vibration, in which the particles, after their vibratory
movement, return to their original place. For purposes of illustration
the first, or translatory motion, may be compared to that which takes
place when a bell is carried along upon a locomotive or a ship; and
the second, or vibratory movement, to what takes place when the bell
is by a blow made to ring. It is with these ringing movements, as we
may term them, that we find ourselves concerned when we undertake the
study of earthquakes.

It is desirable that the reader should preface his study of
earthquakes by noting the great and, at the same time, variable
elasticity of rocks. In the extreme form this elasticity is very well
shown when a toy marble, which is made of a close-textured rock, such
as that from which it derives its name, is thrown upon a pavement
composed of like dense material. Experiment will show that the little
sphere can often be made to bounce to the height of twenty feet
without breaking. If, then, with the same energy the marble is thrown
upon a brick floor, the rebound will be very much diminished. It is
well to consider what happens to produce the rebound. When the sphere
strikes the floor it changes its shape, becoming shorter in the axis
at right angles to the point which was struck, and at the same instant
expanded along the equator of that axis. The flattening remains for
only a small fraction of a second; the sphere vibrates so that it
stretches along the line on which it previously shortened, and, as
this movement takes place with great swiftness, it may be said to
propel itself away from the floor. At the same time a similar movement
goes on in the rock of the floor, and, where the rate of vibration is
the same, the two kicks are coincident, and so the sphere is impelled
violently away from the point of contact. Where the marble comes in
contact with brick, in part because of the lesser elasticity of that
material, due to its rather porous structure, and partly because it
does not vibrate at the same rate as the marble, the expelling blow is
much less strong.

All rocks whatever, even those which appear as incoherent sands, are
more or less set into vibratory motion whenever they are struck by a
blow. In the crust of the earth various accidents occur which may
produce that sudden motion which we term a blow. When we have examined
into