the origin of these impulses, and the way in which they are
transmitted through the rocks, we obtain a basis for understanding
earthquake shocks. The commonest cause of the jarrings in the earth is
found in the formation of fractures, known as faults. If the reader
has ever been upon a frozen lake at a time when the weather was
growing colder, and the ice, therefore, was shrinking, he may have
noted the rending sound and the slight vibration which comes with the
formation of a crack traversing the sheet of ice. At such a time he
feels a movement which is an earthquake, and which represents the
simpler form of those tremors arising from the sudden rupture of fault
planes. If he has a mind to make the experiment, he may hang a bullet
by a thread from a small frame which rests upon the ice, and note that
as the vibration occurs the little pendulum sways to and fro, thus
indicating the oscillations of the ice. The same instrument will move
in an identical manner when affected by a quaking in the rocks.

Where the rocks are set in vibration by a rent which is formed in
them, the phenomena are more complicated, and often on a vastly larger
scale than in the simple conditions afforded by a sheet of ice. The
rocks on either side of the rupture generally slide over each other,
and the opposing masses are rent in their friction upon one another;
the result is, not only the first jar formed by the initial fracture,
but a great many successive movements from the other breakages which
occur. Again, in the deeper parts of the crust, the fault fissures are
often at the moment of their formation filled by a violent inrush of
liquid rock. This, as it swiftly moves along, tears away masses from
the walls, and when it strikes the end of the opening delivers a blow
which may be of great violence. The nature of this stroke may be
judged by the familiar instance where the relatively slow-flowing
stream from a hydrant pipe is suddenly choked by closing the stopcock.
Unless the plumber provides a cushion of air to diminish the energy of
the blow, it is often strong enough to shake the house. Again, when
steam or other gases are by a sudden diminution of pressure enabled to
expand, they may deliver a blow which is exactly like that caused by
the explosion of gunpowder, which, even when it rushes against the
soft cushion of the air, may cause a jarring that may be felt as well
as heard to a great distance. Such movements very frequently occur in