gh the earth takes
place with wonderful rapidity. The elastic wave varies in velocity
from 800 to 1,000 feet per second in sand or clay to three miles per
second in solid granite.

"Sometimes these vibrations are of such a character as to be imparted
to the air, and their transmission through the air outstrips the
transmission through the earth and the ear detects the low rumbling
sounds before the shock is felt.

"If the origin of the shock is under the sea near the coast any
upheaval of the bottom of the ocean that frequently accompanies an
earthquake, gives rise to a great tidal wave that frequently inundates
the neighboring coast with much damage.

"While the phenomena of earthquakes and volcanoes are usually
associated in the same region, one cannot fairly be said to be the
cause of the other. Both are rather effects of a common cause, or
rather of common causes, the chief of which is the shrinking and
readjustment of the rocky strata within the earth. The suggestion that
there is some physical connection between the recent eruptions of
Vesuvius and the earthquake at San Francisco does not accord with the
generally accepted views of geologists concerning these phenomena.

"It is probably true that a critical condition of stress between two
gigantic and contending forces may be touched off, as it were, by any
feeble force originating at a distance. Thus a distant volcanic
eruption or earthquake shock may determine the climax of stress in a
given portion of the earth, which will produce an earthquake.
Observations show that more earthquakes occur near the full and the
new moon than at other times. This is probably due to the fact that at
these times the gravitation of the sun and moon are combined, and
their effect upon the earth is greater. We can see this effect in the
higher tides at new and full moon. But these forces, it will be seen,
are the occasions, and not the causes of earthquakes.

"The probable recurrence of the San Francisco earthquake is a matter
of great uncertainty. In general, whenever the internal stress of the
forces that give rise to earthquakes is relieved there is usually a
long period of quiescence in the strata of the earth, but in the
course of time, especially in regions of recent and rapid geological
changes, such as is the case on the Pacific coast, there is almost
certain to be recurrences of earthquake shocks from time to time.

"The geological forces may, however, gradually adju