ke-wave, when it passes through rocks differing in density
and elasticity, changes in some degree not only its velocity, but its
direction; being both refracted and reflected in a manner analogous to
that of light when it passes from one medium to another of different
density.[3] When a shock traverses the crust through a thickness of
several miles it will meet with various kinds of rock as well as with
fissures and plications of the strata, owing to which its course will be
more or less modified.

(_b._) _Formation of Fissures._--During earthquake movements, fissures
may be formed in the crust, and filled with gaseous or melted matter
which may not in all cases reach the surface; and, on the principle that
volcanoes are safety-valves for regions beyond their immediate
influence, we may infer that the earthquake shock, which generally
precedes the outburst of a volcano long dormant, finds relief by the
eruption which follows; so that whatever may be the extent of the
disastrous results of such an eruption, they would be still more
disastrous if there had been no such safety-valve as that afforded by a
volcanic vent. Thus, probably, owing to the extinction of volcanic
activity in Syria, the earthquakes in that region have been peculiarly
destructive. For example, on January 1, 1837, the town of Safed west of
the Jordan valley was completely destroyed by an earthquake in which
most of the inhabitants perished. The great earthquakes of Aleppo in the
present century, and of Syria in the middle of the eighteenth, were of
exceptional severity. In that of Syria, which took place in 1759, and
which was protracted during a period of three months, an area of 10,000
square leagues was affected. Accon, Saphat, Baalbeck, Damascus, Sidon,
Tripoli, and other places were almost entirely levelled to the ground;
many thousands of human beings lost their lives.[4] Other examples might
be cited.

(_c._) _Earthquake Waves._--We have now to return to the phenomena
connected with the transmission of earthquake-waves. The velocity of
transmission through the earth is very great, and several attempts have
been made to measure this velocity with accuracy. The most valuable of
such attempts are those connected with the Charleston and Riviera
shocks. Fortunately, owing to the perfection of modern appliances, and
the number of observers all over the globe, these results are entitled
to great confidence. The phenomena connected with the Charleston