28 PHYSICAL GEOGRAPHY.

 

more or less straight. Lines joining such a series
may be considered as huge cracks in the crust,
the volcanic phenomena occurring in their weak-
est places.

The frequent occurrence of volcanoes in moun-
tainous districts is caused by the crust being
broken and flexed, so as to admit of an easy
passage for the molten rock.

Where one system of fissures crosses another the
crust becomes weak, the openings numerous, and the
volcanic activity great. The two antipodal points
of the Antilles and the Sunda Islands are excel-
lent examples, and are the most active volcanic
regions on the earth.

Efforts have been made to show some connection be-
tween certain states of the weather and periods of vol-
canic activity; but, so far, these have amounted to mere
predictions of coming changes, based on observations of
the direction of upper currents of air from the clouds
of ashes or smoke ejected by the volcano. No law of
periodicity of eruption has, as yet, been discovered.

49. Other Volcanic Phenomena:

Mud Volcanoes are small hillocks that emit
streams of hot mud and water from their craters,
but never molten rock. They are found in vol-
canic regions.

Solfataras are places where sulphur vapors es-

cape and form incrustations. They occur in vol- |

canic regions.
Geysers are sometimes ranked with volcanic phe-
nomena. They are described under Hot Springs.

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Earthquakes.

50. Earthquakes are shakings of the earth’s
crust, of degrees varying in intensity from
scarcely perceptible tremors to violent agita-
tions that overthrow buildings and open huge
fissures in the ground. They may therefore be
divided into two classes:

(1.) A shaking movement without any perma-
nent change in the surface ;

'(2.) A shaking movement accompanying an
uplift or subsidence.

An earthquake is sometimes called a seismic
shock.

51. Facts concerning Earthquakes—A careful
study of earthquakes appears to establish the fol-
lowing facts:

(1.) The place or origin of the shock is not
deep-seated or far below the earth’s surface, but

 

 

 

 

Fig, 23, Fissures produced by the Charleston Earthquake of 1886.

is near the surface, probably never deeper than
thirty miles, and often much less.

(2.) ‘The area of disturbance depends not only
on the energy of the shock, but also on the depth
of its origin below the surface: the deeper the
origin, the greater the area.

(3.) The shape of the origin is generally that
of a line, often many miles in length.

(4.) The direction of the motion at the surface
is nearly upward over the origin, and more in-
clined as the distance from the origin increases.

(5.) The shape of the area of disturbance de-
pends on the nature of the materials through
which the wave is moving. If these are of
nearly uniform elasticity in all directions, the
area is nearly circular; if more elastic in one
direction than in another, the area is irregular
in shape.

52. The Varieties of Earthquake Motion at the
Earth’s Surface are—

(1.) A wave-like motion, in which the ground
rises and falls like waves in water.

(2.) An upward motion, somewhat similar to
that which follows an explosion of powder below
the surface. This has been known to occur with
sufficient force to throw heavy bodies considerable
distances up into the air. :

(8.) A rotary motion, which, from its destruc-
tive effects, is fortunately of rare occurrence.

Humboldt mentions an earthquake that happened in
Chili where the ground was so shifted that three great

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