SYLLABDS. . 31

 

SYLLABUS.

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The earth was originally melted throughout. It after-
ward cooled on the surface and formed a crust. The earth’s
original fluidity is rendered probable—

(1.) By the spherical shape of the earth;

(2.) By the crystalline rocks underlying all others;
and

(3.) By the greater heat of the earth during geological
time.

The interior is still in a highly-heated condition. This
is proved—Ist. By the increased heat of the crust as we go
below the surface; 2d. By the escape of lava from volca-
noes in all latitudes.

The following opinions are held concerning the condi-
tion of the interior of the earth:

(1.) That the earth has a solid centre and crust, with a
heated layer between.

(2.) That the earth has a solid crust only, and an inte-
rior sufficiently heated to be in a fused or in a pasty con-
dition.

(3.) That the earth is solid throughout, but highly
heated in the interior.

The thickness of the crust is not known. It is probable
that the portions solidified by cooling pass insensibly into
those that are nearly solid from the combined influence
of loss of heat and increasing pressure. The heated
interior, however, must lie comparatively near the sur-
face.

The effects produced by the heated interior on the crust
are—Ist. Volcanoes; 2d. Earthquakes; 3d. Non-volcanic
igneous eruptions ; and 4th. Gradual elevations or subsi-
dences.

Volcanic mountains are of a variety of shapes. Near
their craters the cone shape predominates, and serves to
distinguish these mountains as a class.

The ejected materials of volcanoes are—Ist. Melted rock _

or lava; 2d. Ashes or cinders; 3d. Vapors or gases.

These materials are brought up from great depths into
the volcanic mountain by the force produced by a contract-
ing globe. They may escape from the crater—lst. By the
pressure of highly-heated vapors; or, 2d. By the pressure
of a column of melted lava.

The inclination of the slopes of the volcanic cone de-
pends on the materials of which it is composed. Ash-
cones are steeper than those formed of lava.

Eruptions are of two kinds, ae and non-explo-
sive.

High volcanic mountains are, as a rule, characterized by
non-explosive eruptions.

Volcanoes occur both on the surface of the land and on
the bed of the ocean.

Those on the land occur mainly near the borders of
sinking oceans, where the crust is weakest.

The principal volcanic districts of the world are—1.
Along the shores of the Pacific; 2. On the islands which
are scattered over the Pacific; 3. Scattered over the seas
which divide the northern and southern continents; 4. In
the northern and central parts of the Atlantic Ocean; 5.
In the western and central parts of the Indian Ocean.

The centres of volcanic activity’ are found in the An-
tilles and in the Sunda Islands, where several lines of
fracture cross each other.

 

Subordinate volcanic phenomena are seen in—1. Mud
volcanoes; 2. Solfataras; 3. Geysers.

Earthquakes are snes of the earth’s crust; they may
occur with or without a permanent displacement.

The following facts have been discovered as to earth-
quakes:

(1.) Their place of origin is not very deep-seated.

(2.) The area of disturbance increases with the energy
of the shock and the depth of the origin.

(3.) The shape of the origin is that of a line, and not
that of a point.

(4.) The shape of the area of disturbance depends on
the elasticity of the materials through which the shock
moves.

(5.) The earthquake motion travels through the earth
as spherical waves which move outward in all directions
from the origin of the disturbance.

The movement at the earth’s surface may be—Ist. In
the form of a gentle wave; 2d. An upward motion; 3d. A
rotary motion.

The velocity with which the earthquake motion is trans-
mitted varies with the intensity of the shock and the
nature of the materials through which it is propagated.

There are two distinct kinds of motion accompanying
earthquake waves: the earthquake motion proper, and
the motion producing the accompanying sounds.

As a rule, the earthquake shocks which ‘produce the
greatest damage are of but short duration, generally but
a few seconds or minutes. Slighter disturbances may fol-
low the main shock at intervals of days, weeks, or even
years.

Earthquake shocks are more frequent—lst. In winter
than in summer; 2d. At night than during the day; 3d.
During the time of new and full moon than at any other
phase.

Earthquakes are. mainly caused by the gradually in-
creasing force produced by the contraction of the crust.

Earthquakes are also to be attributed to the forces which
eject the molten matter from the craters of volcanoes.. |

Slight earthquake shocks may be occasioned by the fall-

ing in of masses of rock from the roofs of subterranean.

caverns, or by the settling of upturned strata.

Earthquakes may occur in any part of the earth, but are
most frequent in volcanic and in mountainous regions.

Dykes are masses of rock formed by the gradual cooling
of melted matter which has been forced up through fis-
sures from. the interior.

Basaltic columns are formed by dykes. They owe their
columnar structure to fractures produced on cooling.

The crust of the earth is subject to gradual as well as to
sudden changes of level.

The Scandinavian peninsula is rising on the north and
sinking on the south.

The southern coast of Geeenland is sinking.

The North American coast, from Labrador to New Jer-
sey, is rising.

The range of the Andes near Chili is rising.

The bed of the Pacific in the neighborhood of the Poly-
nesian island chain is sinking.

These movements are caused by the contraction of a
cooling crust.