30 PHYSICAL GEOGRAPHY.

 

are most frequent in volcanic districts. They are

more frequent in mountainous than in flat coun-

tries. They are especially frequent in the high-

est mountains. According to Huxley, fairly pro-

nounced earthquake shocks occur in some part of
’ the earth at least three times a week.

There is, in many instances, an undoubted connection
between volcanic eruptions and earthquakes. Humboldt
relates that during the earthquake at Riobamba, when
some 40,000 persons perished, the volcano of Pasto ceased
to emit its vapor at the exact time the earthquake began.
The same is related of Vesuvius at the time of the earth-
\ quake at Lisbon.

— 61, Phenomena of Earthquakes.—In order to give
some idea of the phenomena by which severe earthquake
shocks are attended, we append a brief description of the
earthquake which destroyed the city of Lisbon, on the 1st
of November, 1755. The loss of life on this occasion was
the more severe, since the shock occurred on a holy day,
when nearly the whole population was assembled in the
churches. A sound like thunder was heard, and, almost
immediately afterward, a series of violent shocks threw
down nearly every building in the city. Many who es-
caped the falling buildings perished in the fires that soon
kindled, or were murdered by lawless bands that after-
ward’ pillaged the city.

The ground rose and fell like the waves of the sea; huge
chasms were opened, into which many of the buildings
were precipitated. In the ocean a huge wave, over 50 feet
high, was formed, which, retreating for a moment, left the
bar dry, and then rushed toward the land with frightful
force. This was repeated several times, and thousands
perished from this cause alone. The neighboring moun-
tains, though quite large, were shaken like reeds, and
were rent and split in a wonderful manner.

This earthquake was especially remarkable for the im-
mense area over which the shock extended. It reached
as far north as Sweden. Solid mountain-ranges—as, for
example, the Pyrenees and the Alps—were severely shaken.
A deep fissure was opened in France. On the south, the
earthquake waves crossed the Mediterranean and destroyed
a number of villages in the Barbary States. On the west,
the waves traversed the bed of the Atlantic, and caused
unusually high tides in the West Indies. In North Amer-
ica the movements were felt as far west as the Great Lakes.
Feebler oscillations of the ground occurred at intervals for
several weeks after the main shock.

62. Non-voleanic Igneous Eruptions.—In re-
gions remote from volcanoes, melted rock has
been forced up from the interior through fissures
in the rocks of nearly all geological formations.
On cooling, the mass forms what is called a dyke.
Dykes vary in width from a few inches to several
yards. They are generally much harder than the
rocks through which they were forced, and, being
less subject to erosion, often project considerably
above the general surface.

From their mode of formation, dykes are gen-
erally without traces of stratification, but by cool-
ing a series of transverse fractures are sometimes

 

 

produced. The dykes thus obtain the appearance
of aseries of columns, called basaltic columns.

Igneous rocks of this description are found in
all parts of the continents, but are especially com-
mon near the borders of mountainous districts.
Fingal’s Cave, in Scotland, is a noted example
of basaltic columns.

 

Fig. 24, Basaltic Columns, Fingal’s Cave, Scotland.

68. Gradual Elevations and Subsidences——Be-
sides the sudden changes of level produced by
earthquakes, there are others that take place
slowly, but continuously, by which large portions
of the surface are raised or lowered from their
former positions. The rate of movement is very
slow—probably never exceeding a few feet in a
century. The following examples are the most
noted : ‘

The Scandinavian peninsula (Norway and Swe-
den) is slowly rising in the north and sinking in
the south.

The southern part of the coast of Greenland is
sinking.

The North American coast, from Labrador to
New Jersey, is rising.

The Andes Mountains, especially near Chili,
are gradually rising.

The Pacific Ocean, near the centre, is sinking
over an area of more than 6000 miles.

The cause of these movements is to be traced
to the warping action caused by gradual contrac-
tion of a cooling crust.