the striking of the ship on a sand bar. On the tenth of July a column of water, like a water spout, was seen rising out of the sea, and soon after a dense mass of steam ascended to the height of one thousand eight hundred feet. On the eighteenth of July, a small island had appeared, with a crater in its center, ejecting volcanic matter. It had, on the fourth of August, attained an elevation of two hundred feet, and a circumference of three miles. After this, it diminished by subsidence and the action of the waves, so that at the end of October, no vestige of the crater remained. In A. D. 1833, a submerged reef, about three-fifths of a mile in extent, existed in its stead.

Graham's Island, as it appeared in August, A. D. 1831.

61. The term lava is applied to any mineral substance which has flowed from a volcano in a melted state. Lavas consist essentially of two minerals, feldspar and augite. When the feldspar predominates, the lava is

5*

UNITY
CALIFO

called feldspathic; it is light colored, and has specific

gravity not exceeding 2.8.

Trachyte is such a lava.

lava is dark colored, has and is called augitic

When the augite prevails, the
specific gravity exceeding 2.8,
basaltic. Lava cooled under great pressure is dense, like
the older rocks; when cooled under pressure of the atmos-
phere only, it is porous, distended by gases. Feldspathic
lava, flowing into water, is converted into pumice, which is
so light as to float on the surface of the water. When
silex enters largely into the composition of lava, it pro-
duces volcanic glass-obsidian, which resembles ordinary
glass, and is of a smoky hue. A portion of the lava of
Kilauea is vitreous, and is sometimes blown by the wind
into minute threads, called by the natives "Pele's hair."
The lava of this volcano is more fluid than that of most
volcanoes. Lava is a very poor conductor of heat, and
consequently the interior portions of a lava stream retain
their heat a great length of time. Lava ejected from
Etna in 1819 was sufficiently hot and fluid to move a yard
a day nine months after eruption. In another instance,
lava was in motion ten years after it was ejected. The
lava of Kilauea, erupted in June, 1840, was so hot in
November that pieces of paper introduced into fissures in
it were immediately inflamed. Lava flows within a crust
which is rapidly formed over its surface. On piercing this
crust, the fluid within flows out, and its course may in this
way sometimes be controlled. In the summer of A. D. 1828, a
mass of ice was discovered on Etna, beneath a bed of vol-
canic ashes and lava, whose non-conducting property had
preserved it for centuries from melting.

63. Earthquakes are movements, more or less violent, of the superficial crust of the earth, consisting usually of

rapidly succeeding undulations, oftentimes accompanied by
sounds, and traceable in particular directions. Three dis-
tinct kinds of motion are recognised. 1st, The perpendicu
lar, which acts from below upward, like the explosion of a
mine. This was witnessed in the destruction of Riobamba,
in A. D. 1797, when many of the bodies of the inhabit-
ants were thrown upon a hill several hundred feet high.
2nd, The horizontal, which takes place in successive undu-
lations, proceeding in a uniform direction. 3rd, The rota-
tory or vorticose, which seems to be due to interferences of
undulations, causing a whirling movement of the Earth, by
which buildings are twisted round, and parallel rows of
trees are displaced without being prostrated. The first
movement is the most common and harmless, while the
third occurs only in the most disastrous earthquakes. A
hollow sound, like a mine-explosion, often accompanies an
earthquake, which is sometimes heard as distinctly at a
great distance from the scene as in its immediate neighbor-
hood. The progression of earthquakes is most commonly
in a linear direction, with a velocity of twenty or thirty
miles in a minute; but sometimes the concussion proceeds
from the center of a circle or ellipse, decreasing in force
toward the circumference. Their duration is very brief;
thousands of lives are sacrificed, and cities and provinces
are reduced to ruins in a few seconds. No country is
entirely exempt from their visitations, but particular re-
gions are subject to severe, continuous, and extensive con
cussions, as Central and South-eastern Asia, South America, ale
and Mexico. Slight shocks are so frequent that there is
reason to presume that the surface is continually agitated
by concussions on some of its points. There are accounts
of no less than three thousand four hundred and thirty-two

!! Sea !

20-303

ware 1"

distinct earthquakes which have occurred in Europe since the commencement of the fourth century of the Christian

era.

63. The most remarkable earthquake on record is that which destroyed the city of Lisbon, in A. D. 1755. It commenced with a sudden subterranean sound: this was immediately followed by violent shocks, which demolished. the greater portion of the buildings, destroying the lives of sixty thousand persons. A large quay, to which hundreds of people had resorted for safety from the falling buildings, was instantaneously engulfed in an unfathomable chasm, from which nothing ever rose to the surface. Vessels were thrown violently aground; the bed of the river was raised to the surface, and immediately afterward the ocean came rolling in, fifty feet higher than usual. The walls of some houses were seen to open from top to bottom more than a quarter of a yard, and close again so accurately as to leave but slight trace of the injury. The movement of this earthquake was undulatory, progressing about twenty miles a minute, agitating a surface four times as large as Europe and nearly one-twelfth of the whole superficies of the globe. The water in the lakes of Scotland rose and fell three feet. The earthquake was perceived at Fahlun in Sweden, Barbadoes, and on Lake Ontario. The sea rose on the West India islands, and a ship one hundred miles west of St. Vincent suffered so severe a shock that the seamen were thrown upon the deck.

64. The great earthquake of Calabria, Southern Italy, in A. D. 1783, was distinguished by the concentration of its violence, heaving the surface like the waves of the sea. Radiating from the town of Oppido as a center, its violence was manifested over an area of five hundred square miles.

Fig. 28.

:

About two hundred towns and villages were destroyed, and nearly one hundred thousand people perished. The movement was rotary, as shown by the twisting of the stones of the obelisks of St. Bruno, which were turned from six to nine inches from their former position. A chasm, a mile long, one hundred and five feet broad, and thirty feet deep, was formed; and another, three-fourths of a mile long, one hundred and fifty feet broad, and one hundred feet deep, with numerous elevations and depressions and many disputes arose respecting the ownership of lands which had shifted position. In the year 1819, a large tract of land at the mouth of the Indus, with villages, was submerged, and another tract, called the Ullah Bund, fifty miles long and sixteen broad, was elevated ten feet. 65. Violent and extensive earthquakes occur in the vicinity of the Andes, in South America. A terrible convulsion was experienced in 1822 on the coast of Chili, by which an area of one hundred thousand square miles was permanently elevated three feet. In 1812, LaGuayra and Caraccas, in South America, were destroyed by an earthquake of great violence. In December of the previous year, a series of convulsions commenced along the valley of the Mississippi, from New Madrid to the mouths of the Ohio and St. Francis rivers. The earth rose in great undulations. Lakes, twenty miles in extent, were suddenly formed, and others were drained. Extensive chasms opened in a direction North-east and South-west, and new islands were formed in