earthquake is a vertical vibration, having an undulatory progression.
An example of the simple bounding movement occurred in 1797, when the
city of Riobamba, in the Quito Valley, was buried under part of a
mountain shaken down by the violent concussion, and men were tossed
several hundred feet. We saw one massive structure which had nearly
turned a somersault. The ordinary vibrations seldom exceed two feet in
height. The wave-movement has a rate of from twenty to thirty miles a
minute, depending on the elasticity of the rock and the elevations on
the surface. When two undulations cross each other, a rotatory or
twisting motion is produced. The waves are generally transmitted along
the lines of primary mountain chains, which are doubtless seated on a
fracture. The Lisbon waves moved from southwest to northeast, or
parallel to the mountain system of the Old World; those of the United
States, in 1843, ran parallel to the volcanic chain in Mexico. In South
America they roll along the Andes. That of 1797 left its tracks along a
westerly line from Tunguragua through Pelileo and Guano. It is a little
singular, that while the late trembling at Quito seemed to come from the
north, the great shock in Peru preceded that in Ecuador by three days.
Though the origin of earthquakes is deep-seated, the oscillation is
mostly superficial, as deep mines are little disturbed. The most damage
is done where the sedimentary plains abut against the hard, upturned
strata of the mountains. The shock is usually brief. That of Caracas
lasted fifty seconds, that of Lisbon six minutes; but Humboldt witnessed
one in South America which continued a quarter of an hour.

Several hypotheses have been advanced to account for earthquakes. Rogers
ascribes them to billowy pulsations in the molten matter upon which the
flexible crust of the earth floats. Mallet thinks they may be viewed as
an uncompleted effort to establish a volcano. Dana holds that they are
occasioned by the folding up of the rocks in the slow process of cooling
and consequent contraction of the earth's crust. In this process there
would occur enormous fractures to relieve the tension; tilted strata
would slip, and caverns give way. All this no doubt takes place; but the
sudden, paroxysmal heavings incline us to refer the cause to the same
eruptive impulse which makes Vesuvius and Cotopaxi discharge pent-up
subterranean vapor and gas. The most destructive earthquakes occur when
the overlying