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AMERICAN GEOLOGY. 
ever, by the position and condition of the resisting medium. 
In its progress, a direct undulation may be converted into a 
gyratory one by an increased resistance in its course, or into a 
vertical one at the place situated immediately above the point 
of impulse. An instance of direct earthquake undulation, 
extending over a large portion of Europe, a part of Africa, the 
Atlantic ocean, and West India isiands, is well known in the 
remarkable visitation of Lisbon in 1755; of the gyratory, or of 
the vertical movement, which took place in the great Calabrian 
earthquake in 1783, by which two obelisks at the convent of 
St. Bruno were twisted on a vertical axis without falling. Of 
the vertical movement, a striking example occurred during the 
earthquake at Riobamba, 1799, where a sudden rise of the 
ground took place, which hurled the bodies of men to a height 
of several hundred feet. 
Considering earthquakes as earth-waves, it is evident that 
when those waves are generated in the ground beneath the 
ocean, their impulse must be communicated to the water above, 
whose motion will partake of the same character. Waves will 
therefore be generated therein, which will travel onward in 
directions which the impulse communicates; but from the 
nature of the medium, the water-wave will travel with less 
speed than the earth-wave. In consequence of this fact, a per¬ 
son upon a shore where the two waves are tending, will expe¬ 
rience first the earth-wave, and soon after the water-wave will 
follow; lastly, another wave will be recognized through the 
medium of the air. In each of these cases the rate of transit 
depends on the nature of the medium receiving the shock. 
Experience proves that the intensity of the shock is very varia¬ 
ble in volcanic districts. It does not, however, seem to depend 
directly on the activity of volcanic action, for according to 
Dana, the activity of Kileaua is not exceeded by that of any 
other volcano; yet earthquakes are rare and feeble, even when 
a force immeasurable by us in its power is manifested in some 
of the results of this action, especially in rending the earth for 
twenty-five miles without having produced an earth-wave 
