
Parr i. Secr.ii] | EARTHQUAKES. 273 
4, Permanent Changes of Level.—It has been observed, 
after-the passage of an earthquake, that the level of the disturbed 
country has sometimes been changed. Thus after the terrible 
earthquake of 19th November 1822, the coast of Chili for a long 
_ distance was found to have risen from 3 to 4 feet, so that along 
shore littoral shells were exposed adhering still to the rocks amid 
multitudes of dead fish. The same coast-line has been further 
upraised by subsequent earthquake shocks. On the other hand, 
many instances have been observed where the effect of the earth- 
quake has been to depress permanently the disturbed ground. For 
example, by the Bengal earthquake of 1762, an area of 60 square 
miles on the coast, near Chittagong, suddenly went down beneath 
the sea, leaving only the tops of the higher eminences above water. 
The succession of earthquakes which in the years 1811 and 1812 
devastated the basin of the Mississippi, gave rise to widespread 
depressions of the ground, over some of which, above alluded to, 
the river spread so as to form new lakes, with the tops of the trees 
still standing above the surface of the water. 
Distribution of Harthquakes.—While no large space of the 
_ earth’s surface seems to be free from at least some degree of earth- 
_ quake-movement, there are regions more especially liable to the 

visitation. As a rule, earthquakes are most frequent in volcanic 
_ districts, the explosions of a volcano being generally preceded or 
~ accompanied by tremors of greater or less intensity. In the Old 
— World a great belt of earthquake disturbance stretches in an east 
and west direction, along that tract of remarkable depressions and 
elevations lying between the Alps and the mountains of northern 
_ Africa, and spreading eastward so as to enclose the basins of the 
- Mediterranean, Black Sea, Caspian, and Sea of Aral, and to rise 
_ into the great mountain-ridges of Central Asia. In this zone lie 
numerous volcanic vents, both active and extinct or dormant, from 
the Azores on the west to the basaltic plateaux of India on the east. 
The Pacific Ocean is surrounded with a vast ring of volcanic vents, 
_ and its borders are likewise subject to frequent earthquake shocks. 
- Some of the most terrible earthquakes within human experience 
have been those which have affected the western seaboard of South 
America. 
Origin of Earthquakes.—Though the phenomena ofan earthquake 
become intelligible as the results of the transmission of waves of 
shock arising from a centre where some sudden and violent impulse 
has been given within the terrestrial crust, the origin of this 
sudden blow can only be conjectured. Various conceivable causes 
may at different times and under different conditions, communicate a 
_ shock to the subterranean regions. Such are the sudden flashing 
into steam of water in the spheroidal state, the sudden condensation 
of steam, the explosions of a volcanic orifice, the falling in of the 
roof of a subterranean cavity, or the sudden snap of deep-seated rocks 
_ subjected to prolonged and intense strain. 
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