272 DYNAMICAL GEOLOGY. —— [Boor II. _ 
porarily affected by earthquake movements, becoming greater or 
2 
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2. Effects upon Terrestrial Waters.'—Springs are tem- _ 
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+> °- a gu eee 

smaller in volume, sometimes muddy or discoloured, and sometimes _ 
increasing in temperature. Brooks and rivers have been observed to 
flow with an interrupted course, increasing or diminishing in size, — 
stopping in their flow so as to leave their channels dry, and then | 
rolling forward with increased rapidity. Lakes are still more 
sensitive. Their waters occasionally rise and fall for several hours, 
even at a distance of many hundred miles from the centre of disturb- 
ance. Thus, on the day of the great lisbon earthquake, many of the 
lakes of central and north-western Europe were so affected as to main-. 
tain a succession of waves rising to a height of 2 or 3 feet above their 
usual level. Cases, however, have been observed where, owing to — 
excessive subterranean movement, lakes have been emptied of their 
contents and their beds have been left permanently dry. On the 
other hand, areas of dry ground have been depressed, and have 
become the sites of new lakes. ‘ 
Some of the most important changes in the fresh water of a 
region, however, are produced by the fall of masses of rock and earth; 
which, by damming up a stream, may so arrest its water as to form 
a lake. If the barrier be of sufficient strength, the lake will be 
permanent; though from the usually loose, incoherent character of © 
its materials, the dam thrown across the pathway of a stream runs a 
ereat risk of being undermined by the percolating water. A sudden 
giving way of the barrier allows the confined water to rush with 
ereat violence down the valley and to produce perhaps tenfold more 
havoc there than may have been caused by the original earthquake. 
When a landslip is of sufficient dimensions to divert a stream from’ 
its previous course, the new channel thus taken may become 
permanent, and a valley may be cut out or widened. ; 
3. Effects upon the Sea—The great sea-wave propagated 
outward from the centre of a sub-oceanic earthquake, and reaching - 
the land after the earth-wave has arrived there, gives rise to much 
destruction along the maritime parts of the disturbed region. As it 
approaches the shore, the littoral waters retreat seawards, sucked up, 
as it were, by the advancing wall of water, which, reaching a height 
of sometimes 60 feet, rushes over the bare beach and sweeps inland, 
carrying with it everything which it can dislodge and bear away. 
Loose blocks of rock are thus lifted to a considerable distance from ~ 
their former position, and left at a higher level. Deposits of sand, 
gravel, and other superficial accumulations are torn up and swept 
away, while the surface of the country, as far as the limit reached by 
the wave, is strewn with débris. If the district has been already 
shattered by the passage of the earth-wave, the advent of the great 
sea-wave augments and completes the devastation. The havoc 
caused by the Lisbon earthquake of 1755, and by that of Peru and 
Ecuador in 1868, was much aggravated by the co-operation of the 
oceanic waye, 
' Kluge, Neues Jahrb., 1861, p. 777. 
