148 
NEWER PLIOCENE PERIOD. 
[Ch. XI. 
We have endeavoured, in a former volume, to point out the 
great power exerted by running water on the land in excavating 
valleys, at those periods when violent earthquakes derange, 
from time to time, the regular drainage of a country *. We 
also explained the manner in which temporary lakes are formed, 
and how the accumulated waters may suddenly escape, when 
the barriers are rent open by subsequent convulsions. 
Erratic blocks. — Blocks of extraordinary magnitude have 
been observed at the foot of the Alps, and at a considerable 
height in some of the valleys of the Jura, exactly opposite the 
principal openings by which great rivers descend from the 
Alps. These fragments have been called « erratic,' and many 
imaginary causes have been invented to account for their trans- 
portation. Some have talked of chasms opening in the ground 
immediately below, and of huge fragments having been cast out 
of them from the bowels of the earth. Others have referred 
to the deluge, — a convenient agent in which they find a simple 
solution of every difficult problem exhibited by alluvial phe- 
nomena. More recently, the instantaneous rise of mountain- 
chains has been introduced as a cause which may have given 
rise to diluvial waves, capable of devastating whole continents, 
and drifting huge blocks from one part of the earth's surface 
to another. 
M. Elie de Beaumont has indulged in the speculation, that 
the sudden c appearance of the Cordillera of the Andes ' may 
have caused ' the historical deluge f ! ' Now, if we were suf- 
ficiently acquainted with the Andes to have grounds for assuming 
that they were not upheaved, like the Alps, at several suc- 
cessive periods ;— if we could assume that they have started up 
at once, so as to attain their actual height in an instant of 
time ; — if, in short, we could embrace the theory of ' paroxys- 
mal elevations,' still we should consider the hypothesis of a 
connexion between the rise of the Andes and the historical 
* Vol. i. chap. xxiv. 
f L'Age relatif ties Montagues, sec. x.— Revue Francaise, No. xv., Mai, 1830, 
p. 55. 
