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 ' appearance of the Cordillera of the Andes ' may 

 have caused c 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 des Montagues, sec. x. Revue Frai^aise, No. xv., Mai, 1830, 

 p. 55. 



