

468 



THEORY PROPOUNDED BY AGASSIZ. 



[Ch. XIX 



We have seen above, p. 462, that when we approach 

 the delta of the Mississippi we find on the coast of the Gulf 



rivers . 



0± 



Mexico, and slightly raised above the level of the sea, 

 swamp deposits, with bnried forests and layers of clay con- 

 taining recent oysters and barnacles. 



In order to explain the great Amazonian formation above 

 described, Professor Agassiz conceives that the whole valley 

 was for a long period converted into a lake, by a large dam 

 or barrier stretching across its seaward extremity, and which 

 has since been removed by the ocean. A similar hypothesis 

 has been advanced again and again to account for the vast 

 extent of old fluviatile and lacustrine deposits, as well as for 

 the inundation-mud called loess, which have once filled the 

 lower portions of the basins of most of the principal rivers of 

 the world, such as the Mississippi, Nile, Danube, and Ehine. 

 I have elsewhere* endeavoured to show that such pheno- 

 mena are the natural result of oscillations in the level of the 

 land, extending over large continental areas, by which the 

 fall of the rivers is lessened at certain periods, giving rise 



matter more or less lacustrine, while 



ccumulations 



movement 



on 



direction, the rivers cut through their old deposits, re-exca- 

 vating the valleys and often eroding them below their original 

 depth. There is nothing new therefore in the character of 

 the Amazonian clays, sands and loess, except the grand scale 

 which they are developed. Geologists have usually been 

 driven to abandon the theory of a seaward dam at the 

 mouths of great rivers, such as the Rhine and Mississippi, by 

 the difficulty of imagining first the construction of such 

 barriers after the valley was 

 quent disappearance. 



forme d 



Professor Aga 



ssiz has hazarded the startling conjecture 



that the Amazonian basin was closed up and converted into 

 a lake by the terminal moraine of a glacier, which stretched 

 for thousands of miles from west to east, and entered the sea 

 under the equator. But this distinguished naturalist can- 

 didly confesses that he failed to discover any of those proofs 

 which we are accustomed to regard, even in temperate la 1- 



* See above, p. 308, and Ant, of Man, p. 333 ; Elements of Geology, V- ll8. 



! 













c* 



ft 



pebble 

 The 



X 



tb 



12 



en' 



el • 



inform 



has _ 

 200 j 



east 1 1 



it is pi 

 the pr 



indei 



From 

 the pr 



length 

 com} 



veller 

 great ; 



bant 



an 





m 



thel 

 agij 



t 





aa 



u 



for 



of eol, , 



\\ 



t. 





S i 



of 



. 



M 





