chap. xi. Theories on its Origin, 351 



the wide surface of the Pampas, when under water : on 

 the other hand, over the whole of Patagonia, the same 

 or another debacle is supposed to have borne nothing 

 but gravel, — the gravel and the fine mud in the neigh- 

 bourhood of the Rios Negro and Colorado having been 

 borne to an equal distance from the Cordillera, or 

 imagined line of disturbance : assuredly directly oppo- 

 site effects ought not to be attributed to the same 

 agency. Where, again, could a mass of fine sediment, 

 charged with calcareous matter in a fit state for chemi- 

 cal segregation, and in quantity sufficient to cover an 

 area at least 750 miles long, and 400 miles broad, to a 

 depth of from twenty or thirty feet to a hundred feet, 

 have been accumulated, ready to be transported by the 

 supposed debacle ? To my mind it is little short of 

 demonstration, that a great lapse of time was necessary 

 for the production and deposition of the enormous 

 amount of mud-like matter forming the Pampas ; nor 

 should I have noticed the theory of a debacle, had 

 it not been adduced by a naturalist so eminent as 

 M. d'Orbigny. 



A second theory, first suggested, I believe, by Sir 

 W. Parish, is that the Pampean formation was thrown 

 down on low and marshy plains by the rivers of this 

 country before they assumed their present courses. 

 The appearance and composition of the deposit, the 

 manner in which it slopes up and round the primary 

 ranges, the nature of the underlying marine beds, the 

 estuary and sea-shells on the surface, the overlying 

 sandstone beds at M. Hermoso, are all quite opposed 

 to this view. Nor do I believe that there is a single 

 instance of a skeleton of one of the extinct mammifers 

 having been found in an upright position, as if it had 

 been mired. 



The third theory, of the truth of which I cannot 



