III.] MAMMALIFEROUS DEPOSITS. 67 



that the deposit has been carried down from the interior of the 

 north by the Parana, Paraguay, and other tributaries of what is 

 now the Rio de la Plata ; but since there is no splitting of the 

 latter river at its estuary, it is evident that the formation cannot 

 properly be called a delta. That it is mainly of freshwater origin 

 seems evident not only from its intrinsic character, but likewise 

 from the vast number of entire skeletons of mammals buried 

 within it, since these creatures must certainly have lived very near 

 to the places where their bones are now entombed. In the more 

 southern part of its area the pampas is, however, probably to 

 a large extent of estuarine origin ; and the presence of layers of 

 marine shells in its uppermost horizon near Buenos Aires proves 

 that at least a portion was submerged beneath the sea before its 

 final upheaval. Whereas the Rio de la Plata now flows in a single 

 channel in a south-easterly direction near the northern limit of the 

 coast-portion of the pampas, it would seem probable that the 

 Parana and Paraguay rivers may have originally continued their 

 southerly course across the southern pampas, through which they 

 may have flowed in a number of streams. Most likely the Pam- 

 pean formation was laid down in a slowly subsiding area, in which 

 the rate of deposition approximately counterbalanced the sinking, 

 so that the greater part of it has been always land until the period 

 of the great submergence. After the latter, the entire area was 

 upheaved to a small degree above the sea-level, when the rivers 

 assumed their present approximate courses. Whereas in certain 

 localities the deposit is barren of mammalian remains, in other 

 spots it appears absolutely crowded with them, and the number of 

 entire skeletons that are entombed in it must doubtless be counted 

 by thousands. 



Somewhat older than the Pampean is a mammaliferous deposit 

 occurring on the coast near Bahia Blanca in a small hill known as 

 Monte Hermoso ; and beds of approximately equivalent age occur 

 in Catamarca at the foot of the Andes. Probably these beds should 

 be regarded as of Pliocene age ; and it may be mentioned that 

 equivalents of these deposits are met with in other parts of 

 Argentina, while representatives of the Pampean occur in Pata- 

 gonia, Chili, Bolivia, etc. Still older than the Monte Hermoso 

 deposits are the Santa Cruz beds of Patagonia, occurring not only 



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