1870.] 423 [Maack. 



further south at Quilrnes, near Colonia del Sacramento and Monte- 

 video, and near Bahia Blanca. 



In all these different sediments, and also at the bottom of the 

 larger rivers, we rarely find pebble stones ; but as soon as we come 

 nearer to the interior chains of mountains, these pebbles are mixed 

 with the alluvial Pampa sand, and in large quantity cover the 

 surface of the land. Especially is this the case in the neighbor- 

 hood of Mendoza and Catamarca, where the specific character of 

 the Argentine alluvium can be very well studied. Of course all 

 these pebble stones are brought down from the chains of the moun- 

 tains by the rivers. On account of the slight slope of the whole 

 country these streams were not able to transport them further than 

 to the Rio Parana, or the mouth of the Rio de la Plata. For this 

 reason we do not find them in the lower part of the Republic. 



But as compensation for this want we find there a large mass of a 

 very fine, sandy sediment, which has accumulated in such a quantity 

 that it has already formed new land; for instance, " las Barracas " 

 at the mouth of the stream "Riachuelo," the islands " San Fer- 

 nando " and " las Conchas," northwards of Buenos Ayres ; in one of 

 which was found, some years ago, a skeleton of a whale, one and one- 

 half feet below the surface covered with large willow trees. This 

 accumulation commences also at the mouth of the Rio de la Plata. 

 The fact is, that this La Plata bay is covered from year to year more 

 and more with this fine Pampa sand, and that the present average 

 depth of the La Plata between Montevideo and Buenos Ayres 

 amounts to only six metres ; to this obstacle to navigation, are to be 

 added the many sand bars already formed in the lower portion of 

 the river. No doubt some hundred years hence a delta like those 

 now found at the mouths of the Mississippi, Ganges, Nile and Indus, 

 will be formed at this place. 



Below the alluvial formation lies the Diluvium, which is called 

 especially "the Pampean formation." {Formation pampeenne, D'Orb. 

 Pampean ??iwdljp)arwin). The color of this formation is always more 

 or less red, and it extends, as we shall see afterwards, not only 

 through the whole Republic, but further north, as well as to the 

 south. Its average thickness is from ten to sixty feet. This forma- 

 tion consists also of sand, clay and lime; the lime forming but a 

 comparatively small part of its substance. The only exception in 

 which lime is the prevalent constituent part, is shown in that curi- 

 ous concretion of this formation which is called " Tosca," a true 



