TKAXSACTIOXS OF SECTION E. 935 



) . 

 an average thickness of 50 feet, it would represent about 4,000 cubic miles of silt. 

 The Andean and Brazilian shales and sandstones were probably disintegrated and 

 dissolved with great rapidity when the rivers which tore them down tiowed into 

 the Pampean Sea. These streams must then have carried, one with another, a 

 quantity of silt which, compared to that carried to-day by the Plata and Madeira 

 liiversj'may safely be estimated at three-fourths the amount per cubic foot of water 

 now carried by the Mississippi Piiver. Allowing for partial loss of silt in the 

 ocean, and from other causes", this would give the Pampean formation an age of 

 about 70,000 years. 



JIozv the "Ancient Mojos Lake zcas Formed. — I find the divortmm aquarian 

 between the Mojos and Plata basin to be only about 170 feet hioher than the 

 ancient lip of Guajar;i-mirim. It is easy to understand that the Rio Grande and 

 the Parapiti have deposited there a quantity of alluvium far exceeding 170 feet in 

 depth. The divide, prior to this and to the general uplifting movement described, 

 could not ha^-e been more than I'OO to bOO feet above sea-level, and thus much 

 lower than even the present lip of the upper- fall of the Madeira. Therefore, 

 through this southern gap the combined streams which now form the principal 

 affluent of the river Amazon found their way. Slowly the highly-charged, 

 yellowish-brown waters sifted out and spread their heavier material over the sandy 

 slope at the head of the Pampean Sea, leaving the iine silt in suspension to be 

 precipitated over the submerged plains. The waters from the north were aug- 

 mented in turn by the Paraguay and Parana, the Pilcomayo, Bermejo, and Salado. 

 From east and west numerous rivers pushed into the sea, stirring up its waters, 

 and thus keeping the greater part of the silt in suspension to be carried far south- 

 ward and deposited principally, and in maximum thickness, over the Pampean 

 region south of latitude 30°. 



The Grande and the Parapiti entered the plain with a northern trend to 

 contest with the great river of the north the possession of the gap. They struck 

 it almost at a right angle, and slowly pushed their rival eastward over against the 

 Chaco base of the Chiquitos sierras. Here the final conflict must have taken place, 

 as the Grande and Parapiti threw their dam across the outlet of the Mojos River, 

 thus cutting off its exit into the ancient sea. No doubt the giant stream waged fierce 

 war for thousands of years to keep its channel open, alternately sweeping away the 

 barrier and again yielding to the ceaseless volume of sand and clay which, visible 

 to-day, confirms the victory of the Grande and Parapiti. The dam having finally 

 become permanent, the formation of the ancient Mojos Lake was assured (see 

 physical map). When it reached the level of the lip of Guajara-mirim, its waters 

 commenced to tumble over it and carve their way to the Amazon. Since then 

 huge volumes of alluvium have poured down the northern slopes of the Bolivian 

 Andes; the ancient lake is now almost loaded with material, but is not yet 

 entirely obliterated. The muddy silt which covers the surlace of the basin is so 

 fine that, when an Indian goes up stream to the mountains, his friends ask him to 

 bring back a stone that they may see what it is like. 



Since forming the dam, the Rio Grande has slowly been returning westward 

 down the counterslope which its own alluvium creates. 



Sliffht Tidal Action of the Pampean Sea.— OS the mouth of the Rio de la Plata, 

 the tides which flow south along the Brazilian coast meet those making north from 

 the coast of Patagonia, counterbalance each other, and maintain the liquid mass 

 at nearly the same level, so that the average tide at Montevideo is only about 

 three feet. Thus the Plata River is able to pile its silt in a direct line further seaward 

 than either the Orinoco or the Amazon. 1 have shown that the Ventana Range 

 acted as a massive breakwater to the Pampean Sea, and that the Cordova sierra 

 almost divided this into a northern and southern half. Hence the prevailing 

 conditions appear to have permitted but little tidal action in the great basin. Had 

 it been otherwise, as, a priori, one might suppose, the fierce contests which the 

 tides would have waged when meeting the large rivers would have ripped up the 

 Pampean beds and washed them into the ocean, and, in their place, we should now 

 find nothing but a clayey, sandy, and shingle-covered waste. 



In 18G0 I located the Northern Railway from Buenos Ayres to San Fernando. 



