93-1 REPORT— 1898. 



must have covered large areas of Paraguay, Corrientes, Eiitre llios, and Uruguay, 

 and, before the uplifting of the country, it extended soutii-west to the rivers 

 Chadi-Leofu and the Colorado, lapping round the southern slope of the Ventana 

 Range until the curved rim, concave to the north-east, which connects this with 

 the Sierra de Cordova, was sufficiently elevated to completely cut off its south- 

 western extension. This rim, for the first fifty miles, starting at the Ventana, is 

 about 700 to 750 feet above the sea, and shows much tosca rock near the surface. 

 It afterwards rises rapidly towards the Cordova sierra. 



The Uplifting of the Pampean Beds.— The Pampean beds were apparently 

 laid down in shallow water. Their present irregularity and elevation may be 

 attributed to pronounced local uplifting, followed by an extremely slow general 

 upheaval of the Andes from west to east, which was communicated to the whole 

 bed of the Pampean Sea, raising it ultimately to its present level. Gradually, as 

 the Ventana, Tandil, and Cordova sierras were lifted, the mud settled upon their 

 slopes until they ceased to exist as lofty islands, and with their connecting-rim of 

 high ground formed a vast breakwater against any inroad of the ocean from the 

 south or south-west. South of latitude 30° the force which raised the Andes has 

 not shown the same vigour which it has to the north of that parallel, where it 

 appears to have been greater in proportion to the broadening and swelling up of 

 the mountain masses. As a resultant of this and its slope to the eastward, the 

 northern section seems to be upheaved from the north-west, while, southward of 

 latitude 30°, the real west to east action is apparent. Here the great distance of 

 the Atlantic coast from the Andes has caused the eastern part of the province of 

 Buenos Ayres to be raised but little, not sufficiently to lift the Pampean beds en- 

 tirely out of the sea. In the north the gradually decreasing distance between the 

 Cordillera and the Brazilian highlands confined the force and gave the plain its 

 maximum lift at the great gap, at about 18° latitude, where the leverage of the 

 Andes must then have ceased, due to the fact that at this point they take a sharp 

 turn to the north-west, leaving the basin of the Beni and Mojos undisturbed. 



This local and general upheaval determined the course of the Paraguay, the 

 Lower Paranii, and the Plata Rivers, which were naturally pushed over against the 

 more ancient Brazilian formation. It also, as the Pampean Sea retired, caused the 

 Pilcomayo and Bermejo to take their south-east course across the Gran Chaco and 

 find their present outlet. 



The Cordova Range has lifted the Pampean mud to an elevation of about 

 1,300 feet above sea-level. 



The Tandil Range has brought up, to a height of 900 feet, beds identical with 

 the tertiary deposits 100 feet below the surface at Buenos Ayres. This indi- 

 cates an upheaval of 1,000 feet since tertiary times. 



Lying between the Bermejo River and the Salado, say the southern third of the 

 Gran Chaco, we appear to have an almost undisturbed part of the bottom of the 

 ancient sea — the present boundary belt between northern and southern climatic 

 influences. 



Age of the Pampean Formation. — The United States engineers, Humphreys 

 and Abbot, estimate the amount of silt discharged yearly by the Mississippi River 

 to be one cubic mile in twenty-two years. Although it carries, per cubic foot of 

 water, much more silt than the Plata, I doubt if it exceeds that of the river 

 Madeira, which now drains the Mojos basin, and is a very turbid stream. 



The mean flow of the Mississippi at New Orleans is 675,000 cubic feet per 

 second, but its maximum at flood is about 1,000,000. The minimum flow of the 

 Plata, past Buenos Ayres, is 534,000, the maximum 2,145,000. It may, therefore, 

 be fairly assumed that the yearly flow of the great North-American river is not 

 superior, and may be inferior, to that of the Plata ; and if it be admitted, as I 

 believe, that the Madeira branch of the Amazon, at the falls, annually carries an 

 amount of water equal to that passing Buenos Ayres, it will be evident that the 

 total cubic volume of the streams which poured into the Pampean Sea must have 

 been equal to twice that which the modern Mississippi contributes to the Gulf of 

 Mexico. 



Estimating the Pampean mud to cover an area of 400,000 square miles, with 



