932 REPOKT— 1898. 



Section 9 shows oue of the lines of the Southern Railway, from Buenos Ayres 

 to Bahia Blanca, and the gradual rise of the country south-westward, up the 

 slope of the Curamalal Mountains. 



Section 10. This i.s of great intere.^t. It starts at the Atlantic coast, and is 

 roughly parallel to and south-west and west of the Plata estuary and Parana 

 River, and from 70 to 100 miles distant from them, until reaching a point 

 about 100 miles above the mouth of the river Pilcomayo ; thence to the great gap, 

 between the Bolivian Andes and the Chiquitos sierras, and thence to the lip of 

 the first fall of the river Madeira — a total length of about 1,770 geographical miles 

 (about 3,300 kilometres). This line, from the great gap to the Atlantic coast, was 

 approximately that of least resistance to the flow of the Pampean mud. I have 

 called attention to the very gradual north-eastern swell due to the Cordova sierra. 

 This section clearly shows it, and indicates, moreover, that the Salado, or Jura- 

 mento, River flows along its lowest margin, and serves as the boundary of the 

 most southern part of the Gran Chaco. In fact, the Salado occupies the south- 

 west side of a very level depression, 300 miles across, and only 240 feet above the 

 sea, along the north-eastern edge of which runs the Bermejo. The northern 

 undefined limit of the Gran Chaco is probably the Chiquitos sierras. From the 

 Bermejo, northward, the section shows the slope of this Chaco district. It is the 

 natural iucline which the waters gave it as the sand was poured in from the 

 north. 



I have been obliged to estimate the height of the present water-divide between 

 the Plata Valley and the Mojos Basin. For the first 240 miles north of the divide 

 I allow a slope of 9 inches to the geographical mile, and thence down the Mamor^ 

 River to the present lip of the Madeira Falls, a descent of about 4 inches to the 

 mile. Like Keller, I found the Mamore to be a very sluggish stream, ' the in- 

 clination very small.' Barometric measurements I could not take, owing to the 

 failure of some instruments and the loss of others. The height of the upper fall of 

 the Madeira is .shown to be 547 feet above the sea. It has served as my starting- 

 point in estimating the summit of the water-divide at 817 feet. Minchin gives a 

 few widely separated barometric measurements on his map of the neighbouring 

 country to the east, and I judge that he makes the divide perhaps 100 feet higher 

 than I do. I should be glad if so able an engineer would give us further in- 

 formation about it. 



Formation of the Bed of the Pampean Sea. — A vast area of the Plata Valley 

 is covered, to a depth of from 20 to 100 feet, by a bed of reddish-yellow, semi- 

 plastic, argillaceous earth, varying in colour, hardness, and constituent parts. It 

 is mixed with a little sand, and has traces of titanic iron and olivine. Due 

 probably to underground percolation from the lime-carrying rivers, it frequently 

 merges into a marly rock full of calcareous nodules. This rock is found over 

 immense areas of the cnuntr)^ and is at times apparently stratified. Great 

 numbers of the Calomys Biscacha burrow in famiUes under the rocky caps which 

 are near the surface, and thus expose them to view. ' For this reason,' according 

 to Heusser and Claraz, * the Pampa Indians call the hard material " trui-cura," 

 or Biscacha stone (tnii, Biscacha ; and ci/ra, stone) ; but the country people have 

 given it the name of tosca, of which the literal translation is tufa, whether it be 

 the bed itself or the calcareous nodules contained in the clay. The Pampas are 

 entirely without stones or pebbles. In general the Pampa clay becomes more 

 and more sandy as one goes west. It is the same towards the south, starting 

 from the Quejen Salado. There is mvich gypsum and carbonate of lime in the 

 deposits, and much fine debris of a volcanic nature.' 



In 1859, in a small cave excavated under a tosca cap at Nueva Roma, north- 

 west of Bahia Blanca, I found stalactites from 6 to 12 inches long. Pelleschi 

 tells me that at Puan, where a small stream has cut out a bowl-shaped depression 

 (785 feet elevation above the sea by instrumental survey), there is a cap of tosca 

 covering the district which is well filled with shells, closely cemented, but be does 

 not know the species, and that on an island in a neighbouring lagoon excellent 

 lime is made from a limestone rock found there. 



A broad saddle of Pampean formation lies at an elevation of over 600 feet 



