58 Geology of South America. 



up to a height of 300 feet. Its thickness is often pretty con- 

 siderable. In an artesian well, bored in 1837 at Buenos 

 Ayres by order of Governor Rivadavia, it was found at a depth 

 of nearly 100 feet ; beneath, the Patagonian tertiary sands 

 were met with, which afforded abundance of water. 



From Buenos Ayres to San Pedro, a distance of upwards of 

 90 miles, the Pampean bed is seen forming, without interrup- 

 tion, the pretty elevated /a/««6'e* of the Plata and the Parana. 

 The falaises exhibit, when the river is low, immense beds, 

 known in the country by the name of Tosca. These consist 

 of the same clay, more or less indurated, always cavernous, 

 or filled with calcareous nodules, and containing bones of 

 raammifera. At Santa- Fe-Bajada, on the left bank of the 

 Parana, the Pampean deposit reposes on the Patagonian ter- 

 tiary formation, which is filled with marine remains. It also 

 forms the right bank, and continues to do so upwards as far 

 as Goya and Correntes. It ceases to present itself generally 

 throughout the plains of Chiquitos, of Santa-Cruz-de-la- Sierra, 

 and of Moxos ; but it seems to exist there under the alluvial 

 covering ; nay, it probably occupies in these provinces a sur- 

 face equal to that which it forms in the Pampas. From thence 

 it appears to connect itself on the south with the superficial 

 deposit of the Pampas, and on the north with the upper basin 

 of the Amazon. 



The Pampean bed does not present itself solely in the low 

 plains, for beyond the regions explored by himself, M. d'Obigny 

 thinks he recognises it in the lower bed of diluvium which, 

 according to M. Clausen, fills a portion of the caverns of the 

 Province of Minas-Geraes in Brazil. According to M. Lund, 

 the interior of the caverns of Brazil is more or less filled with 

 a red earth, identical with the red earth which forms the su- 

 perficial bed of the country. This bed, which varies from 10 

 to 50 feet in thickness, covers indiscriminately, and without in- 

 terruption, the plains, the valleys, the hills, and even the gentle 

 slopes of the highest mountains, up to a height of 6500 feet. 

 It consists chiefly of clay, containing subordinate beds of 

 gravel, and of quartz pebbles. It is often so ferruginous, that 

 the particles of iron become transformed into a pisolitic mi- 



