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Report on M. Alcide d^Orhigny's Memoir^ entitled General 

 Considerations on the Geology of South America. By M. 

 Elie de Beaumont. 



Regarded as a whole, the portion of the American Conti- 

 nent situated to the south of the equator, exhibits a great 

 variety of orographical configuration. On the east we have 

 an immense group of low mountains, forming a mass whose 

 branches extend from some degrees south of the Line as far as 

 the mouth of La Plata ; while on the west, we have the Cor- 

 dillera, whose elevated summits commence near the Straits 

 of Magellan, and extend into Columbia, forming a ridge which 

 follows different directions, and from which rise the highest 

 peaks of the New World. Between these two great systems, 

 commencing from the south of Patagonia, a nearly level plain 

 skirts the Cordillera, occupies the intervening space comprised 

 between that important chain and the mountain mass of Brazil, 

 passes from the basin of the Plata into that of the Amazon, 

 afterwards expands to the east, and embraces to a long dis- 

 tance the two banks of that vast river. 



Gneiss Formation. — In South America, as over the surface 

 of the whole globe, the rocks which constitute the first forma- 

 tions of the series of stratified rocks are crystalline ; they are 

 chiefly gneiss. These rocks are especially developed in the 

 eastern part of the Continent, where the modern geological 

 products are less prevalent than in the western. All geolo- 

 gists who have visited Rio Janeiro have pointed out the gneiss 

 formation. Messrs Clausen and Pissis have ascertained its 

 existence over the greater part of the surface comprised be- 

 tween the course of the Rio San-Francisco and the sea, from 

 the 16th to the 27th degree of southern latitude. M. d'Orbigny 

 has found it again at Maldonado, at Monte- Video, and in the 

 Banda Orientale. M. Parchappe recognised it in the chain 

 of the Tandil. M. d'Orbigny discovered an immense belt of 

 the same rock, occupying a mean breadth of half a degree, 

 having a length of upwards of 340 English miles, and tra- 

 versing the whole province of Chiquitos. 



The old rocks are almost every where composed of the 

 same elements ; and these are, at Rio-Janeiro and in the pro- 



