122 Geology of South America. 



groups could not then have been yet connected together by 

 the great continuous chain of the Cordilleras. This vast chain 

 is sinuous, like our Alps. It presents different portions which 

 are very variously disposed : without speaking of those which 

 M. D'Orbigny refers to the Columbian and the Fuegian sys- 

 tems, and without leaving the region examined by himself, 

 two very distinct directions are observed. From the Straits 

 of Magellan to Bolivia, over a space of 35°, which embraces 

 the whole length of Chili, the Cordillera runs from S. 5°. W. 

 to N. 5° E. ; afterwards, in Bolivia itself, it makes a sudden 

 bend to the west, and runs from SE. to NW. On entering 

 South Peru, the mountains preserve a constant parallelism to 

 those of Bolivia, as far as the fifth parallel of south latitude ; a 

 fact which permits us to suppose that the geological lines ob- 

 served by M. D'Orbigny in the Bolivian system, to the east of 

 the Cordillera, properly so called, extend as far as that latitude, 

 thus embracing a total extent of 15°. Further north, the 

 chain again changes its direction, and takes that of the Cordil- 

 lera of Chili. 



Thus, in the region comprised between the Straits of Ma- 

 gellan and the equator, the Andes present two great systems 

 of mountain-chains and valleys. These two systems, which 

 M. D'Orbigny designates the Bolivian system and the Chilian 

 system, cross each other nearly in the same manner as the 

 systems of the Western Alps and of the principal chain of 

 the Alps cross in Europe, and they appear likewise to be the 

 result of successive dislocations. 



The circumstance that the Cordillera, in the region between 

 Terra-del-Fuego and Quito, is composed of several large frag- 

 ments variously placed, and probably of different origins, is con- 

 nected with a curious fact, which confirms, in a remarkable man- 

 ner, the reality of the distinction founded on the difference of 

 directions. No earthquake has ever been felt on the great 

 Bolivian plateau. This, at least, was what M. D'Orbigny was 

 informed, and it corresponds with his experience in the lati- 

 tude of Arica. It is natural to ask, if the presence of the 

 Bolivian system in this latitude has not some influence con- 

 nected with the limitation of earthquakes. It appears, in fact, 

 that very violent shocks are experienced in the centre of the 



