126 Geology of South America. 



company porphyritic rocks. In Bolivia, they only present 

 themselves on the Great Bolivian PlateaUy on the Western 

 Plateau, and on the western side of the Cordillera. No one 

 has observed them in Brazil. 



M. D'Orbigny admits that, on the western side of the long 

 ridge which formed the first outline of the Cordillera, and con- 

 sisted of elements belonging to the different systems men- 

 tioned above, a new opening took place, and incandescent 

 trachytic matters, pushed with violence towards this vast out- 

 let, escaped in all directions, dislocated the porphyries and the 

 cretaceous rocks, and invaded the whole summit of the chain. 



In the immense mass of Bolivia, events apparently more 

 complicated took place. The lines of dislocation of the Chilian 

 system meeting with reliefs existing previously to the Bolivian 

 system, and not being able to fracture this great mass, ex- 

 tended to the west, as the porphyritic rocks had done pre- 

 viously. The trachytes and their conglomerates, which, ac- 

 cording to M. De Humboldt, form an immense dome on the 

 Plateau of Quito, formed, according to M. D'Orbigny, another 

 dome on the Western Plateau of Bolivia. Moreover, these 

 rocks issued through fissures in the sedimentary rocks, along 

 that line, so interrupted by trachytic eminences, which, to the 

 east of the Great Bolivian Plateau, borders the foot of the dis- 

 locations of the Devonian rocks from Achacoche to Potosi. 

 They are not the primary cause of the Bolivian system, but 

 t;hey upraised some portions of it, while, at the same time, 

 they perhaps communicated to the Chilian Cordillera the 

 greater part of its external shape. The trachytes thus acted 

 in the New World as in Southern Italy and in Greece, where 

 their lines of eruption followed those of systems of mountains 

 of more ancient origin, especially of the system of the Pyrenees. 



A dislocation of 50°, or upwards of 3400 English miles in 

 length, which produced one of the highest chains in the world, 

 and which elevated above the ocean all the marine tertiary 

 formations of the Pampas over an immense extent, could not 

 have taken place without causing a proportional displacement 

 in the waters of the sea. It was then, according to M. 

 D'Orbigny, that the latter, being agitated with violence, in- 

 vaded the continent, destroyed and transported the great ter- 



