Formation of Trachytes^ ^c, 125" 



the west by the tract occupied by the tertiary formation of 

 Chili, which runs along the whole extent of the Chilian Cor- 

 dillera. 



M. D'Orbigny connects this event with the appearance of 

 the trachytes which were erupted in the axis of this Cordillera, 

 and which completed its relief at a very modern epoch. 



By studying the position of the trachytes, and of the tra- 

 chytic conglomerates, M. D'Orbigny has been able to convince 

 himself that these two species of rocks have performed very 

 different parts. His maps shew, in fact, that the solid trachytes 

 must have risen in an incandescent state, at different times, 

 over great lines. Sometimes elevated in pasty masses nearly 

 solid, they have given rise to those obtuse cones so remark- 

 able, and at the same time so characteristic, which, at the 

 summit of the Cordilleras, have absolutely the same form as 

 in Auvergne. If at other points these rocks have a stratified 

 appearance, this evidently results from the eruption of more 

 or less liquid masses, which have spread themselves out in 

 sheets. Of this we have an example in the section left by the 

 Rio Maure, where the author distinctly remarked the alterna- 

 tion of masses of trachyte with pumice conglomerates; and 

 also on the coast near Tacna, where the pumice conglomerates 

 cover thin layers of trachytes. With the exception of the 

 alternation observed near the Rio Maure, M. D'Orbigny has 

 always found the trachytes under the conglomerates. The 

 former present very various asperities, while the latter every- 

 where form masses like beds, nearly horizontal, which level 

 these asperities. The pumice conglomerates are composed of 

 alternate beds of pumice, more or less considerable, or of frag- 

 ments of obsidian, and the component ingredients are not 

 united by any kind of tement, a circumstance which leads us 

 to believe that the conglomerates have been ejected in the 

 state of cinders, during the eruption of the trachytes, or pos- 

 terior to it. We may even ask if all the conglomerates be- 

 long to the same epoch as the trachytes, and if their superior 

 position does not shew that they sometimes belong to a more 

 modern period. 



In South America, the trachytic rocks only shew themselves 

 in the chain of the Cordilleras, and there most frequently ac- 



