26 EARLV LETTERS LChap. I 



Letter 7 To J. S. Henslow. 



Lima, July 12th, 1 835. 

 This is the last letter which I shall ever write to you 

 from the shores of America, and for this reason I send it. 

 In a few days time the Beagle will sail for the Galapagos 

 Islands. I look forward with joy and interest to this, both 

 as being somewhat nearer to England and for the sake of 

 having a good look at an active volcano. Although we 

 have seen lava in abundance, I have never yet beheld the 

 crater. I sent by H.M.S. Conway two large boxes of 

 specimens. The Conway sailed the latter end of June. 

 With them were letters for you, since that time I have 

 travelled by land from Valparaiso to Copiapo and seen 

 something more of the Cordilleras. Some of my geological 

 views have been, subsequently to the last letter, altered. I 

 believe the upper mass of strata is not so very modern as 

 I supposed. This last journey has explained to me much 

 of the ancient history of the Cordilleras. I feel sure they 

 formerly consisted of a chain of volcanoes from which 

 enormous streams of lava were poured forth at the bottom 

 of the sea. These alternate with sedimentary beds to a 

 vast thickness ; at a subsequent period these volcanoes must 

 have formed islands, from which have been produced strata 

 of several thousand feet thick of coarse conglomerate. 1 

 These islands were covered with fine trees ; in the con- 

 glomerate, I found one 15 feet in circumference perfectly 

 silicified to the very centre. The alternations of compact 

 crystalline rocks (I cannot doubt subaqueous lavas), and 

 sedimentary beds, now upheaved fractured and indurated, 

 form the main range of the Andes. The formation was 

 produced at the time when ammonites, gryphites, oysters, 

 Pecten, Mytilus, etc., etc., lived. In the central parts of 

 Chili the structure of the lower beds is rendered very 

 obscure by the metamorphic action which has rendered 

 even the coarsest conglomerates porphyritic. The Cor- 

 dilleras of the Andes so worthy of admiration from the 

 grandeur of their dimensions, rise in dignity when it is 

 considered that since the period of ammonites, they have 



1 See Geological Observations oti South America (London, 1S46), 

 Chap. VII.: "Central Chile; Structure of the Cordillera." 



