20 



EARLY LETTERS 



[Chap. I 



Letter 5 



Letter 6 



proceed further south than C. Tres Montes ; from which point 

 we survey to the north. The Chonos Archipelago is delight- 

 fully unknown: fine deep inlets running into the Cordilleras — 

 where wc can steer by the light of a volcano. I do not 

 know which part of the voyage now offers the most attrac- 

 tions. This is a shamefully untidy letter, but you must 

 forgive me. 



To J. S. Henslow. 



April iSth. 1835. Valparaiso. 

 I have just returned from Mcndoza, having crossed the 

 Cordilleras by two passes. This trip has added much to my 

 knowledge of the geology of the country. Some of the 

 facts, of the truth of which I in my own mind feel fully 

 convinced, will appear to you quite absurd and incredible. 

 I will give a very short sketch of the structure of these 

 huge mountains. In the Portillo pass (the more southern 

 one) travellers have described the Cordilleras to consist of 

 a double chain of nearly equal altitude separated by a con- 

 siderable interval. This is the case ; and the same structure 

 extends to the northward to Uspallata ; the little elevation 

 of the eastern line (here not more than 6,000 — 7,000 ft.) has 

 caused it almost to be overlooked. To begin with the 

 western and principal chain, we have, where the sections are 

 best seen, an enormous mass of a porphyritic conglomerate 

 resting on granite. This latter rock seems to form the 

 nucleus of the whole mass, and is seen in the deep lateral 

 valleys, injected amongst, upheaving, overturning in the 

 most extraordinary manner, the overlying strata. The stratifi- 

 cation in all the mountains is beautifully distinct and from 

 a variety in the colour can be seen at great distances. 

 I cannot imagine any part of the world presenting a more 

 extraordinary scene of the breaking up of the crust of the 

 globe than the very central parts of the Andes. The upheaval 

 has taken place by a great number of (nearly) N.and S. lines ; 

 which in most cases have formed as many anticlinal and 

 synclinal ravines ; the strata in the highest pinnacles are 

 almost universally inclined at an angle from 70° to 80°. I 

 cannot tell you how I enjoyed some of these views — it is worth 

 coming from England, once to feel such intense delight ; at an 

 elevation from 10 to 12,000 ft. there is a transparency in the air, 



