st you, or at least those from Patagonia where
I collected every one in flower. There is a bottle clumsily but I think
securely corked containing water and gas from the hot baths of Cauquenes
seated at foot of Andes and long celebrated for medicinal properties. I
took pains in filling and securing both water and gas. If you can find
any one who likes to analyze them, I should think it would be worth the
trouble. I have not time at present to copy my few observations about
the locality, etc., etc., [of] these springs. Will you tell me how the
Arachnidae which I have sent home, for instance those from Rio, appear
to be preserved. I have doubts whether it is worth while collecting
them.

We sail the day after to-morrow: our plans are at last limited and
definite; I am delighted to say we have bid an eternal adieu to T. del
Fuego. The "Beagle" will not proceed further south than C. Tres Montes;
from which point we survey to the north. The Chonos Archipelago
is delightfully unknown: fine deep inlets running into the
Cordilleras--where we can steer by the light of a volcano. I do not
know which part of the voyage now offers the most attractions. This is a
shamefully untidy letter, but you must forgive me.

LETTER 6. TO J.S. HENSLOW. April 18th, 1835. Valparaiso.

I have just returned from Mendoza, 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 considerable 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 feet.) 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 stratification in all
the mountains is beautifully distinct and from a variety in the c