26th July 1835

LETTER 7. TO J.S. HENSLOW. Lima, July 1835.

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.

These islands were covered with fine trees; in the conglomerate, 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 Cordilleras 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 formed a marked feature in the geography of the globe. The geology
of these mountains pleased me in one respect; when reading Lyell, it had
always struck me that if the crust of the world goes on changing in a
circle, there ought to be somewhere found formations which, having the
age of the great European Secondary beds, should possess the structure
of Tertiary rocks or those formed amidst islands and in limited basins.
Now the alternations of lava and coarse sediment which form the upper
parts of the Andes, correspond exactly to what would accumulate under
such circumstances. In consequence of this, I can only very roughly
separate into three divisions the varying strata (perhaps 8,000 feet
thick) which compose these mountains. I am afraid you will tell me
to learn my ABC to know quartz from feldspar before I indulge in such
speculations.

I lately got hold of a report on M. Dessalines D'Orbigny's
labours in S. America (7/2. "Voyage dans l'Amerique Meridionale, etc."
(A. Dessalines D'Orbigny).); I experienced rather a debasing degree of
vexation to find he has described the Geology of the Pampas, and that I
have had some hard riding for nothing, it was however gratifying that my
conclusions are the same, as far as I can collect, with his results. It
is also capital that the whole of Bolivia will be described. I hope to
be able to connect his geology of that country with mine of Chili.
After leaving Copiapo, we touched at Iquique. I visited but do not quite
understand the position of the nitrate of soda beds. Here in Peru, from
the state of anarchy, I can make no expedition.

I hear from home, that my brother is going to send me a box with books,
and a letter from you. It is very unfortunate that I cannot receive this
before we reach Sydney, even if it ever gets safely so far. I shall not
have another opportunity for many months of again writing to you. Will
you have the charity to send me one more letter (as soon as this reaches
you) directed to the C. of Good Hope. Your letters besides affording
me the greatest delight always give me a fresh stimulus for exertion.
Excuse this geological prosy letter, and farewell till you hear from me
at Sydney, and see me in the autumn of 1836.