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. (7/1. See "Geological Observations on South America"
(London, 1846), Chapter VII.: "Central Chile; Structure of the
Cordillera.") 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