Mr. Darwin, who observes that it would be
too rash to assign to the eastern chain of Bolivia and Central Chile, a
later origin than the western chain (the nearest the Pacific), but that
the circumstance of the rivers of a lower mountain chain having forced
their way through a higher chain seems, without this supposition, to be
enigmatical. Mr. Darwin is of opinion that the phenomenon is assignable
to a periodical and gradual elevation of the second mountain line (the
Andes); for a chain of islets would at first appear, and as these were
lifted up, the tides would be always wearing deeper and broader
channels between them.

In the heights of the Cordillera the effect of the diminished
atmospheric pressure on the human frame shows itself in intolerable
symptoms of weariness and an extreme difficulty of breathing. The
natives call this malady the _Puna_ or the _Soroche_; and the Spanish
Creoles give it the names of _Mareo_ or _Veta_. Ignorant of its real
causes they ascribe it to the exhalations of metals, especially
antimony, which is extensively used in the mining operations. The first
symptoms of the veta are usually felt at the elevation of 12,600 feet
above the sea. These symptoms are vertigo, dimness of sight and
hearing, pains in the head and nausea. Blood flows from the eyes, nose,
and lips. Fainting fits, spitting of blood, and other dangerous
symptoms, usually attend severe attacks of veta. The sensations which
accompany this malady somewhat resemble those of sea-sickness, and hence
its Spanish name _mareo_. But sea-sickness is unaccompanied by the
distressing difficulty of breathing experienced in the veta. This
disorder sometimes proves fatal, and I once witnessed a case in which
death was the result. Inhabitants of the coast and Europeans, who for
the first time visit the lofty regions of the Cordillera, are usually
attacked with this disorder. Persons in good health and of a spare habit
speedily recover from it, but on plethoric and stout individuals its
effects are frequently very severe. After an abode of some time in the
mountainous regions, the constitution becomes inured to the rarefied
atmosphere. I suffered only two attacks of the veta; but they were very
severe. The first was on one of the level heights; and the second on the
mountain of Antaichahua. The first time I ascended the Cordillera I did
not experience the slightest illness, and I congratulated myself on
having escaped the veta; but a year af