Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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These Tertian Agues Are Attended With Great
Debility Of The Muscular System; Yet We Find Poor Ecclesiastics On The
Orinoco, Who Endure For Several Years These Calenturitas, Or
Tercianas:
Their effects are not so fatal as those which are
experienced from fevers of much shorter duration in temperate
climates.
I have just alluded to the noxious influence on the salubrity of the
atmosphere, which is attributed by the natives, and even the
missionaries, to the bare rocks. This opinion is the more worthy of
attention, as it is connected with a physical phenomenon lately
observed in different parts of the globe, and not yet sufficiently
explained. Among the cataracts, and wherever the Orinoco, between the
Missions of Carichana and of Santa Barbara, periodically washes the
granitic rocks, they become smooth, black, and as if coated with
plumbago. The colouring matter does not penetrate the stone, which is
coarse-grained granite, containing a few solitary crystals of
hornblende. Taking a general view of the primitive formation of
Atures, we perceive, that, like the granite of Syene in Egypt, it is a
granite with hornblende, and not a real syenite formation. Many of the
layers are entirely destitute of hornblende. The black crust is 0.3 of
a line in thickness; it is found chiefly on the quartzose parts. The
crystals of feldspar sometimes preserve externally their reddish-white
colour, and rise above the black crust. On breaking the stone with a
hammer, the inside is found to be white, and without any trace of
decomposition.
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