Revisiting Alexander von Humboldts Initiation of Rock Coating ...

Journal of Geology V O N HUMBOLDT AND ROCK COATING RESEARCH 5
Figure 2. Secondary electron image of varnish formed on quartz, where the thinnest coating appears to be incipient
deposits !100 nm thick.
grow together into larger botryoidal forms with diameters
of ∼5 mm. Some hand samples of the Orinoco
varnish do not display a strong luster, and
these contain more areas botryoidal textures.
An Accretion or Derived from Weathering
of the Underlying Rock?
The debate over whether rock varnish forms by
Figure 3. Energy-dispersive spectroscopy spectra of point 6 in table A2, available online or from the Journal of
Geology office. Note the presence of Ba.