Posted by: Rob Viens | March 14, 2012

Rock Varnish, Lead Plants, and Swedish Chemists

At long last – on March 14th, after 8 days stuck on the ship, Darwin is back in the field again! What a glorious day.  He kept it simple (I imagine he was still limping), noting:

“Hired a boat & went some miles up the harbour. — I found some interesting geological appearances & spent some pleasant hours in wandering on the beach.” (Mar 14)

I may be reading a lot into it but I suspect that by “pleasant” he meant, “thank god the pain is manageable and I can be outside again!”.

I’m not sure what he saw on the beach that particular day, but this jaunt may have been the origin of a curious rock “varnish” that he described in Voyage:

“On a point not far from the city, where a rivulet entered the sea, I observed …[that the] syenitic rocks are coated by a black substance, appearing as if they had been polished with plumbago. The layer is of extreme thinness; and on analysis by Berzelius it was found to consist of the oxides of manganese and iron. In the Orinoco it occurs on the rocks periodically washed by the floods, and in those parts alone where the stream is rapid; or, as the Indians say, ‘the rocks are black where the waters are white.’ ” (Voyage of the Beagle)

Darwin is describing a type of varnish that forms on the surface of the rocks.  As he points out, it is commonly composed of a combinations of manganese oxide and iron oxide (the first being blackish, the later being more brown or red).

This varnish is frequently found in arid or semi-arid regions, where it is called desert varnish.  It forms where a thin layer of very fine-grained, wind-blown clay accumulates on the rock surface. The clay, in combination with periodic moisture, acts like fly paper and captures very small grains of manganese and iron oxides. The heat of the sun then helps “bake” the varnish. On the Colorado Plateau of the SW US, where rock varnish is common, some Native American peoples carved petroglyphs into the varnished surface that is still visible today.

Petroglyphs carved in varnish in the Valley of Fire, Nevada (Wikipedia commons):


Of course, the area around Bahia is not a desert, and the river banks and beaches Darwin describes are rather wet, so this may be a slightly different type of rock varnish. Some studies suggest that under these conditions microbes may play a role in facilitating the deposition of manganese oxide on the rock surface.

So in the end, Darwin’s comment that, “the origin, however, of these coatings of metallic oxides, which seem as if cemented to the rocks, is not understood” is still somewhat true.  We have a better idea today about how varnishes form but we have yet to fully understand the process.

There are two additional references in Darwin’s description that are worth mentioning – plumbago and Berzelius.

Plumbago is the name of a genus of tropical plants (named after the latin word for lead (plumbum).  (Yes – that is where the word “plumber” comes from, too.). This seemed a bit odd, until I found that one of the characteristics of this plant is that it secrets a sap that stains the color of lead (i.e,, black). Because it stains skin, some cultures have even used it for tattoos.  So I think Darwin is suggesting the surface looks like it has been polished with a black stain.

Plumbago auriculata – Cape Leadwort (from Wikipedia Commons):


Darwin’s reference to Berzelius, must be to Jöns Jacob Berzelius, a Swedish chemist who was involved in working out how to write out chemical formulas (e.g.,  MnO2 – the formula for manganese oxide). Again I am interpreting here, but it would appear Darwin sent his samples to Berzelius for analysis upon his return to England. Either that, or Berzelius had done earlier work on the subject.  If anyone knows the answer to this, please let me know.

When Mars Pathfinder roamed the surface of Mars in 1997, it found desert varnish on the surface of Mars.  Considering that it often forms in the prescience of water, and possibly in the presence of microbes, I can only wonder what would Darwin have had to say about that! (RJV)


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