Posted by: Rob Viens | June 1, 2013

Geologizing Around Minas

Darwin would continue to describe the birds he encountered in Uruguay, but for the next couple of days his focus seemed to turn “downward” – to the rocks. It seems only natural considering he was in a town named after mines.  On the 13th and 14th he wrote:

“In the evening arrived at a Pulperia North of the Rio Polanco. — it was my furthest point: its distance in a straight line from Maldonado is not much more than 70 miles; but this distance was much lengthened by our route.— I here saw what I wanted in the geology & in the morning returned to near our former sleeping place” (May 13)

“the country continues very much the same; it was about the Polanco more level & the hills less steep & there were a few trees about the rivers, chiefly of the willow kind.” (May 14)

Darwin’s descriptions in his geologic notebooks suggest that the rock around Minas was similar to the type of rock he has been seeing up and down the coast of South America – old metamorphic rocks of the continental shield. In particular he notes the presence of “pale & blue slate”, “fine white marble”, “imperfect gneiss”, and an abundance of “quartz” (possibly quartzite?) – all metamorphic rock.  As you can see from the geologic map below (which maps only the age of the rocks), the region around Minas is Precambrian in age (something Darwin would not have known in his time).

Geologic Map of Uruguay from

geologic map of Uruguay

The Precambrian is a geologic time period that encompasses more than 80% of the history of the Earth (ranging from its origin about 4.6 billion years ago, to the beginning of the Phaneozoic Eon about 545 million years ago).  That certainly covers a lot of ground! Research suggests that the rocks in this part of Uruguay are part of the Río de la Plata Craton and are between about 1700 and 1800 million years old – a little less than half the age of the Earth. The Río de la Plata Craton forms the foundation of a large part of Uruguay, however, it is only in the southern part of the country where it is exposed at the surface (what geologists call the “continental shield”).  In other areas it is covered by younger sedimentary and igneous rock (what we call a “continental platform”). For more on cratons and continental shields be sure to see Geologizing on the South American Craton.

On a side note, Darwin also mentions the geologic reason that the town is called Minas:

“A few years since Gold mines were opened in several places, in these mountains from the very small quantities found. the works have ceased. … I believe Las Minas takes its name from some gold found in alluvial beds.” (Geological Notebooks)

Whenever you have miners, geologists and cowboys you can be sure that trouble is afoot.  Apparently this stereotype (at least through Darwin’s European filter) help true in Uruguay, too:

“In the evening I saw rather a curious scene: an old Paraguay man, who had been our guide in the morning, got very drunk, & being offended at a man present was drawing his knife under his poncho; a Gaucho who sat by him knew what he was about & stopped him, & took his knife from him. — After this, to frighten the old gentleman, the others in jest pretended to stab him. — the method with which they dashed across the room, struck him upon the heart & then sprang out of the door, showed it to be the result of practice, at least in them.— The only manner of fighting amongst the Gauchos is thus stabbing each other; & this little scene showed me very plainly the way in which it takes place. I wear a large clasp-knife, in the manner of sailors fastened by a string round my neck; I had often noticed that the Gauchos seemed to think this practice of confining the knife very strange.” (May 14)

The next few days would find Darwin waiting out the weather with his new knife-wielding friends… (RJV)

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