Posted by: Rob Viens | May 15, 2013

Discovering Marble and Practicing Patience

Darwin spent a lot of his time in Maldonado out exploring the countryside. One of his first excursions wasn a trip to look at some rocks located about 10 miles west of the city:

“I rode about four leagues into the Camp to the head of a large fresh water lake called Laguna del Potrero. The object of my ride was to see a white marble, from which lime is manufactured. — The day was beautiful & it was a pleasant ride over hill & dale of turf & surrounded by endless flocks of cattle, sheep & horses.” (May 4)

I say “appears to be” for a couple of reasons. First, the only reference I can find to Laguna del Potrero is a small region labeled on the Google Map below (the larger body of water to the North of that location is an arm of Laguna del Sauce):

Map of region west of Maldonao

Secondly, I’m having a hard time finding and reference to a marble quarry in this area, nor can I see one zooming in on satellite views of the area (via my friend Google Maps). However, I do know that there are some classic “marbles” sold as decorative stone for tabletops, etc. that comes from Uruguay.  Is there a connection? Probably not, as the marble found at this specific location seems to be a relatively small deposit that is not well exposed, but you can bet I’m going to say a few things about it anyway.

In the world of decorative stone, almost everything is called either granite or marble. However, as I  often point out to my students, names used for marketing stone are not always the same as rock names used in geology. To geologists, marble has a much more specific meaning – it is a metamorphic rock that forms from the exposure of limestone (a sedimentary rock) to high temperature and pressure. Limestone is made of basically one mineral – calcite (calcium carbonate). So when that rock is metamorphosed, there are not a lot of different elements present to rearrange into new minerals.  So during metamorphism the atoms simply recrystallize, forming new calcite crystals which are, in many cases, coarser then the calcite grains they started out as in limestone. (For a lesson on metamorphic rocks see Old Schist and Other Gneiss Rocks.) 

Interestingly, limestone most commonly forms in relatively shallow, warm ocean water.  That means that anywhere you find limestone or marble today, there was once a shallow tropical ocean.  Something to ponder, considering that you can find these rocks distributed throughout the world today.

Some Uruguay marble that can be found for sale at stonecontact.com.

Marble from Uruguay

Pure marble is white (think about the famous Italian marble used in sculptures). Any color in the rock comes from impurities (e.g., reds from rust minerals) or layers of other minerals. Darwin notes that marble at this location “contains asbestus, large crystals of Hornblende. & mica in planes”. The presence of hornblende and mica (presumably biotite) suggest it may have looked something like the first sample in the picture above.

Darwin’s Geologic Notebooks tells us a little bit more about this particular layer of marble.  In the area west of Maldonado he notes that there is a lot of gneiss (a metamorphic rock that forms under high temperature and pressure). Within the gneiss he describes a layer of “limestone” – almost certainly marble, since it would have been exposed to the same high temperatures and pressures as the surrounding gneiss. Darwin goes on to add in his Geologic Notebook that this small layer of “limestone” “was discovered & worked by the present owner of a Lime-kiln.”  Lime is essentially the term used to describe calcium-bearing oxides (such as CaO). It is frequently extracted from limestone (hence the name) or marble in a kiln – a type of oven that “cooks” the lime out of the limestone.) Today, lime is a main ingredient in cement.

For the next several days, heavy rain kept Darwin from exploring any more rocks.  But it did give him some time to practice something that he frequently mentions was one of his weak points – Spanish:

“During the greater number of these days, there has been torrents of rain & heavy thunder storms. — The whole country is in a state of inundation, even so that many lives have been lost. — the oldest inhabitants have never seen such weather before. — It has necessarily prevented me from making a trip into the country which I had intended to have almost finished by this time. In consequence of these delays & the bad weather I have scarcely been able to set about anything. It anyhow has afforded me some good lessons in being patient & in speaking Spanish.” (May 5-8)

Nicer weather was on the horizon, and soon Darwin would be back on horseback exploring the countryside… (RJV)

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Responses

  1. I am continually amazed on Darwin’s many talents and interests, including geology!

  2. I’ve enjoyed your geology lesson very much-it amuses me to imagine Slovenia with shallow tropical ocean !

    • Thanks Tamara! It may add more to know that the limestones in Western Slovenia are somewhere between 50 and 150 million years old. That would have been the sea covered the area. It is amazing – you can find limestones all over the earth including on Mt. Everest and in the polar regions. It really tells the story of a changing planet!

      • I don’t know which limestones from Slovenia do you mean. I had much fun visiting Dovžanova soteska (http://www.slovenia.info/?ucna_pot=2048&lng=2), where I collected some limestone with leaves in it, but didn’t have luck to find any fossils! But today I’ve learned the Ilex plants occurring in nature are in Slovenia since the time when there was ocean ! This planet is evidently changing 😉


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