There are geological mysteries in Charles Darwin’s works. I’m still reading Geological Observations on South America, off and on between other things, and there are times when I want to poke him in the chest and say, “Ha! I know exactly why that is! You see, there’s this thing called plate tectonics… and that’s why you’re seeing those fossils correlate when they shouldn’t possibly!”

Of course, he’s been dead for 130 years, so it’s a little hard to get his attention. I’m rather sure, after having sampled a broad swath of his work, that if he were alive today, I wouldn’t be explaining a damned thing to the man. He combined native genius with keen observation and a rather obsessive evidence-collecting habit, and though he scoffed at the idea of continents sailing around way back when there wasn’t nearly enough evidence to make such a thing plausible, I think today he’d be one of those scientists looking at the more intractable bits of plate tectonic theory, rearranging a few things, giving others a twist, and then handing us a revolution in understanding. It’s not his fault he lived long before colliding continents became a probability, or that evolution distracted him from his geological observations. Okay, yes, the last was his fault, but still.)

Anyway. Mysteries. Right. There are a few I can solve. There are many more I can’t. I’m not well-versed enough in the various sorts of rocks to figure out strata that puzzled him simply from his description. In Chapter V, I came across a teaser. It’s in a footnote, and in the interests of geological detection, I’ll reproduce paragraph and puzzle here:

At the northern extremity of the island, near S. Carlos, there is a large volcanic formation, between 500 and 700 feet in thickness. The commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal with calcareous spar and bole: most even of the darkest varieties fuse into a pale-coloured glass. The next commonest variety is a rubbly, rarely well characterized pitchstone (fusing into a white glass) which passes in the most irregular manner into stony grey lavas. This pitchstone, as well as some purple claystone porphyry, certainly flowed in the form of streams. These various lavas often pass, at a considerable depth from the surface, in the most abrupt and singular manner into wacke. Great masses of the solid rock are brecciated, and it was generally impossible to discover whether the recementing process had been an igneous or aqueous action.* The beds are obscurely separated from each other; they are sometimes parted by seams of tuff and layers of pebbles. In one place they rested on, and in another place were capped by, tuffs and gritstones, apparently of submarine origin.

* In a cliff of the hardest fragmentary mass, I found several tortuous, vertical veins, varying in thickness from a few tenths of an inch to one inch and a half, of a substance which I have not seen described. It is glossy, and of a brown colour; it is thinly laminated, with the laminæ transparent and elastic; it is a little harder than calcareous spar; it is infusible under the blowpipe, sometimes decrepetates, gives out water, curls up, blackens, and becomes magnetic. Borax easily dissolves a considerable quantity of it, and gives a glass tinged with green. I have no idea what its true nature is. On first seeing it, I mistook it for lignite!

This is on Chiloé Island, just off the coast of Chile. And it’s intriguing. What is it that he saw? Is his description enough to identify it? Your mission, should you choose to accept it, is to solve Darwin’s geological mystery.

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Good luck, geos!