On a remote tropical island in the Indian Ocean lies a geologic enigma. Some 4 million years ago, volcanic eruptions on the seabed piled lava upward almost two miles, until it broke above the waves. Then it kept piling up, to form what is now the craggy, densely vegetated island of Anjouan. Like all islands formed this way (think Hawaii) Anjouan is 100 percent dark volcanic basalt. Except for the part that is not. That part—a mass of pure white quartzite, apparent remains of a river or beach deposit formed on some faraway, long-ago continent—is not supposed to be there.