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HUNDRED thousand years ago, when the Gulf of Mexico extended up the Mississippi Valley to the mouth of the Ohio, and the ice-sheet covered New York, there was no need of digging a Panama Canal, for there was no Isthmus of Panama. Instead, a broad strait separated South and Central America, and connected the Atlantic and Pacific oceans. This was the strait that the early European navigators were to hunt for in vain, for long before their time it had been filled up, mainly by the lava and ashes poured into it by the volcanoes on its banks.

But though the formation of the Isthmus is for the most part volcanic, it has very few volcanoes of its own, and all of these have been extinct for untold centuries. The so-called volcano in the Gaillard Cut, about which so much was once said in the American newspapers, was nothing but a small mass of rock that had become heated in a curious and interesting way. The intense heat of the sun—thermometers have registered a hundred and twenty degrees in certain parts of the Cut at noon—caused the spontaneous combustion of a deposit of