Thursday, July 21, 2005

A wreck of a world

The tip of Ireland's Dingle Peninsula is the westernmost point of Europe. There was a time, some 200 million years ago, when I could have walked home from here without getting my feet wet. As everyone now knows, the ocean that separates Europe and North America is a relatively recent artifact of the slip and slide of the Earth's crustal plates.

But here's something I bet you didn't know.

In July 1747, Ben Franklin wrote to Jared Eliot, a Connecticut clergyman: "The great Appalachian Mountains, which run from York River back of these Colonies to the Bay of Mexico, show in many Places near the highest Parts of them, Strata of Sea Shells, in some Places the Marks of them are in the solid Rocks. 'Tis certainly the Wreck of a World we live on!"

And what caused this "wreck" that heaved the floors of oceans high into the air, lifting sea shells to mountain peaks? Franklin found other clues during his travels in Britain. In a coal mine at Whitehaven in northern England he observed the leaves and branches of ferns impressed upon slates which formed the natural roof of the mine, deep beneath the present surface of the Earth. Elsewhere in England he found oyster shells mixed with the rocks of a mountain top. Evidently, surface marshes had been depressed and the ocean floor thrust upwards.

Franklin wrote: "Such changes in the superficial parts of the globe seemed to me unlikely to happen, if the earth were solid to the centre. I therefore imagined, that the internal parts might be a fluid more dense, and of greater specific gravity than any of the solids we are acquainted with, which therefore might swim in or upon that fluid. Thus the surface of the globe would be a shell, capable of being broken and disordered by the violent movements of the fluid on which it rested."

This strikes me as a pretty accurate description of the fundamental notion of plate tectonics, written a generation before James Hutton founded the modern science of geology with his "Theory of the Earth" in 1785, and more than two hundred years before the theory of plate tectonics changed our way of thinking about the Earth.

I came across this info in Ronald Clark's biography of Franklin, but I don't recall ever seeing it mentioned in the scientific literature. Does anyone know of a geology text that gives the great man credit?