En route down the Yellowstone River in August of 1806, William Clark wrote about it:
The Pumies Stone which is found as low as the Illinois Country is formed by the banks or Stratums of Coal taking fire and burning the earth imedeately above it into either pumies Stone or Lavia. This Coal Country is principly above the Mandans.1
Clark was perhaps relying on Owen's Dictionary, published in 1754, which was among the books in the small reference collection Lewis had assembled in Philadelphia to take along on the expedition. Owen's defined pumice as "a flag or cinder [residue of burning] of some fossil, originally bearing another form, and only reduced to this state by the action of the fire, though generally ranked by authors among the native stones."2 Lewis had also purchased a copy of the second edition (1784) of Richard Kirwan's Elements of Mineralogy, which probably was of only marginal use on this point. Although Kirwan had been largely responsible for establishing the new science of mineralogy on a solid basis, Elements was primarily a laboratory manual, and Lewis and Clark lacked the chemicals and instruments to test their specimens.3
Actually, pumice is not related to coal at all. It is superheated lava spewed into the air and cooled so quickly that the mineral component forms threads and fibers, which trap bubbles of air. Depending on what minerals are in the lava, its color ranges from white to gold to brown and even, in Hawaii, black. With all those bubbles inside, it is comparatively light in weight, and when it falls in water, or is washed into a river by rain or flood, it may float many miles from the point where it fell to earth. And that is why Lewis and Clark could find pumice far from any coal beds.
Easily ground into powder, pumice has been used throughout human history as a mild abrasive, and today it is also used in plaster, concrete, and acoustic tiles. Small pumice rocks are sometimes used to smooth and soften caloused skin. In Owen's dictionary, Clark could read that "the great use of pumice among the antients [sic], seems to have been as a dentifrice, and at present it is retained in the shops on the same account."4 (Today's toothpaste uses a milder abrasive, finely ground chalk—calcium carbonate).
Actually, Thomas Jefferson almost had it right, as he wrote in Notes on the State of Virginia:
A substance supposed to be pumice, found loating on the Missisipi [sic], has induced a conjecture, that there is a volcano on some of its waters: and as these are mostly known to their sources, except the Missouri, our expectations of verifying the conjecture would of course be led to the mountains which divide the waters of the Mexican Gulph from those of the south Sea; but no volcano having ever yet been known at such a distance from the sea, we must rather suppose that this floating substance has been erroneously deemed pumice.5
Perhaps his instruction to Meriwether Lewis to watch for volcanos was motivated by his curiousity about pumice. In this connection, let us look at the specimen of the material that Lewis collected.
1. Moulton, ed., Journals, 8:414.
2. A New and Complete Dictionary of Arts and Sciences; comprehending all the branches of useful knowledge, with accurate descriptions as well of the various Machines, instruments, tools, figures, and schemes necessary for illustrating them, as of the classes, kinds, preparations, and uses of natural productions, whether animals, vegetables, minerals, fossils, or fluids; together with the kingdoms, provinces, cities towns, and other remarkable places throughout the world. Illustrated with above three hundred copper-plates, Curiously engraved by Mr. Jeffreys, geographer and engraver to his Royal Highness the prince of Wales. The whole extracted from the best authors in all languages, by a Society of Gentlemen. London: Printed for W. Owen, at Homer's Head, in Fleet-street.1754, Vol. III, Part II, 2592. Noah Webster's Compendious Dictionary of the English Language (1806) defined pumice simply as "a spongy stone full of holes."
3. Kirwan had freed "the gross indications of the unassisted senses" from their attendant fallacies by rendering "more refined chemical tests" more conclusive. The appeal of his work to the gentlemen of Jefferson's circle is reflected in Kirwan's preface to the second edition: "Nations in the full enjoyment of the most considerable natural advantages" had begun paying fuller attention to those resources, and "many have been immediately rewarded by the discovery of substances within their own territories, before imported with great difficulty and expence from other countries." Richard Kirwan, Elements of Mineralogy, 2 vols, 2nd ed. (London: Printed for Elmsly, in the Strand, 1796), v. Lewis and Clark were compelled to rely mainly on those old "gross indications of the unassisted senses."
4. Owen's, s.v. Pumice.
5. In Merrill D. Peterson, ed., The Portable Thomas Jefferson (New York: Penguin Books, 1975), 50. Jefferson's Notes was first published in 1785.
Funded in part by a grant from the Montana Committee for the Humanities