Pumice Stone

En route down the Yellowstone River in August 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]Moulton, ed., Journals, 8:414.

William Clark was perhaps relying on Owen’s Dictionary, published in 1754, which was among the books in the small reference collection Meriwether 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]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 … Continue reading 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]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 … Continue reading

Geological Description

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]Owen’s, s.v. Pumice. (Today’s toothpaste uses a milder abrasive, finely ground chalk—calcium carbonate).

Jefferson’s Observations

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]In Merrill D. Peterson, ed., The Portable Thomas Jefferson (New York: Penguin Books, 1975), 50. Jefferson’s Notes was first published in 1785.

Perhaps his instruction to Meriwether Lewis to watch for volcanos was motivated by his curiosity about pumice. In this connection, let us look at the specimen of the material that Lewis collected.

Lewis’s Specimen

Among the numerous papers and collections shipped back east from Fort Mandan on the barge (called the ‘boat’ or ‘barge’ but never the ‘keelboat’) in April 1805 was a mineralogical collection of sixty-eight specimens. Lewis’s list of the collection’s contents is among the “Fort Mandan Miscellany,”^[6]Moulton, ed., Journals, 3:472-378. but only seven of the existing specimens remain, and only one of those, a “Petrified Jawbone of a fish” found by Sergeant Gass on the Missouri River in August 1804, still has the original label attached. The provenance of the rest cannot be verified.

Three of the sixty-eight descriptions in Lewis’s list are of interest in the present context. Number twenty-four was “Carbonated wood found on the Std. side of the Riv near fort mandane 60 feet above high water mark in the Bank Strata 6 Inch thick.” Number sixty-four was “Specimen of Carbonated wood with the loose Sand of the sand-Bars of the Missouri & Mississippi, it appears in considerable quantaties in many places.” The whereabouts of those specimens are currently unknown.

The third is number sixty-two. Lewis’s annotation in the list reads:

Specimen of the pummice stone found amongst the piles of drift wood on the Missouri, sometimes found as low down as the mouth of the osage river. I can hear of no burning mountain in the neighborhood of the Missouri or its Branches, but the bluffs of the river are now on fire at Several places, particularly that part named in our chart of the MissouriThe Burning Bluffs.^[7]This may refer to either the “Mineral Bluffs” or the “Hot Bluff” shown upstream from the Big Sioux River, on the map Clark drew at Fort Mandan during the winter of 1805. the plains in many places, throughout this great extent of open country, exhibit abundant proofs of having been once on fire . Witness the Specimens of Lava and Pummicestone found in the Hills near fort mandon. . . .

Unfortunately, the association between that description and the specimen pictured above is only anecdotal.

Clearly, Lewis confused cold coal clinker with pumice. But the riddles of the earth’s crust, and of the fire at the core, were yet to be solved by geologists—who in the decade of the expedition had not even agreed upon the name of their science.