Geology and mineralogy, as well as anthropology, botany, and zoology, were part of Meriwether Lewis’s job description when he set out on his expedition to the Pacific Ocean. In the extensive marching orders he received from Thomas Jefferson, Lewis was ordered to document more than just the Native American tribes, plant, and animal life he observed along the route. “Other objects worthy of notice will be the soil & face of the country,” Jefferson wrote. “…the mineral productions of every kind; but more particularly metals, limestone, pit coal & salpetre; salines & mineral waters, noting the temperature of the last, & such circumstances as may indicate their character.” Lewis took these instructions very seriously—so seriously that he endangered his own life.
Clark’s journal entry of August 22, 1804 tells the tale. The Corps of Discovery was passing along some bluffs near present-day Vermillion, South Dakota, when Lewis’s scientific investigations went awry.
the High land near the river for Some distance below. This Bluff contain Pyrites alum, Copperass & a Kind Markesites also a clear Soft Substance which Capt lewis was near being Poisened by the Smell in pounding this Substance I belv to be arsenic or Cabalt.
Capt. Lewis took a Dost of Salts this evening to carry of the effects of (arsenec) or cobalt which he was trying to find out the real quallity.
Lewis had evidently taken a mineral sample from the bluff, and in attempting to analyze it by pulverizing the rock, either inhaled or ingested enough of the resulting powder to poison himself.
So why would Lewis do such a thing? Before leaving for the west, Lewis had studied geology and mineralogy with Thomas Jefferson, Benjamin Smith Barton, and Andrew Ellicott. For reference, he brought along Richard Kirwan’s two-volume Elements of Mineralogy to consult during the expedition. In the absence of the equipment and chemicals necessary to do a proper mineral analysis, smell, and taste were (and still are) legitimate scientific techniques to determine a rock’s composition. But they can also be dangerous.
As Lewis found out, the rocks he ingested contained some poisonous substance. Based on current mineral analysis in the area of the bluffs he was sampling, it is unlikely that the mineral was cobalt, as Clark suspected. However, South Dakota is loaded with pyrite and marcasite, and under the right circumstances, especially when combined with iron sulfides, these two minerals can produce traces of arsenic. Low exposures of arsenic can produce headaches, vertigo, nausea, and acute diarrhea—the last symptom probably not alleviated by the “Dost of salts” Lewis took “to work off the effects of the Arsenic.”
Lewis was lucky. In more serious cases, the symptoms of arsenic poisoning can include difficulty in swallowing, burning pain, vomiting, throat constriction, diarrhea, dehydration, renal failure, liver failure, pulmonary edema, gastrointestinal distress, headache, drowsiness, confusion, delirium, seizures, and finally, death.
In spite of his efforts to purge his system, Lewis was still feeling poorly two days later, though it didn’t stop him from accompanying Clark and a number of other men to see the famous “Spirit Mound” supposedly populated by tiny devils with large heads. Clark wrote in his journal on Saturday, August 25, 1804:
a Cloudy morning Capt Lewis & my Self Concluded to go and See the Mound which was viewed with Such turrow by all the different Nation in this quarter droped down to the mouth of White Stone River where we left the Perogue with two men and at 200 yards we assended a riseing ground of about Sixty feet, from the top of this High land the Countrey is leavel & open as far as Can be Seen, except Some few rises at a Great Distance, and the Mound which the Indians Call Mountain of little people or Spirits this mound appears of a Conic form & is N. 20° W. from the mouth of the Creek, we left the river at 8 oClock, at 4 miles we Crossed the Creek 23 yards wide in an extensive Valley and continued on at two miles further our Dog was So Heeted & fatigued we was obliged Send him back to the Creek, at 12 oClock we arrived at the hill Capt Lewis much fatigued from heat the day it being verry hot & he being in a debilitated State from the Precautions he was obliged to take to provent the affects of the Cobalt, & Minl. Substance which had like to have poisoned him two days ago, his want of water, and Several of the men complaining of Great thirst, deturmined us to make for the first water which was the Creek in a bend N. E. from the mound about 3 miles—
Despite the fatigue and strain of the day, Lewis made it to the top of the hill, as well as walking nine miles back to camp in the blazing heat. He was slowly getting back to his old self, as evidenced by his own journal entry for the day:
on our return from the mound of sperits saw the first bats that we had observed since we began to ascend the Missouri— also saw on our return on the Creek that passes this mound about 2 M. distant S. a bird of heron kind as large as the Cormorant short tale long leggs of a colour on the back and wings deep copper brown with a shade of red. we could not kill it therefore I can not describe it more particularly.