SE of Vermillion, SD The boats sail twenty miles stopping southeast of present-day Vermillion, South Dakota. Lewis is overcome by caustic fumes and when the captains poll the men for a new sergeant, Patrick Gass receives the most votes.
Caustic Fumes
Capt lewis was near being Poisened by the Smell in pounding this Substance I belv to be arsenic or Cabalt. [He] 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
—William Clark
Mineral Specimens
This Bluff contain Pyrites alum, Copperass & a Kind Markesites . . . . arsenic or Cabalt . . . . Cops. ans [copperas] and almin [alum] pure & Straters of white & brown earth of 6 Inch. thick.
—William Clark