Daniel T. Potts of Montgomery County, Pennsylvania, one of Sublette’s men on this expedition, is now identified as the long-mysterious author of the letter which first appeared in the Philadelphia Gazette and Daily Advertiser, September 27, 1827, reprinted in Niles’ Register of October 6, which contains the earliest known description of any portion of present Yellowstone National Park by an American. The original document came to light in 1947 when Mrs. Kate Nixon and Miss Anne G. Rittenhouse of Washington, D. C., collateral descendants of Potts, made themselves known to officials of the National Park Service. It has been fittingly acquired for posterity by the Yellowstone Library and Museum Association at Mammoth Hot Springs. The cover is addressed to “Mr. Robert T. Potts, High Street, Philadelphia” and stamped “St. Louis, Missouri.” Dated July 8, 1827, at the “Sweet Lake” or Bear Lake (Utah) rendezvous, it describes how the Potts party, no members of which are identified, went north after the Salt Lake rendezvous of 1826: A few dass sinci our trader arived by whom I received two letters one from Dr. Lukens the other from yourself under date of January 1827 which gives me great congratulation to hear that you are both happy wilst I am unhappy also to hear from my friends shortly after writing to you last year I took my departuri for the Black-foot Country much against my will as I could not maki a party for any other rout. We took a northerly direction about fifty miles where we cross Snake River or the South fork of columbia at the forks of Henrys & Lewis’s forks at this place we was dayly harrased by the Black-feet from thence up Henrys or North fork which bears North of East thirty miles and crossed a large ruged Mountain which sepparates the two forks from thence East up the other fork to its source which heads on the top of the great chain of Rocky Mountains which sepparates the water of the Atlantic from that of the Pacific. At or near this place heads the Luchkadee or Calliforn Stinking fork Yellow-stone South fork of Massuri and Henrys fork all those head at an angular point that of the Yellow-stone has a large fresh water lake near its head on the verry top of the Mountain which is about one hundrid by fourty miles in diameter and as clear as crystal on the south borders of this lake is a number of hot and boiling springs some of water and others of most beautiful fine clay and resembles that of a mush pot and throws its particles to the immense height of from twenty to thirty feet in height The clay is white and of a pink and water appear fathomless as it appears to be entirely hollow under neath. There is also a number of places where the pure suphor is sent forth in abundance one of our men Visited one of those wilst taking his recreation there at an instan the earth began a tremendious trembling and he with dificulty