Originally included in the Shoshone Indian Reservation Treaty of 1868, the Hot Springs later gained a reputation for health-giving properties, and the U.S. Congress sought to set aside the area as a national park or reservation. In 1896, acting under congressional authority, Indian Commissioner James McLaughlin negotiated a treaty for the purchase of the Hot Springs, securing the cession of approximately 10 square miles of the reservation to the United States Government for $60,000; participants in the signing included Shoshone Chief Washakie and Arapahoe Chief Sharp Nose. Most of the water for the Hot Springs comes from an underground flow from the Owl Creek Mountains west of this location, where rainfall enters porous rock, filters downward through underground formations, and emerges heated and mineralized by rock and subterranean volcanic gases. The Big Horn Hot Springs is the largest spring, flowing at approximately 3,6000,000 gallons per day at a temperature of 135 degrees fahrenheit. To the west are terraces formed over thousands of years by mineral deposits, chiefly lime and gypsum layers known as travertine, whose coloring comes from many species of algae that thrive in the warm waters.