Mystery Photo 38 shows a donkey figurine in an exhibit at the Fishing Bridge Museum & Visitor Center in Yellowstone National Park.  Like other objects in the exhibit, the little donkey has a thin white coating of travertine that it acquired by being intentionally exposed to the calcareous waters of Mammoth Hot Springs back in the early days of Yellowstone tourism when the practice was allowed.   

Travertine, a light-colored porous calcite (CaCO[sub]3[/sub]), is deposited from solution quite rapidly on objects that are immersed or sprayed with water from some hot springs, such as those at Mammoth.  Objects like those in this exhibit could become completely travertine-encrusted in several days.  In the 1880s, when travertine-coated items were sold as souvenirs, many Yellowstone tourists purchased or brought along items that were coated at Mammoth Hot Springs while they toured the park, an experience that typically lasted about five days.  Commemorative pieces with names and/or dates were among the popular choices.