This is a postcard my dad received in 1921. It does not say who sent it. The White part of Mammoth Hot Springs is from the calcium carbonate in the hot water that flows over Mammoth Hot Springs daily.
Postcard was mailed at Yellowstone post office June 27, 1921. Too bad it was not signed. It may be from Duane & Carrie Taylor, my dad's older sister and brother in law. They got married a week before this and were honeymooning in Yellowstone after the marriage.
The Festival of Postcards can be found here.