Well, I just reviewed the book I was using as a reference for this question and my answer. The title of the book I'm citing is, A Trout & Salmon Fisherman For 75 Years(1946, by Edward R. Hewitt (1866-1957). On pages 15 through 20 he describes the trip out west he was taken on by his father. The trip took place in 1881 when he was only 15 years old. I should add that Mr. Hewitt was born with a 'silver spoon' in his mouth and his family was extremely well-connected. Yellowstone "at that time was a complete wilderness which had not yet ben opened to the public". Among the party was a Senator who was also the Secy of the Interior, an Army General, and a Captain. They traveled to the end of the Northern Pacific RR at Billings, Montana and hooked up with General Phil Sheridan. According to the author, General Sheridan assigned 30 cavalrymen to accompany some of the party about 300 miles to Yellowstone National Park. Mr. Hewitt describes his fly fishing in Yellowstone River, Boulder Creek and the Snake River. He describes the immense number and weight of Cut-throat trout that were caught and used to feed the entire party as well as Indians at an encampment. He also writes that he again visited Yellowstone in 1914, fishing the area waters as well as the Madison and then fished some spring water at the HF Bar Ranch in Wyoming.
Hewitt listed members of the party: His father, Sir John Pender (business owner), Senator/Secy of Interior, Mr. Bayard, Gen. Lloyd Bryce, Capt. Gorringe who brought over and set up the obelisk in Central Park, and of course the young Mr. Hewitt.
Very interesting book.