Orson Pratt recounts Joseph's teachings concerning the lost ten tribes; concludes that he is not sure if Joseph was speaking by revelation or "it was a matter of mere speculation."
Orson Pratt, letter to John Hall, December 13, 1875, Historian's Office correspondence files, 1856-1926 , CR 100 394, Church History Library
The Prophet Joseph once in my hearing advanced his opinion that the ten tribes were separated from the earth; or a portion of the earth was by a miracle broken off, and that the ten tribes were to him away with it, and that in the latter days it would be restored to the earth or be let down in the Polar regions. Whether the Prophet founded his opinion upon revelation or whether it was a matter of mere speculation with him, I am unable to say.