JS refers to the plains of the Nephites, bones as proof of BOM.
Joseph Smith, Letter to Emma Smith, 4 June 1834, The Joseph Smith Papers website, accessed July 26, 2021
The whole of our journey, in the midst of so large a company of social honest and and sincere men, wandering over the plains of the Nephites, recounting occasionaly the history of the Book of Mormon, roving over the mounds of that once beloved people of the Lord, picking up their skulls & their bones, as a proof of its divine authenticity, and gazing upon a country the fertility, the splendour and the goodness so indescribable, all serves to pass away time unnoticed, and in short were it not at every now and then our thoughts linger with inexpressible anxiety for our wives and our children our kindred according to the flesh who are entwined around our hearts;