The Robert Lenox Belknap papers are a fragment saved by his descendants. The papers include Belknap's private letterbooks for the final years of his career (1892-1895), although thirty-three earlier volumes have been lost. They include both business and personal correspondence and give a good picture of the life of a New York financier of the second rank.

Belknap travelled extensively until the last year of his life managing his various enterprises. Many of the letters are written to and from the field between Belknap and W. P. Stevenson, who appears to have been his confidential secretary. The letters cover Belknap's activities in the West Superior enterprises and the street railways in Asbury Park and Indianapolis, as well as his later work on behalf of the National Guard, the Presbyterian Church, and several social clubs. There are several letters concerning his summer homes at Sea Bright, New Jersey, and Huntington, Long Island.

There is also a separate inventory of Mr. and Mrs. Belknap's assets and liabilities in 1872, when he was just starting his business career.

The largest part of the papers concerns the Kesler Mining Company, a silver-mining venture at Big Cottonwood, Utah, about twenty-three miles from Salt Lake. This firm was organized in June 1879 after New York investors purchased a bankrupt mine and claim. It was one of Belknap's early ventures and not particularly successful. The papers cover the organization of the firm from 1878 to 1881, with scattered materials down to 1895. Of particular interest are a series of detailed reports and statements sent by Ellsworth Daggett, the company's agent and superintendent. They describe the difficulties in operating a small western metal mine under absentee ownership.

Other correspondence is between Belknap and the firm's Utah bankers and with members of the New York law firm of De Forest & Weeks, who were associated with Belknap as stockholders and officers. There are maps and sections showing the mine workings, payrolls giving names, occupations and wages of people employed at the mine, legal papers concerning disputes over claims, and a geologist's report.