Assessment work
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
Under the mining laws of the United States, the holder of an unpatented mining claim on the public domain is required, in order to hold his claim, to do labar or make improvements upon it to the extent of at least one hundred dollars in each year.
Rev. St. U. S. § 2324 (U. S. Comp. St. 1901, p. 1426).
This is commonly called by miners "doing assessment work."