Entries existing on , and allowed prior to , under the Stock Raising Homestead Act of (Thirty-ninth Statutes at Large, page 862) [ et seq.], for land withdrawn as valuable for oil or gas, but not otherwise reserved or withdrawn, are validated, if otherwise regular: , That at date of entry the land was not within the limits of the geologic structure of a producing oil or gas field.43 U.S.C. 291February 7, 19251925-02-07April 1, 19241924-04-01December 29, 19161916-12-29Provided