Addressing this precise issue, the lessors contend: (1) "marketable" is an unambiguous word of art not requiring the consideration of any parol evidence; (2) that allowing testimony of prior negotiations varies the terms of the lease in contravention of the parol evidence rule. Whatever significance the lessors attach to this lease possibly being a sale of coal in place, see, e.g., Walter Estate, 437 Pa. 544, 265 A.2d 368 (1970), clearly ignores both the realities of strip mining and the "marketable" clause. While the parol evidence rule precludes, in the absence of fraud, accident or mistake, the admission of prior or contemporaneous agreements of the parties that alter, contradict or vary the terms of their writing, "it is well established that, where a written instrument is ambiguous, either party may produce oral evidence to resolve the ambiguity, such evidence being admitted, not to add to or detract from the writing, but merely to ascertain the meaning of the parties.
Smith, supra, 347 Pa. at 302, 32 A.2d 227. Potts Run's interests under the lease may be described as personal property in the form of a right to royalties, and a contingent right of entry upon condition broken. Essex Coal CompanyAppeal, 411 Pa. 618, 192 A.2d 675 (1963); Walter Estate, 437 Pa. 544, 265 A.2d 368 (1970). Neither of these interests constitute an estate in land.