Opinion by

Plaintiffs offered in evidence their abstract whereupon defendants admitted title in Hartzell. Plaintiffs then proved the lease by Hartzell to them, their entry under it, and the putting down of one well. They thus made out a complete prima facie case, which called on defendants to show a better title. But it is said, and the court below appears to have adopted this view, that plaintiffs went further and gave evidence of an abandonment of their possession. This however will not support a compulsory nonsuit. Abandonment is a mixed question of acts and intention, and therefore is ordinarily for the jury, and there is nothing in this case to take it out of the rule, for the abandonment was denied in plaintiffs’ abstract, and was therefore incumbent on defendants to prove before they could avail themselves of it. Whether the evidence on the part of plaintiffs showed an abandonment by them did not depend exclusively on the length of time that operations had ceased but also on the intention, and that again was largely dependent on the •agreement and understanding of the parties. The plaintiffs ■denied the intention and the court therefore could not declare the abandonment as a matter of law but should have left it as ■ a fact to the jury.

The plaintiffs further offered evidence by Burton and Hart-.zell to show the agreement of the parties as to due diligence • and abandonment. This should have been admitted. It was •not in any sense an attempt to modify or alter a written contract. The lease provided that the work when commenced ■should “be prosecuted with due diligence until completion or •abandonment,” but what should constitute due diligence was .a question of fact on which the parties might agree, either at the time or afterwards, and if they did, then neither court nor jury ¡had any right to adopt a different standard. If due diligence lhad been defined in the writing no pretence of such right could ihave been set up. But parol proof of the understanding and ■agreement of both parties was just as effective as a writing, for it went not to a construction of the paper, but to the definition ■of a fact referred to. Whether any such understanding not *329expressed in the paper would affect others subsequently acquiring rights from one of the parties, without notice, we need not consider, for there is no evidence that any such others are before us. The ease was not allowed to get that far.

But the nonsuit was erroneous for another reason. Even if there had been a want of due diligence and an abandonment by plaintiffs, no one could take advantage of it but the lessor or one succeeding to his rights, and there was no such party yet in the case. So far as the evidence showed, defendants had neither Hartzell’s title nor any other beyond that of mere squatters. As against them plaintiffs as already shown had made out a prima facie case. The error of the appellees, and apparently of the court below, was in regarding the estate of plaintiffs under the lease as terminated ipso facto by the failure in diligence or the abandonment. But it required the act of the lessor. Repeated decisions of this court from Wills v. Gas Co., 130 Pa. 222, to Cochran v. Pew, 159 Pa. 184, have established that the clause of forfeiture or termination of the estate is for his benefit, and no act of the lessee can produce that result without his concurrence. Still less can any action of a stranger enforce it. As against any but the grantor, an abandonment is not complete until the statutory period of limitation, or the end of the term granted, and possession may be resumed by the grantee at any time previous.

There is nothing in the cases cited, Munroe v. Armstrong, 96 Pa. 307, and Venture Oil Co. v. Fretts, 152 Pa. 451, in conflict with these views. In the former the lessor had expressly declared the forfeiture and made a new lease to the plaintiff, and in the latter he had asserted it by implication by a new lease to parties who defended under his title.

But it is said that appellees here defend under Hartzell’s title, and that their abstract shows the fact. The abstract however was not in evidence as it should have been if relied on for affirmative proof. The practice in the analogous case of statements and affidavits of defence, which under rule of court are to be taken as admissions of all material facts not denied therein, is shown in Neely v. Bair, 144 Pa. 250. The respective abstracts of title in ejectment filed by the parties under the rule of court in the present case, stand upon the same basis, having the same object as stated by the present Chief Justice *330in Neely v. Bair, to narrow the trial down to material matters which are really disputed. The defendants may be in possession under Hartzell’s title, and the case therefore analogous tq Venture Oil Co. v. Fretts, but it has not yet been shown to be so. The issue was determined prematurely.

Judgment reversed, and procedendo awarded.