A subsequent written contract upon the same subject matter merges a prior oral one even though it has been performed in part. ( Housekeeper Pub. Co. v. Swift, 97 Fed. 290; Denny-Renton Clay Coal Co. v. Johnson, 88 Wn. 251, 152 P. 1017; Goss v. Northern Pac. Hospital Assn. of Tacoma, 50 Wn. 236, 96 Pac. 1078; McCabe Const. Co. v. Utah Const. Co., 199 Fed. 976; Spreckels Bros. Co. v. Bender, 30 Or. 577, 48 P. 418; Bourn v. Dowdell, 5 Cal. Unrep. 820, 50 P. 695; Sherman v. Sweeney, 29 Wn. 321, 69 P. 1117; 4 Page on Contracts, last edition, par. 2492, p. 4399.) The question of merger is to be determined from the instrument and is a matter for determination by the court.