Watkins and Son (P) agreed to excavate a cellar for Carrig (D) for a stated price. The contract provided that all material was to be removed. There were no contingencies in the contract for rock and D did not mislead P regarding the nature of the grounds upon which the work was to be performed. The parties did not take rock as being a contingency of the contract. When P started to work, he discovered that two-thirds of the space was solid rock. P's manager notified D, and the parties orally agreed that P should remove the rock at a unit price about nine times higher than the unit price for excavating upon which the gross amount to be paid according to the written contract was calculated. When the work was finished, P sought to recover the increased payments. D argued that the oral agreement was not supported by consideration; P's promise was illusory because P had a preexisting duty to excavate the cellar. A referee found that the oral agreement superseded the written contract. That is the parties agreed to rescind the written contract as though it had not been made and entered into an oral one as though it were the sole and original one. D appealed.