delivered the opinion of the court.
There is no doubt that by the terms of the contract the plaintiffs were bound to make all the excavations necessary to grade the road, of whatever materials they were composed; and, had they agreed to make the excavations at a certain price, they would have been bound to do it, notwithstanding the material of the excavation was different from what it was supposed and the removal of it was attended with a greatly increased cost. This is the principle of the case of Boyle v. Agawam Canal Co. 22 Pick. 381, relied on by the defendant, and in which we entirely concur. This is a familiar principle and applicable to all contracts of the kind.
But the question in this case is, not what work the contract contemplated was to be performed, but what was the price to be paid for it. It is scarcely necessary to observe *376that the terms of a written contract can not be varied or added to by parol evidence. But this principle does not prevent the showing that a contingency has arisen not provided for in the contract. The evidence of the engineer satisfies us of the correctness of the view we take of this contract. He says it was supposed that the excavations would consist of ordinary earth; that among engineers and contractors there was a marked distinction between common earth and indurated or cemented earth and gravel; and that earth excavation would not be held to include cemented gravel; that it was worth more to excavate the latter than the former.
By the contract, sixteen cents for the cubic yard was the price agreed upon for earth excavations. It is clear from the evidence that this was the usual price of excavating ordinary earth, for all the witnesses testify that the excavating of in-durated earth or gravel was worth a sum greatly exceeding the cost of excavating ordinary earth. It would thus appear that the defendants did. not make a contract involving any hazard. They did not for a given sum undertake to do the excavation, assuming the risk of the material of which it was composed, for the price fixed was such a one as was usually paid for removing ordinary earth. The contract then only fixed a price for excavating ordinary earth. As it was supposed that there would be nothing but ordinary earth to remove, the contract only provided a price for such excavations, leaving excavations of a more costly material unfixed as to price, as it was not supposed that there would be any such to bo made. This view of the subject is fully shown by the evidence of the engineer of the company, who surveyed the road in company with one of the plaintiffs, and who we must suppose disclosed all his knowledge to those who employed him.
Although by the terms of the contract the defendants were bound to make all the excavations necessary for grading the road, of whatever material they were composed, yet it does not follow that the price for removing one kind of material was that agreed upon for every kind. The obvious and *377most natural sense of tbe words “ excavation of earth,” is the ordinary earth; for, if it was intended by the word “ earth” to include all materials whatever found beneath the surface of the ground, there would have been no necessity for any specification of the material, for the word “ excavation,” without any other limiting its signification, would have been most appropriate to express the minds of the parties. The expression of one thing is the exclusion of another. As among contractors and engineers the terms “ excavation of rock,” “ excavation of indurated earth or gravel” and “ the excavation of earth,” meaning ordinary earth, were known, signifying different kinds of work varying greatly in cost, can any thing be plainer than that the use of one of these terms is an exclusion of the other ? The law appropriates no such signification to the terms “ excavation of earth,” nor are they technical words whose sense may not be controlled by extrinsic evidence when they are used in connection with any particular trade or business. So the ordinary and customary sense of the terms of the contract are against the defendants.
If no price was fixed for the excavation of the indurated gravel, then it was not within the contract, and the engineer had no authority to make an estimate of it. The defendant would be entitled to recover what it was reasonably worth. The engineer allowed extra compensation for excavating a portion of the indurated gravel because he thought it was right. If it was right to pay for a part, why was it not right to pay for the whole ? If the estimate of the engineer was conclusive, as was contended for the defendant, we do not see on what ground the plaintiffs did not recover the amount of it, if it was right in part. From the evidence of the engineer it appears that he was of the opinion that it would be right to pay the plaintiffs an extra compensation for excavating the indurated gravel.
We are of the opinion that the clearing of the road-bed admits of different considerations from those arising out of the excavating the indurated gravel. The trees standing in *378the road were visible and it was known that they must be removed. The specifications forming a part of the contract provided that the trees should be removed, and the excavation of the earth could not be done without first taking them away.
The other judges concur.Reversed and remanded.