9th Interrogatory—In what manner have you estimated for that portion of the work, which will be tunnelled? In what time can said Tunnel be constructed? Must not your estimates for said Tunnel be conjectural? And may not a difference in the formation through which said Tunnel passes, vary the expense many thousands of dollars—that is to say, should the formation be entirely granite rock, will not the expense be much greater than if it should prove to be of clay, limestone, slate or coal?
Answer—With the same care that we have used in forming other parts of our estimates...A difference of the formation would of course, affect the expense: it might increase, or it might diminish the cost, several thousand dollars. I can say that it is not only improbable, but that it is impossible that we shall meet with granite in the constructions of the tunnel, the geological character of the country forbids it.
10th Interrogatory.—Upon what description of formation or strata have you based your estimates for the tunnel, and what certain reasons have you to suppose, that particular formation or strata exists, upon which you have based your estimates?
Answer—Our estimate is based upon a formation of clay, slate and sandstone, in layers, alternating; and occasionally earth, clay, slate, predominating. The Direction of the tunnel and that of the strata form an angle of about 29 degrees. We cannot, I think, be mistaken in the strata that we expect to meet with. The Potomac, for a few miles above, and for several miles below, has its channel back and forth repeatedly across the strata in the direction of the tunnel, these strata we find invariably in the same relative position, parallel to each other, and in the same continuous lines. (Emphasis added)