The shale, schist, and trachyte of the Isthmean ridge is of variable consistence. Many places exhibit friable, seamy strata, disintegrating upon exposure to the atmosphere. A tunnel of the dimensions to admit the passage of ships, when carried through rock of this character, will require a lining of masonry to prevent falling material from obstructing the way. To pass ships with the topmast struck, the intrados of the arch should be 100 feet above the surface of the water. A semi-ellipse with semi-transverse, and conjugate diameters of 100 feet, added to the canal prism of thirty feet in depth, will give an area of tunnel equal to 10,104 superficial feet, or to 1,976,263 cubic yards per mile. [Illustration: PROFILE OF THE HOOSAC TUNNEL] Assuming that the cost of tunneling through the Isthmus can be executed at $10 per cubic yard, we shall have 19,762,630 dollars as the cost of one mile of tunnel. Estimating the excavation alone at present contract price, $5.40 per cubic yard for small tunnels, one mile of ship tunnel will cost $10,670,820. An open canal upon the line of the canal proposed by General Michler, uniting the Atrato with Humboldt Bay, will cost, according to the estimate of that officer, $1,792,202 per mile. This amount, taken from the careful and elaborate estimates contained in General Michler’s report, may be assumed as a basis of comparison of the two proposed methods of intermarine communication, viz.: by uniting the two oceans upon one level by a tunnel, or by means of a moderate number of “lift-locks.” Eight locks, four at each end of the canal, or sixteen locks, eight at each end of the canal, will raise the summit fifty feet above tide in the first case, and one hundred in the second, and will cost eight millions, and sixteen millions respectively. Since two guard locks will be requisite for either method of communication (_i. e._ by “strait,” or canal with lift-locks), their cost should be excluded from the above sums, which are thereby reduced to six millions, and fourteen millions of dollars. These sums are fixed as the probable limits of the cost of a system of lift-locks sufficient to overcome the divide of the Isthmus, and also to supply the reader with a standard, by which he may judge of the merits of different routes. The construction of a ship tunnel is, as has been said, “a herculean task,” and it is not apparent that “the prejudice against it will be removed by the operations at Mt. Cenis.” A moderate number of lift-locks seems preferable to a tunnel of one mile in length, which, in turn, would be more economical than an excessive number of locks. A greater number than we have mentioned may be deemed excessive.