The whole length of this adit, from its mouth, which is situated in the valley of Tepetongo, about a league from Tlalpujahua, to the point at which it intersects the great Coronas Vein, will be 5,176 yards. At 4976 yards it cuts the Vein of La Borda, upon which some of the most promising mines of the Company are situated; and, in both instances, a depth of 132 yards below the deepest of the old workings will be obtained. The Gallery is two yards wide, and three and a half high, being constructed in such a manner as to allow of the use of small carts. In order to facilitate its construction, twelve lumbreras, (air-shafts, puits d'airage,) have been opened, at intervals of from 350 to 450 yards from each other; and from each of these, again, two galleries extend to the right and left upon the line of the adit, so as to admit of the greatest possible number of workmen being employed at the same time. The depth of the twelfth of these air-shafts, which is the deepest, is 300 yards; the first is only sixty-three yards deep; but the aggregate depth of all amounts to 2,332 yards. It is in this that the great expence of the undertaking consists, as well as its difficulty. The aggregate depth of the air-shafts constructed for the Tunnel at Bolaños, is only 180 yards; and the tunnel itself, being merely a water course, is upon a much smaller scale than the Tlalpujahua adit, (four feet deep, by six wide).
