Where there is no outcrop in the tract to be mined, and the coal lies below water level, the best mode of making an entrance to it is by shaft. In the Wyoming region, since the upper veins have been so generally mined out, nearly all the openings are by shaft. The location of the shaft at the surface should be such that when it is completed its foot shall be at the bottom, or nearly at the bottom, of the synclinal valley into which it is sunk. As will be more readily seen hereafter, this is necessary in order to carry the water of the mine to the foot of the shaft, to facilitate the transportation of coal under ground, and to get room to open up the greatest possible working area. The depth to which a shaft must be sunk depends on the seam to be reached, and on the district in which it is located. At Carbondale, in the northeasterly extremity of the Wyoming basin, the average depth to the conglomerate or bed of the lowest coal seam is 250 feet. From Scranton to Pittston it is from 500 to 600 feet. At Wilkes Barre it is 1,200 feet. It reaches its greatest average depth a mile northeast of Nanticoke, where it is from 1,500 to 1,600 feet. This will be the limit of depth for shafts in the Wyoming region. At present the average depth is from 300 to 400 feet, and there are few that are more than 800 feet deep. The red-ash vein to which most of the shafts are now being sunk is, at Pittston in the middle of the general basin, from 450 to 650 feet below the surface. In the southern anthracite region the average depth of shafts is somewhat greater, the maximum depth being reached in the vicinity of Pottsville, where the Pottsville deep shafts are about 1,600 feet in depth. In beginning to open a shaft a rectangular space is staked out on the ground from four to eight feet wider and longer than the proposed dimensions of the shaft; and the soil and loose stones are thrown out from this larger area until bed rock is reached, which is usually done, except in the river bottom lands, within a depth of twenty feet. From this rock as a foundation a cribbing of solid timber, twelve inches square, is built up to the surface on the four sides of the opening to prevent the earth from caving in. Sometimes heavy walls of masonry are built up instead of the timber cribbing, and though the original cost is greater, the purpose is far better answered by the stone curbing. When this has been completed, sinking through the rock goes on by the ordinary process of blasting, plumb lines being hung at the corners of the shaft to keep the opening vertical.