Though varying slightly in details, the methods of working the mines are identical in principle. From the steeply inclined shaft horizontal galleries are run diagonally right across the mine, the vertical interval between successive galleries being 40 feet. From each gallery side galleries are run at right angles to it and parallel to the working face. The blue ground is worked systematically backwards from the working face. The mass is stoped, _i.e._ drilled and broken from the bottom upwards, until only a thin roof is left. As soon as the section is worked out and the material removed, the roof is allowed to fall in, and work is begun on the next section of the same level; at the same time the first section on the level next below is opened out. Thus work is simultaneously carried on in several levels, and a vertical plane would intersect the working faces in a straight line obliquely inclined to the vertical direction (Fig. 60). When freshly mined, the blue ground is hard and compact, but it soon disintegrates under atmospheric influence. Indeed, the yellow ground itself was merely decomposed blue ground. No immediate attempt is made, therefore, to retrieve the precious stones. The blue ground is spread on to the ‘floors’ (Plate XXI), _i.e._ spaces of open veldt which have been cleared of bushes and inequalities, to the depth of a couple of feet, and remains there for periods ranging from six months to two years, depending on the quality of the blue ground and the amount of rainfall. To hasten the disintegration the blue ground is frequently ploughed over and occasionally watered, a remarkable introduction of agricultural methods into mining operations. No elaborate patrolling or guarding is required, because the diamonds are so sparsely, though regularly, scattered through the mass that even of the actual workers in the mines but few have ever seen a stone in the blue ground. When sufficiently broken up, it is carted to the washing and concentrating machines, by means of which the diamonds and the heavier constituents are separated from the lighter material. [Illustration: FIG. 60.—Vertical Section of Diamond Pipe, showing Tunnels and Stopes.]