In other chapters the geological characteristics of the diamonds are dealt with, and the best authorities quoted at considerable length; in this connection, therefore, it will suffice to say that the “yellow ground” only extends to a certain depth; this is friable, and was easily broken up by means of shovels and clubs known as “beaters.” Beneath this ground lies the hard blue, the discovery of which, as I mention elsewhere, caused no small consternation amongst the diggers, who feared when reaching it that they had come to what in alluvial gold digging would be called the “bed-rock.” This blue ground requires careful manipulation, and 1875 witnessed the general adoption of the “rotary” already introduced for the quicker manipulation of the blue. The machine in question may be thus briefly described: A species of “pay mill,” a round, stationary pan with two concentric rings of sheet iron about nine inches in height; in the centre an upright spindle driving six or eight arms, in which are fixed a number of flattish tynes, not unlike the prongs of a trident, placed in a diagonal direction. Into the circular trough formed by the concentric rings the ground was poured, water was supplied, and the mud kept in agitation by means of the tynes, the overflow passing through a small door or opening in the inner ring. This opening could be regulated by a slide according to the nature of the ground manipulated, rapidity of the revolution of the tynes, etc., etc. The machine was kept in work by means of cog-wheels at the top of the spindle. At opposite points of the circumference of the machine Kafirs were stationed to supply the motive power, and stood there turning and turning all day long. Later on horse power was used for turning the rotaries, which were much enlarged, and now are almost universally driven by steam power. These machines, originally about four feet in diameter, and capable of washing some forty loads per diem, are now made as much as fourteen feet in diameter and from 400 to 800 loads, according to the nature of the ground manipulated, can be washed daily by a single machine. A highly important adjunct to the washing machine proper is the cylinder, which was introduced after the discovery of the blue ground. This was frequently found to be too insufficiently disintegrated, even after long exposure on the depositing floors, to allow of its being rapidly washed without more or less serious loss of diamonds.[34] To prevent this loss the contrivance to which I allude was invented. It is a rotatory cylinder, fixed at an angle of some ten degrees, upon which a stream of water plays from a perforated tube directly above it. The upper part of the cylinder is of fine mesh, through which the finer particles of the wetted ground or mud fall upon a table, in the centre of which is an inclined trough leading to the washing machine, and along this the “magma” or slush mentioned flows by natural gravitation.