Without going into a description of the different deposits in which diamonds are found, it will suffice to say that in the Kimberly district the diamonds occur distributed through the materials that fill peculiar funnel-shaped depressions called _pipes_ which extend vertically downward to unknown depths. The rock that fills a pipe consists of an entirely different material from that in which the pipe occurs. The upper extremity of the pipe is generally slightly elevated above the general surface for a few yards. The pipes vary in diameter from twenty to 750 yards, diameters of from 200 to 300 yards being quiet common. In 1892, the diamond-bearing material found in the pipes of the Kimberly mines had been excavated vertically downwards a distance of 1,261 feet, without any signs of its being exhausted. Now, the materials which fill the pipe of the great Kimberly mine are practically the same in all the mines in the neighborhood. At the upper part of the pipe the materials show the action of weathering by exposure to the air. Here the ground is of a yellowish color. Below, the materials have a blue color. According to Bauer the diamond-bearing material that fills the upper part of the pipe consists of a soft, sandy material of a light yellow color, known to diamond miners as _yellow ground_, or _yellow stuff_. In the case of the Kimberly mine, the yellow ground has a thickness of about sixty feet. Below it the material has a blue color and is known as the _blue ground_. This latter material possesses the character of a volcanic _tuff_, which is a hardened clay. It is of a green or bluish green color and has the appearance of dried mud that holds or binds together numerous irregular, tough, and sometimes rounded fragments of a green or bluish black serpentine. The diamonds are found near the surface in the yellow ground, as well as downwards through the blue ground. It was at one time thought that most of the diamonds existed in the yellow ground, and that they would soon disappear entirely at short distances below where the blue ground began. Under this belief some of the most valuable claims changed hands at prices far below their true value. It was soon found, however, that large and valuable stones existed in the blue ground, and, indeed, this ground has never been mined to a depth below where valuable diamonds appear. The diamonds occur in very small quantities spread through the yellow and blue grounds. The following statement by Bauer will show this: "A striking illustration of their sparing occurrence is furnished by the fact that in the richest part of the richest mine, namely, in the Kimberly mine, they constitute only one part in 2,000,000, or 0.00005% of the blue ground. In other mines the proportion is still lower, namely, one part in 40,000,000, a yield which corresponds to five carats per cubic yard of rock."