The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

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."

Of course, you will desire by this time to know the manner in which the pipes of the diamond mines of South Africa have become filled with the diamond-bearing rocks, and particularly what diamonds have to do with a book on volcanoes and earthquakes.

Dr. Emil Cohen, who has made a study of these regions, regards the pipes as volcanic vents or chimneys, and that the materials filling the pipes have been brought up from below by volcanic forces. He says:

"I consider that the diamantiferous ground is a product of volcanic action, and was probably erupted at a comparatively low temperature in the form of an ash saturated with water and comparable to the materials ejected by a mud volcano. Subsequently new minerals were formed in the mass, consequent on alterations induced in the upper part by exposure to atmospheric agencies, and in the lower by the presence of water. Each of the crater-like basins, or, perhaps, more correctly, funnels, in which alone diamonds are now found, was at one time the outlet of an active volcano which became filled up, partly with the products of eruption and partly with ejected material which fell back from the sides of the crater intermingled with various foreign substances, such as small pebbles, or organic remains of local origin, all of which became imbedded in the volcanic tuff. The substance of the tuff was probably mainly derived from deep-seated crystalline rocks, of which isolated remains are now to be found, and which are similar to those which now crop out at the surface, only at a considerable distance from the diamond fields. These crystalline rocks from which the diamonds probably took their origin, were pulverized and forced up into the pipes by the action of volcanic forces, and imbedded in this eruptive material, these diamonds either in perfect crystals or in fragments are now found."

So far as the volcanic origin of the diamonds of the Kimberly diamond fields is concerned, Cohen's theory has been generally accepted with the following modifications: that the pipes were not filled by a single volcanic eruption, but by successive eruptions, and that in the case of the Kimberly mine, the pipes contain the results of as many as fifteen successive eruptions. There has, however, been another and more important modification proposed to Cohen's theory, which is far more probable. It will be noticed that Cohen's theory regards the action of the volcanic eruption as only serving to bring fragments of a deep-seated mother rock that contained the diamonds up from below with the material that fills the pipe. Now, Prof. Carvill Lewis proposes the following very important change in Cohen's theory: that the blue ground does not consist of fragmentary material or tuff, but was forced up from below in the pipe in a molten mass and consolidated on cooling. In other words, the blue ground is filled with an ordinary igneous rock that was solidified in place in the vent or pipe.

In the great Kimberly mines the surface of the pipe is divided into numerous separate claims, each consisting of a small square lot. There are so many of these claims in the Kimberly mine that its surface is honey-combed by numerous square pits. The work is done largely by native Kaffirs employed there since the '70's. As the material was removed from the pit, the adjoining claims were separated from each other by high vertical walls.

At a later date, in order to remove the material and separate the lots, high staging provided with ropes and hauling machinery was erected. The number of these ropes is now so great that the mine has the appearance of a huge cobweb.

A very extensive series of investigations has been made at a comparatively recent date by Prof. Henri Moissan of France on various chemical products that are obtained under the influence of the high temperatures of the electric furnace. When a powerful electric current is caused to pass through a highly refractory material, that is to say, a material difficult to fuse, such as carbon, it raises it to an extremely high temperature. A still higher temperature can be obtained by causing a powerful current to flow between two carbon rods that are first brought into contact, and then gradually separated from each other, just as they are in the ordinary arc lights employed for lighting the streets of our cities. In the latter way a temperature that is estimated as high as 3,500° C. (6,332° F.), can be readily obtained. Under these very high temperatures some very curious chemical products have been obtained in electric furnaces. These furnaces consist of small chambers made of highly refractory materials closely surrounding the incandescent carbon, or the carbon voltaic arc. Among some of the most curious of these products are artificially produced diamonds.