In tracing the causes of these disappearances, Dana points out that, perhaps, the principal cause was a decrease in the temperature of the ocean, since the destructions were limited in large measure to marine life. He regards, however, the other most probable cause as traceable to earthquake waves; for the making of a great mountain range along the entire length of the continent resulted in displacements of the rock formations along lines hundreds of miles in length. Such displacements must have been attended by a succession of earthquakes of unusual violence, causing the destruction by sudden shocks beneath, and resulting, directly and indirectly, in waves sweeping over the continent. Since at this time the land was still low for the greater part, the huge waves must have repeatedly swept over the greater part of the land, leaving only the smaller species of animals and the vegetation. It is evident, therefore, that during the geological past earthquakes occurred that were probably vastly greater than any that have occurred on the earth during more recent times. CHAPTER XXXVI THE KIMBERLY DIAMOND FIELDS AND THEIR VOLCANIC ORIGIN The elementary substance carbon occurs in three forms, i. e., _charcoal_, _graphite_, and the _diamond_. The commonest form of carbon is to be found in charcoal, as well as in bituminous coal, anthracite coal, and _lignite_. Graphite, also known as _plumbago_, or _black lead_, is the substance you have seen so often in the lead of pencils. The diamond, as every one knows, is the highly prized precious stone that sparkles so brightly in the light, and is so hard that it is capable of scratching almost any other substance. Diamonds are found in various parts of the world. We are now interested in them, however, only as they occur in certain parts of the world, as in the great Kimberly diamond fields in Southern Africa. Dr. Max Bauer in his book on precious stones says that the discovery of diamonds in South Africa was made by a traveller named O'Reilly, who, in 1867, saw a child sitting in the house of a Boer named Jacobs, playing with a shining stone. Jacob's farm was a short distance south of the Orange River near Hopetown. This stone proved to be a diamond weighing some twenty-one and three-tenths carats and was afterwards sold for $2,500. The incident led to the discovery and consequent development of the Kimberly diamond fields.