coarser agglomerate, as in many cases in the Drakensberg. It may be filled with volcanic ash of the second stage, or it may be filled with a plug of lava of the third stage. The Kimberley diamond pipes are of the last kind, but here the lava plug has been altered by the percolation of superficial water of the fifth stage,
Fig. 18. Section through a Volcanic Neck, the ash cone of which has been swept away by denudation. Mimosa, near Port Elizabeth
which has changed the rock into a serpentine, expanding it in the process and forcing it to move upwards in the vent. At the same time the carbon gases of the fourth stage have been working through the mass, and from them are derived the diamonds by a process of deposition which is similar to that which is responsible for the formation of many ordinary minerals occurring in less extraordinary rocks, for instance, andalusite from granite. Volcanoes do not always issue from single cracks;
