114 



of the experiments by Moissan on the artificial formation of 

 diamonds, stirred some melted olivine with a stick of graphite and 

 found that the silicate on cooling- contained very numerous but 

 microscopical crystals, which he took to be diamonds. The blue 

 ground in a similar manner dissolves diamonds when they are 

 strongly heated with it. 



The original source of the carbon of the diamond, it is 

 suggested, was either graphite or carbon combined as petroleum, 

 inorganic or organic in source, in underlying rocks, igneous or 

 sedimentary, which was dissolved out of these rocks by the in- 

 trusive volcanic magma on its way to the surface. 



Petroleum sometimes occurs on the surface of waters in the 

 neighbourhood of Kimberley, and a solid hydrocarbon was actu- 

 ally extracted by Sir Henry Roscoe from the blue ground. 



The pipes in the Kimberley district are not, Sir William 

 Crookes shows, ordinary volcanic pipes. The surrounding walls 

 show no sign of igneous action and are not shattered where touch- 

 ing the clay. It seems pretty certain that the pipes were filled from 

 below after they had been pierced, the diamonds having been 

 formed at some previous period in other than volcanic rocks and 

 injected together with other rock debris into the pipes by a mud 

 volcano. According to most South African geologists volcanic 

 activity of this sort was prevalent all over South Africa during the 

 Cretaceous period. It is, however, only in the northern parts of 

 the country that the pipes as well as fissures and dykes are filled 

 with the diamantiferous blue ground; elsewhere they are filled 

 with volcanic agglomerate, breccia and tuffs and in places with 

 basic lavas. 



Cases of the occurrence of diamond in what might be the 

 original matrix are remarkably few in number. It has been found 

 with corundum in a decomposed pegamite vein traversing granite 

 and also in a hornblende diabase intersecting granite as a dyke. 

 Professor T. G. Bonney found eclogite in the blue ground studded 

 with diamond crystals. This crystalline rock consists of pyroxene, 

 garnet and a little olivine. It seems probable that a holo-crys- 

 talline pyroxene garnet rock like this was one source at least of 

 the diamond. But of course the eclogite may not be the truly 

 original matrix, but has simply caught up the diamonds from some 

 other matrix now completely disintegrated, either from some 

 deep seated crystalline rocks or as a concretion in the blue ground. 



V. 



The Uses of Carbon. 



By Hubert Painter, B.Sc, F.C.S. 



(Delivered as a general Lecture at the Society's Eoom, April 29th, 1916). 



TN the first lecture of this series I explained that carbon, exists 

 in the free or uncombined state in three or more forms or 



