J 94 SYSTEMATIC DESCRIPTIOxN OF PilECIOUS STONES 



iminerals associated with diamond in the " blue ground " are iron-pyrites, sapphire, kyanite 

 topaz, and on very rare occasions colourless olivine. Apatite has been detected by chemical 

 tests, and gold was once found enclosed in eclogite at the J agersfontein mine. Under the 

 microscope, graphite, tourmaline, rutile, and perofskite, among other minerals, have been 

 detected. The common mineral quartz, on the contrary, has never yet been observed. 



The majority of the minerals mentioned above occur in all the mines, but some are 

 confined to particular pipes. Thus gold has been met with only in the Jagersfontein mine, 

 and up to the present the occurrence of sapphire also is confined to the same mine. 



The parti-coloured residue left by washing the " blue ground " after sorting out the 

 large rock-fragments consists largely of grains of red garnet and zircon, the green minerals 

 of the pyroxene and amphibole groups, and black ilmenite and magnetite mixed with small 

 fragments of diabase. The absence from this residue of the other minerals mentioned 

 above is explained either by their rarity or by their having been lost in the washing 

 process. Diamonds are of course present, and are picked out by hand. 



All the minerals mentioned above are original constituents of the " blue ground," and 

 were already formed at the time it first filled the pipes. There are others, however, which 

 are of secondary formation, owing their origin to the weathering processes undergone by 

 the upper layers of the " blue ground." Such a secondary mineral is calcite, a not 

 unimportant constituent of the rock-mass, and occurring also in veins and crevices and as 

 crystals encrusting the walls of cavities in the rock. Other secondary minerals are zeolites, 

 especially mesolite and natrolite, sometimes found in beautiful groups of acicular crystals ; 

 also in places rough fragments of a bluish hornstone. Barytes, which is of rare occurrence, 

 is also probably a later-formed mineral. All these minerals of secondary origin, but 

 particularly the zeolites, are found most abundantly in the upper part of the pipes, which 

 is more exposed to the action of atmospheric agencies. At successively lower levels they 

 diminish in amount and finally disappear. 



Stanislas Meunier has described a total of eighty different species from the " blue 

 ground," but the existence of some of these as distinct mineral species probably requires 

 confirmation. 



One other rock found in the "blue ground" of De Beer's mine remains still to be 

 mentioned, but is of no great importance. It penetrates the "blue ground" as a dyke 

 five to seven feet thick, and on account of its tortuous path is known locally as "the 

 snake." It is a compact greenish-black rock of much the same composition and consisting 

 of essentially the same minerals as the " blue ground," but it contains no diamonds. 



The manner in which the pipes have been filled with the material we have 

 been considering has been explained in many and various ways. The first investigator to 

 formulate a theory in accordance with all the observed facts was Emil Cohen, and the 

 views he propounded in 1873 have never been seriously controverted by any one of the 

 numbers of observers who have followed him in this field of inquiry. 



He regards the pipes as volcanic vents or chimneys comparable with those, also 

 extinct, of the Eifel, and considers that the serpentine breccia now filling the pipes was 

 brought up from below by the action of volcanic forces, but at what period of geological 

 history this took place neither he nor later authorities can say. To quote Cohen's own 

 words : 



" I consider," he says, " 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 material ejected by a mud volcano. Sub- 

 sequently new minerals were formed in the mass, consequent on alterations induced in the 



