174 DIG” BONNEY [SEPTEMBER 
anomalous optical character, to be in a condition of strain, and they are 
sometimes only fragments of crystals. 
The matrix, in which the above-named minerals are rather 
irregularly scattered, consists of serpentine, somewhat fragmentary in 
aspect, mixed with about 16 per cent of a carbonate—calcite or dolomite, 
granules of iron oxide and perovskite ; sometimes tiny flakes of brown 
mica—apparently of secondary origin—are generally disseminated. To 
some investigators the rock seems to be porphyritic, to others brecciated, 
several of the minerals looking rather rounded. Angular rock frag- 
ments—shales, grits, diabases, and the like (the first of these sometimes 
apparently a little altered)—are also present, though in variable quantity. 
The country rock is a shale, often dark, interbedded with hard grits, and 
associated with flows or-sills, and with dykes of igneous rocks, mostly 
basalt or diabase. Dykes also occasionally cut the diamantiferous 
rock. The latter occurs in pipes which bear a general resemblance to 
voleanic necks. These vary in size, the largest, named Du Toit’s Pan, 
being about 45 acres in area. 
This very brief sketch of the circumstances under which the South 
African diamonds have been hitherto found may suffice for our present 
purposes, since so much has now been written on the subject.’ The 
facts which have been briefly summarised have received very diverse 
interpretations, though all admit that the rock has been considerably 
affected by secondary mineral changes, which have been brought about, 
in all probability, by the action of heated water. Some writers, how- 
ever, maintain that the rock is a breccia, and that the diamond, like 
the garnets, pyroxenes, olivines, etc, was formed elsewhere, the parent 
rock or rocks having been shattered by some form of explosion. 
Others, while taking the same view as to the character of the blue 
eround, believe that the diamond was formed in situ, probably by the 
action of highly heated water (under considerable pressure) on the 
carbonaceous material of the country rock (Karoo shale*). Others, 
again, agree with the late Professor Carvill Lewis in regarding the 
“blue ground” as a serpentinised and otherwise altered peridotite of 
somewhat peculiar form. For this he proposed the name Kimberlite, 
thus defining it “a porphyritic volcanic peridotite of basaltic structure, 
or, according to Rosenbusch’s nomenclature, the palaeovolcanic ‘ Erguss 
form’ of a biotite-bronzite-dunite, being an olivine-bronzite-picrite- 
porphyrite, rich in biotite... it is a rock sw generis, dissimilar to 



1 | think it needless to attempt a bibliography. The earlier more important papers, 
with some which cannot be so designated, will be found in Carvill Lewis’s ‘‘The Genesis 
and Matrix of the Diamond,” 1897. Some of later date are mentioned in my paper on 
‘«The Parent Rock of the Diamond in South Africa,” read to the Royal Society on Ist June 
of this year. The classic paper of Professor Maskelyne and Dr. W. Flight (Quart. Journ. 
Geol. Soc., xxx. 1874, p. 406) contains the first thorough investigation of the associated 
minerals, and much information will be found in De Launay, ‘‘Les Diamants du Cap,” 
Paris, 1897, and in Max Bauer, ‘‘ Edelsteinkunde,” Leipzig, 1896, both of them most 
valuable works of reference. 
2 This is referred to the Triassic period. 
