668 REPORT— 1886. 



shale as to resemble a breccia. Recent excavatious have shown that large quan- 

 tities of this shale surround the mines, and that they are so highly carbonaceous 

 as to be combustible, smouldering for long periods when accidentally fired. Mr. 

 Paterson states that it is at the outer portions of the pipes, where the ' blue 

 ground ' is most heavily charged with carbonaceous shale, that there is the richest 

 yield of diamonds. 



Mr. Dunn regards the ' blue ground ' as a decomposed gabbro, while Mr. 

 Hudleston, Professor Rupert Jones, and Mr. Davies regard it as a sort of volcanic 

 mud. Mr. Hudleston considers that the action was hydrothermal rather than 

 igneous, the diamonds being the result of- the contact of steam and magnesian mud 

 under pressure upon the carbonaceous shales, and likens the rock to a * boiled plum- 

 pudding.' 



The earlier theories as to the origin of the diamond have, in the light of new 

 facts, quite given way to the theory that the diamonds were formed in the matrix 

 in which they lie, and that that matrix is in some way of volcanic origin, either in 

 the form of mud, ashes, or lava. 



The exact nature of this matrix becomes, therefore, a matter of great interest. 

 The rocks now to be described are from the deeper portions of the De Beers mine, 

 and were obtained through the courtesy of Mr. Hedley. They are quite fresh and 

 less decomposed than any previously examined. Two varieties occui", the one dia- 

 mantiferous, the other free from diamonds, and the lithological distinction between 

 them is suggestive. The diamantiferous variety is crowded with included frag- 

 ments of carbonaceous shale, while the non-diamantiferous variety is apparently 

 free from all inclusions, and is a typical volcanic rock. 



Both are dark, heavy, basic rocks, composed essentially of olivine, and belong to 

 the group of peridotites. Both are of similar structure and composition, diflering 

 only in the presence or absence of inclusions. The rock consists mainly of olivine 

 crystals lying porphyritically in a serpentinic ground-mass. The olivine is remark- 

 ably fresh, and occurs in crystals which are generally rounded by subsequent corro- 

 sion. The principal accessory minerals are biotite and enstatite. The biotite is in 

 crystals, often more or less rounded, and sometimes surrounded by a thin black rim, 

 due to corrosion. Similar black rims surround biotite in many basalts. The bio- 

 tite crystals are usually twinned according to the base. The enstatite is clear and 

 non-pleochroic. Garnet and ilmenite also occur, the latter often partly altered to 

 leucoxene. AU these minerals lie in the serpentinic base, originally olivine. 



This rock appears to difler from any heretofore known, and may be described as 

 a dunite porphyry or saxonite porphyry. The diamond-bearing portions often con- 

 tain so many inclusions of shale as to resemble a breccia, and thus the lava passes 

 by degrees into tuff or volcanic ash, which is also rich in diamonds, and is more 

 readily decomposable than the denser lava. 



It seems evident that the diamond-bearing pipes are true volcanic necks, com- 

 posed of a very basic lava associated with a volcanic breccia and with tuff, and that 

 the diamonds are secondary minerals produced by the reaction of this lava, with 

 heat and pressure, on the carbonaceous shales in contact with and enveloped by it. 



The researches of Zirkel, Bonney, Judd, and others, have brought to light many 

 eruptive peridotites, and Daubree has produced artificially one variety (Iherzolite) 

 by dry fusion. But this appears to be the first clear case of a peridotite volcano 

 with peridotite ash. 



Perhaps an analogous case is in Elliot County, Kentucky, where Mr. J. S. 

 Diller has recently described an eruptive peridotite which contains the same acces- 

 sor}' minerals as the peridotite of Kimberley, and also penetrates and encloses frag- 

 ments of carboniferous shale, thus suggesting interesting possibilities. 



5. On the Metamorpliosis of the Lizard Gahhros.^ 

 By J. J. H. Teall, M.A., F.G.S. 



After recapitulating the facts established by previous observers as to the 

 mineralogical and structural characters of the Lizard gabbros, the author pointed 



> Published in full in the Geol. Mag. Dec. 3, vol. iii. p. 481, 1886. 



