Meviews — Prof. H. C. Leicis — Genesis of the Diamond. 369 



is essentially a mixture, the name " Kimbei'lite," as applied to this 

 rock, though it may be convenient as a mere desiguation, ca,ii 

 scarcely possess any scientific value. 



The so-called porphyritic brecciated structure Lewis attributed not 

 to pressure, but to mechanical disturbances during and after cooling, 

 and according as the porphyritic or brecciated structure predominated 

 so did people differ in their opinion as to whether it was a lava or 

 a tuff. He assigns nine reasons why it should be a true igneous 

 lava, and not a mud or ash. In this respect he had in mind the mud 

 volcano theory which was held by Erskine. Certainly the diamond- 

 rock itself is not exactly a true igneous lava, whatever portions of it 

 may have been ; but the "floating reef" and " snake " of De Beers, 

 which Lewis probably would call " Kimberlite proper," better 

 retain the character of igneous erupted masses. These unfortunately 

 are not diamantiferous. 



There can be little doubt, as long ago pointed out by Chaper, that 

 these pipes bear the marks of several eruptions, as of course must 

 have been the case in volcanic necks. There is nothing in this 

 which should cause the Kimberley pipes to differ from other necks. 

 It is the presence of diamonds in association with broken-up 

 peridotites which constitutes at once the interest and the peculiarity. 



Specimens from two localities in the United States, described in 

 Section III, are said to bear a close resemblance to the diamond- 

 rock of South Africa. These are peculiar peridotites, which, in 

 spite of so much of the structure being secondary, preserve the 

 shape of crystals, olivine and enstatite being the most conspicuous 

 together with the peculiar brown biotite already mentioned. These 

 rocks occur in dykes which have been forced through beds of 

 Carboniferous age : they contain fragments of shale, and now consist 

 mainly of serpentine with a considerable proportion of carbonates. 

 Plate ii (2) is a magnified section of a portion of one of the 

 American rocks, the matrix consisting of granules of serpentine, 

 iron-oxide, perofskite, etc., along with crystals of biotite, as in 

 the Kimberley rock, whilst the grains are mostly olivine in 

 various stages of change. The editor remarks that the specimens, 

 both from New York State and from Kentucky, are perhaps a little 

 more like a porphyritic igneous rock than anything he has seen from 

 Kimberley, owing to the absence of distinct rock fragments, and the 

 more uniform size of the included minerals. No diamonds are 

 found, but carbonadoes are said to occur in the Kentucky rock. 



With respect to the Kimberley rock, the editor, whilst admitting 

 that it differs from any rock, whether peridotite or serpentine, does 

 not agree with the author in his conclusions as to its origin. 

 Professor Bonney considers that the apparent glassy inclusions, 

 "together with all the large minerals (not excluding the diamonds), 

 are true 'fragments,' like the pieces of shale which are sometimes 

 found associated with them, produced by the explosive destruction of 

 more coarsely crystalline rocks of earlier consolidation, and not the 

 result of a fluxional movement in a magma, which had previously 

 reached a stage of partial separation and incomplete consolidation, 



DECADE IV. VOL. IV. — NO. VIII. - 24 



