180 T. G. BONNE Y [September 



the bastite group, and I have no doubt it is the one present in the 

 boulders just mentioned. The specimen accordingly represents a very 

 coarse garnet-bearing bastitite. 1 



One more boulder still remains, though it requires only a passing 

 notice. It is a compact greenish rock with spots of a light-coloured 

 mineral. This proves on examination to be a rather felspathic diabase, 

 with amygdaies consisting chiefly of calcite, with chlorite, and a few 

 small groups of zeolite. 



These diamantiferous plots in West Griqualand, though on a 

 smaller scale than at the older mines near Kimberley, occur in a 

 similar way, and are formed of a rock practically identical. Those 

 now beiiJLr worked are three in number, two at least of them being 

 connected by a line of fissure. The rock has now been proved, and 

 galleries have been driven to a depth of over 300 feet, and the 

 boulders above mentioned were found at various levels down to this 

 from nearly 100 feet. A section obtained just south of the middle 

 " pipe " is interesting. Here a gallery was driven between two walls of 

 diabase (? dykes) about four yards apart, and in the interval were four 

 ribs of blue ground, parted by country rock, which is a grey mudstone, 

 sometimes pebbly. The total amount of the two was nearly the same, 

 but the thinnest rib of " blue " (very decomposed) was about an inch 

 in width, while the thickest was rather under four feet. It is strange 

 that the characteristic " breccia " (though rather a finer variety than 

 usual) should have penetrated into so narrow a fissure. 2 The principal 

 areas, however, appear to be " blow-holes," formed in the same way as 

 parasitic cones along a crack on the flank of a volcano. 



Thus the diamond has been found to be a constituent of an 

 eclogite, and the parent rock occurs as boulders in the ordinary 

 diamantiferous material (blue ground). I have no hesitation in 

 claiming this coarsely holocrystalline eclogite as an igneous rock, 

 though I am aware that some uncertainty has been expressed on this 

 point ; but, as it happens, I have had several opportunities of studying 

 eclogites, not only under the microscope, but also in the field, and am 

 convinced that they are as truly igneous rocks as granites, syenites, or 

 diorites. They are, indeed, rather closely allied with the last named, 

 perhaps also with certain dolerites. The relationship may be expressed 

 by the homely direction : " Put some salt into the magma of an ordinary 

 eclogite and it will crystallise as one of the less acid diorites." 



The diamond then is shown to be an accidental constituent of the 



1 Pyroxenites (diallagite, bastitite, etc.) not unfrequently run very coarse, but (so far 

 as I happen to have seen) in rather thin dykes or veins. See Quart. Jour. Geol. Soc. vol. 

 lv. (1899), p. 290. 



- It will be remembered that the Kimberlite of Elliot County, Kentucky, appears to 

 occupy a branching fissure (Lewis, " Genesis and Matrix of the Diamond," p. 64). As 

 this section was obtained in a gallery at a depth of 300 feet it may possibly be misleading, 

 and some of the blocks of mudstone may not be in situ, but only great fragments which 

 have fallen into the fissure. 



