286 Report S.A.A. Advancement of Science. 



encloses abundant small crystals of a colourless mineral, probably a 

 pyroxene. These shells are of less regular structure than the so- 

 called kelyphite borders, which are so common in round garnets, but 

 they are undoubtedly of the same nature. The biotite occurs in large 

 plates, and calls for no special remark. 



This rock agrees in mineral composition with the special type 

 of enstatite-augite-peridotite, to which the name Lherzolite* is 

 applied, although it contains more garnet than the original rock from 

 the Pyrenees. 



The other interesting boulder, from Dutoitspan (516) is of a very 

 different nature. It consists essentially of plagioclase feldspar, a 

 peculiar form of pyroxene, and a good deal of iron ore. The general 

 texture is coarse, but between the large crystals there is a small 

 quantity of interstitial ground mass, so that, strictly speaking, the 

 rock is porphyritic. 



The feldspar builds large, tabular crystals, usually quite idio- 

 morphic, and showing strongly-developed twinning on the albite and 

 pericline laws. The twin lamellae are broad, as is often the case 

 in basic feldspars. The extinction angles range up to 45°, although 

 the majority are about 30°. Investigation by means of the quartz 

 wedge shows that the vibration direction lying nearest to the twin- 

 lamellse is the axis of greatest elasticity, so that the feldspar belongs 

 to labradorite and bytownite, and not to anorthite, as suggested by 

 its general appearance. 



The pyroxene includes two distinct varieties, which are inter- 

 grown in the same crystal. One is quite colourless, with oblique 

 extinction at a high angle, as in normal diopside. The other form 

 shows a very good example of diallage structure, having rows of 

 minute inclusions parallel to the basal plane. 



Some twinned crystals show the resulting " herring-bone " 

 structure very clearly. As a consequence of this, the hand specimen 

 shows strong schillerization. The diallage has a general yellowish 

 brown colour in a slice, and its extinction is much masked by inter- 

 ference. There is a good deal of iron ore in large, irregular plates. 



The small amount of space left between the large idiomorphic 

 crystals is filled by a beautiful eutectic of quartz and feldspar, and in 

 many cases the feldspar of the eutectic can be seen to be in optical 

 continuity with the large crystals. 



This rock may perhaps be most satisfactorily described as an 

 augite-porphyrite, although the amount of ground mass is so small 

 that it might be regarded as a gabbro. 



IV.— SUMMARY AND CONCLUSION. 



The general succession of the rocks through which the diamond- 

 bearing pipes have been formed, is thus seen to be on the whole very 

 simple. At the lowest level yet reached we have a granite of 

 unknown age. Resting on this is a thin sedimentary bed, and then a 

 great thickness of acid volcanic rocks, which are compared by Rogers 



* Rosenbusch, Gesteinlehre, p. 173. Marker, Petrology, p. 95. 



