Petrography of Rocks. 285 



aspect, showing intersertal rather than ophitic structure. The 

 occurrence of these two slightly differing rock-types may be explained 

 in either of two ways. The doleritic type may form sills intruded 

 into basalt flows in a later phase of activity, as in the Tertiary igneous 

 series of N.W. Europe; or, alternatively, the whole may form parts 

 of one phase, consisting of lava flows of considerable thickness, so 

 that the inner parts of individual flows may have taken on characters 

 resembling those of the hypabyssal rocks. 



Mr. H. S. Harger* seems to imply, wifhout explicitly stating 

 the fact, that the whole of this series is intrusive, although he calls 

 some of the rocks basalts, and the petrographical character of the 

 rocks on the whole favours this idea. 



So far as can be judged from the specimens in our collection, 

 the general succession of the rocks in this group of mines, viz., 

 Bultfonlein, Dutoitspan, and Wesselton, is the same as at Kimberley 

 and De Beers, although, as before stated, the thicknesses are very 

 different. At Dutoitspan the melaphyre appears to be very thin 

 indeed. It w-ould therefore cause needless repetition to describe the 

 specimens in detail. 



4. Boulders. 



Included in the collection are three specimens of boulders, one 

 labelled (409) " Open Mine, Bultfontein," the other two (516) " 470 

 ft., Dutoitspan." 



One of the latter is of no particular interest, being a very 

 ordinary oligoclase — augite rock, probably belonging to the " mela- 

 phyre " series. The other two, however, are worthy of particular 

 description. 



The boulder from Bultfontein (409) is of a rounded form, seven 

 or eight inches in diameter, and much decomposed for some distance 

 from the surface. Its central parts, however, are fresh, and show a 

 coarse texture and general greenish colour. The most conspicuous 

 minerals are olivine, green pyroxenes, brown mica and pink garnet. 

 In a slice the dominant constituent is seen to be olivine, which is 

 partly serpentinised, and shows " mesh-structure " in great perfec- 

 tion. The pyroxenes include both rhombic and monoclinic forms, 

 which are fresh and colourless, so that they may be described as 

 enstatite and diopside respectively. There is also a small quantity of 

 grass-green pyroxene, probably the chrome-bearing omphacite referred 

 to by Bonney and others. 



The garnets show only the very faintest pink tinge in a good 

 light ; they are quite isotropic, and full of the usual irregular cracks. 

 F^ach is surrounded by a shell of a greenish brown colour, and having 

 a kind of dusty appearance. 



Enclosed in this shell, and scattered through the mass of the 

 garnets are abundant small octahedral crystals of a yellowish or 

 reddish brown mineral, of high refringence and isotropic. This 

 is undoubtedly a spinel, and probably one of the chrome-bearing 

 varieties, such as picotite. The shell surrounding the garnets also 



* Trans. Geol. Soc. South Africa, VIII., 1905, p. 112. 



