16 
Transactions of the Royal Society of South Africa. 
of the Selundi range. The ohvine in the rock is decomposed at the 
surface for a few feet, rendering it soft, laminated, and friable, but where 
the river has cut through this surface layer the rock is remarkably fresh 
and scarcely affected by serpentinization. The chromite occurs as a 
series of some half-dozen gently inclined bedded veins or seams averaging 
a few inches in thickness and not exceeding 14 inches. Their trend is 
parallel with that of the Selundi range. The seams were not observed to 
exceed 100 yards in length, and they are mostly much less than this. 
Some have flat surfaces, others are corrugated. 
The veins consist entirely of chromite in some instances ; in others a 
greater or lesser admixture of one or more of the following minerals is 
present with the chromite : Olivine, magnesite, magnetite, and enstatite. 
The chromite is always well crystallized, commonly forming an even 
granular aggregate of fairly equidimensional grains about a milhmetre 
in diameter, in other specimens forming a fine aggregate of well-shaped 
crystals averaging half a millimetre. 
Olivine is commonly present in hypidiomorphic crystals interspersed 
evenly and forming some 20-30 per cent, of the volume of the rock. 
In such outcrop specimens the olivine is decomposed to a pale brown 
serpentine containing very little magnetite, or to a whitish pseudomorph 
containing rather abundant minute grains of magnetite. In other rocks in 
which the olivine predominates and forms a rather coarse aggregate with 
interstitial chromite, magnesite may become a prominent constituent in 
the form of minute anastomosing veins. This alteration and replacement 
has proceeded so far in some instances as to form a white black-speckled 
rock consisting entirely of magnesite and chromite, the latter mineral 
being embedded in the former, which forms crusts round the chromite 
crystals. The magnesite is intermixed with opal in some instances. 
It may be noted here that veins of impure siliceous (opaline) magnesite 
also occur in various places in the enstatite pyroxenites, and it is believed 
may attain considerable dimensions in other districts. 
In some of the chromite seams massive amorphous magnetite is 
present. Enstatite is less common. 
In places the chromite seams are bounded by two flat surfaces which 
sharply demarcate the seams from the harzburgite country. In other 
parts only one flat surface is present, and the other side of the seam 
grades somewhat rapidly into ordinary harzburgite free from chromite. 
The evidence obtained makes it fairly certain that the above described 
chromite rocks may be regarded as ultra-basic segregations of the Great 
Dyke magma. 
The Granite Veins and Norite Pegmatites. — A type of rock very 
commonly found in the form of narrow veins and small lenticular bosses 
in the basic margin of the Great Dyke is one which may be termed 
