pai't 2] GEOLOGl OP THE MABBLE DELTA. I'^l 



chondrodite and a core of a colourless mineral that can only 

 be forsterite (sec PI. IX. fig. 3). It is orthorhombic, and can 

 hardly be colourless humite, as it has a higher refractive index 

 and higher double refraction ; a careful separation of some of the 

 pale greyish-green mineral from the yellow shell enabled the 

 absence of fluorine from the former to be proved chemicall}'. 

 The principal planes of the two minerals appear to be coincident: 

 both alter to clear serpentine without the separation of magnetite. 

 but that from the chondrodite may possess an extremely faint 

 yellowish tint. The only reference to such a relationship in the 

 olivine-humite group that I can find is one by A. Scacchi, 1 which 

 is not available to me. There is in the section an abundance of 

 phlogopite always bounded by the basal pinacoids, clear calcite, 

 a little diopside, and some sphene. 



A typical contact at this spot on F.O. shows a diopside-rich 

 layer alongside the granite, followed by a few inches of calcite 

 full of deep-green serpentine pseudomorphs after olivine, succeeded 

 in turn by a rock very similar in texture, except that cbondrodite 

 takes the place of the olivine; this again is succeeded by prac- 

 tically pure calcite. The chondrodite hereabouts is in grains and 

 aggregates up to about 1 cm. across. 



Spinel is fairly abundant in this strip of marble, rather 

 erratically distributed, and much commoner in the chondrodite- 

 bearing varieties than in any other. It varies in colour from 

 a deep bottle-green to a pale bluish tint, but the majority of 

 individuals are grey and transparent, colourless in thin section. 



The smallest spinels are in more or less octahedral forms, but 

 all the larger individuals are irregular, or possess the markedly 

 skeletal characters shown in PI. IX, fig. 2. Some patches over 

 2 cm. across were found, full of inclusions of the matrix, chiefly 

 calcite. 



The mineral is very irregularly distributed, sometimes in narrow 

 bands generally rich in chondrodite and phlogopite, sometimes 

 associated with forsterite. but the larger individuals are frequently 

 embedded in nearly pure calcite. That the mineral is spinel and 

 not periclase was proved, it may be remarked, by its hardness and 

 resistance to acids ; neither periclase nor brucite, it is important 

 to note, were seen in any of the marbles. 



Chondrodite was also observed in quantity along the left bank 

 of the Umzimkuluwana River on The Forest, near the gneiss- 

 contact, but in close proximity to several granite-tongues; the 

 individuals attain to a size of 1 cm. The mineral had also 

 previously been recorded from the marble strip high up the 

 CTmzimkulu River, near the Cape border. 8 



1 Neues Jahrb. 1876, p. 637, quoted by C. Hintze, ' Handbuch der Minora- 

 logie' vol. ii (1897) p. 397. 



- Ann. Rep. Geol. Surv. S. Africa for L913, p. 



Q. J. G.S. No, 298. i 



