154 MOLYP.DEXUM IN NATAL. 



these cut the pebbly zone with which the Molybdenum is asso- 

 ciated ; the rock was highly ]nritic. 



The exact boundaries of the dei)osit have not been deter- 

 mined, but the body appears to be irregular and small in size. 



The ore on the dump is a peculiar friable black sandstone 

 flecked with white, and carrying a fair amount of sulphide of 

 iron ; the latter is often very abundant in the form of s])herical 

 concretions from one to two inches across, and is mostly the 

 mineral marcasite. Along one horizon they are crowded to- 

 gether, are about half an inch in diameter, and each concretion 

 includes large numbers of cjuartz grains enclosed in the solid 

 marcasite. Such concretions are ])resent in fair number in the 

 Indwe Sandstone generally, but usually they have been oxidised 

 and give rise at the outcro]) to hollow, spherical, ferruginous 

 masses. 



The black sandstone is made up chiefly of dull rounded and 

 sub-rounded grains of Quart::, to which a black matter adheres 

 that is insoluble in acids, and is principally carbonaceous in 

 composition. Along certain bands in the sandstone the quartz 

 grains are set in a minutely crystalline matrix, generally white, 

 but occasionally a jjale blue, due no doubt to the presence of 

 molybdenum compounds. It is very soft, and w'as found to 

 consist of minute ])lates of Kaolinitc derived from the alteration 

 of the potash felspar which is commonly a conspicuous con- 

 stituent of the sandstone. More usually, though, by loss of the 

 cementing material, the rock has been converted into a porous 

 friable mass. 



The sulphide of iron occurs as small crystalline aggregates ; 

 Pyritc is certainly present, but, from the fact that some of the 

 specimens show an efflorescence of sul])hates, there must be 

 Marcasite as well. 



The black amorphous matter surrounding the quartz grains 

 is in great part, as already stated, carbonaceous, and as shown 

 by the readiness with ^^■hich it can be oxidised, cannot be 

 gra])hite. It is insoluble in xylol, but with carbon disulpiiide 

 or ether a small amount of livdrocarboii can be extracted which 

 is not vohitile, and the material is not unlikely to be of the nature 

 of coal, streaks f)f which, it may be remarked, are not unusual 

 in this sandstone. Lassaigne's test (with sodium), curiously 

 enough, gave no reaction for nitrogen. 



The molybdenum is associated with this black material, and 

 can only be present in the form (jf the black sulphide, 

 Molybdenite, MoS. ; minute scales of this mineral were observed 

 upon microscopic examination of the powder, and confirmation 

 of this was obtained by the following analysis of a sample of the 

 rock made by Mr. C". < iarihausen in the laboratorx' of the Geo- 

 logical Survey : — 



Silica ^}>-^7(' 



Aluminji 0.8% 



