Complete Rock-disintegration by Weathering. 63 
there is little doubt that the bulk of the fragments has been 
derived from the disintegration of the quartzite and vein-quartz 
pebbles under the influence of weathering. 
In order to ascertain the nature of the heavy minerals in this 
sand, the whole of the sample was carefully panned, and the con- 
centrates, after drying, introduced into bromoform of the density 
of 29. By this means a separation of the minerals with a density 
greater than 2°9 from the quartz which floated on the surface of 
the liquid was effected). A minute quantity of a heavy powder 
was obtained, the subsequent examination of which under the 
microscope disclosed the following minerals: 
Zircon. Chalcopyrite. 
Zine blende. Rutile. 
Galena. Tourmaline. 
Tron pyrites. 
Of these heavy constituents, the first three constitute about 
95 per cent. 
In addition to the above minerals, diamonds are found in the 
course of the washing operations conducted at the spot. I had 
an opportunity of examining 40 of these stones: they ranged in 
weight from 0°68 to 4°42 metric carats*, and showed the character- 
istic rounded shape of the diamond, the prevalent forms being the 
octahedron, the rhombic dodecahedron, the 3-faced octahedron 
and the 6-faced octahedron, either alone or in combination. The 
octahedral faces are usually smooth and brilliant, but triangular 
pittings are common on them. The majority of the stones have 
a characteristic greenish tint, but this is only “skin deep,” and 
does not appear in the cut stones. Examination under the micro- 
scope shows that this colouration is due to the presence of small 
spots and flecks of some green mineral (chlorite?) in the super- 
ficial layers of the stones. There is little to indicate the original 
source of these diamonds. Although found in the conglomerate 
they are certainly older than that rock. It appears likely that 
there is some community in origin between them and the soap- 
stone, which, as already stated, probably represents a decomposed 
olivine-rock of igneous origin. 
Origin of the Material. The material is an ancient con- 
glomerate, that has suffered prolonged weathering. So com- 
pletely have the weathering agents performed their work that 
not only has the cement of the conglomerate been entirely 
removed, but the pebbles themselves have been profoundly 
affected. As already stated, many of the pebbles are rounded 
fragments of an older quartzite formation. The material that 
* 1 metric carat=200 milligrams. 
