744 R. A. F. PENROSE, JR. 
none of the outlying districts have become very great producers, while 
even in the Witwatersrand district it is in only certain parts that 
highly remunerative results have been obtained. The conglomerates 
seem to be richest in the central part of the district, and to decrease 
in value on the east and the west ends. A large percentage of the 
gold comes from the mines along some twenty-five or thirty miles of 
the central part of the district, but for a distance of some fifty miles, 
east and west, there are scattered considerably over a hundred mines. 
Such a record, both for the number of mines in a given distance on 
one deposit, and for the percentage of profitable ones among this 
number, is probably unprecedented anywhere else. 
Nature of the ore-—The ores of the Witwatersrand district con- 
sist, as already stated, of conglomerates impregnated with gold, and 
are frequently known by the Boer term “‘banket.’’ The conglomerates, 
in their general character, do not differ from many conglomerates in 
other parts of the world except in their content of gold. The pebbles 
are well rounded and vary from a small fraction of an inch to several 
inches in diameter, most of them probably ranging from about a 
quarter of an inch to about an inch and a half or two inches. They 
are imbedded in a sandy matrix cemented by secondary silica, which 
knits them into a solid mass, and often forms small lenses, or irregular 
bodies of quartz, in the conglomerate. The rock thus cemented is 
massive and compact, and when broken the fracture often passes 
through the pebbles as readily as around them. Iron pyrites and 
marcasite are abundant, and a greenish chloritic or sericitic material 
often occurs encircling the pebbles and impregnating the matrix. 
Flakes of muscovite are not uncommon, and under the microscope 
other minerals, including rutile, zircon, magnetite, corundum, tour- 
maline, etc., are to be seen. 
The pebbles are mostly of a transparent, white, or smoky character, 
while more rarely some have the appearance of chalcedony, jasper, 
or chert. Sometimes fragments of quartzite and slate occur in the 
conglomerate, but they are few as compared with the quartz pebbles. 
In some places it is found that the coarser the pebbles, the richer 
the conglomerate in gold, but this does not always hold good, and 
sometimes the finer conglomerates are the richer. All the Witwaters- 
rand ores are more or less impregnated with iron sulphides, pyrite 
t Hatch and Corstorphine, op. cit., p. 136. 
