152 JOURNAL OF SCIENCE 
The diamond mines of South Africa lie in a vast, undulating, 
arid plain, through which the Vaal River meanders sluggishly. 
It is treeless, except near the river. There are a few hills, but 
none of them are more than 400 or 500 feet in height. Local 
depressions, mostly without any outlet, are common, and are 
called “Pans.” Some of these pans are two or three miles in 
length, but generally they are smaller. They are quite sterile, 
owing to the quantity of salt impregnating the soil. The plain 
has evidently been subject to great denudation, and is covered 
by a fine sand-drift a few feet in thickness, below which there is 
usually a tufaceous limestone of varying thickness and purity. 
The origin of this limestone has been explained as due to the 
rapid evaporation, which takes place after heavy rainfalls, bring- 
ing to the surface the lime which has been dissolved out of the 
underlying rocks. 
The fundamental rock of the plains is shale with vegetable 
remains, probably of triassic age, and is known as the “ Karoo 
beds ;” but from the geological structure of the western part of 
the continent, there is reason to think that beds of gneiss and 
other schistose rocks underlie the Karoo beds at some unknown 
depth. The shales are horizontal, except where locally disturbed 
by the intrusion of eruptive rocks. These eruptive rocks are 
numerous, chiefly in the form of dykes, which stand out above 
the surface of the plain, and form the ridges and hills. There 
are, however, some old volcanic “pipes” formed of softer 
materials. The rocks are for the most part dolerite and gabbro, 
sometimes altered into serpentine, but two or three exposures of 
augite-andesite are known. ‘There are no acidic rocks. 
Diamonds were first found in 1867 in the alluvial gravels of 
the Vaal, and between that year and 1871 they were traced for 
about 230 miles along the banks of the river, and for 70 miles 
down the Orange River below its junction with the Vaal. By 
far the greater quantity, however, were obtained in the middle 
Vaal. In 1870 the mother-rock of the diamonds was discovered 
in Du Troit’s pan, about 15 miles east of Pneil, on the middle 
Vaal, and the “ river diggings ” were very soon abandoned for the 
“dry-diggings.” De Beere’s and Bulfontein were discovered the 
same year, and Kimberley in 1871. These four are all near to- 
gether, within a radius of two or three miles, and from that day 
to this no mines of any importance have been found, except two 
small ones in the Orange Free State, about 60 miles distant. 
This is the more remarkable, as for thousands of square miles 
the geological conditions appear to be similar. 
There is, of course, nothing extraordinary in the discovery of 
diamonds in alluvial deposits. It is on the dry-diggings or mines, 
that all the interest concentrates; and it is these alone that I 
shall describe. 
The Kimberley mine is in the pipe of an old volcano, the 
upper portion of which has been removed by denudation. In 
form it is an ellipse of about 1000 feet by 600, and it covers an 
area of about nine or ten acres. Originally it appears to have 
