away
from the banks was equally rich, and this led them finally to the
kopjes and dry diggings, resulting in the discovery of the immense
chimneys of diamond-bearing clay which have yielded such enormous
quantities of the precious material.
The
tops of these chimneys were filled with a yellow clay called " yellow
ground" to a depth of fifty to sixty feet. Beneath this lay a quantity
of " rusty ground" sixteen to twenty feet thick, which, as it became
distinct from the " yellow ground," yielded no diamonds, and it was
feared that the end of the diamond-bearing clay had been reached.
However, on going through this, another clay of a greenish-blue color,
fully as rich, if not richer, than the yellow ground, was discovered,
and this has been continuously worked since. This is called the "blue
stuff."
In
the Du Toits Pan the clay is in the form of a horseshoe about two
thousand two hundred and forty-seven feet long by six hundred and seven
feet broad; in the Bultfon-tein it is circular and has a diameter of
one thousand and eighty feet. The De Beers is elliptical, nine hundred
and fifty-eight feet east and west by six hundred and thirty feet north
and south, and the Kimberley is an oval eight hundred and eighty-six
feet long by six hundred and fifty-six feet broad, with an eastern spur
extending one hundred and eleven feet farther. The Kimberley narrows as
it goes down.
These
chimneys of diamond-bearing clay appear to have been forced upward
through intervening strata from nature's retort deep in the earth. Some
of the crystals bear evidence of having been shattered in the grinding
process of volcanic upheaval. Splinters and microscopic crystals are
abundant in the " blue earth." The upper portion of each of these pipes
consisted of the sandy soil peculiar to the country, under which was a
light deposit of tufaceous rock, to which occasional diamond crystals
were found adhering. Lower down, volcanic rock, often broken up and
oxidized. This, from its
