Dr.
Stelzner. This ground, he says, must be designated as a breccia. Most
of the small or large angular-edged or rounded fragments of this
breccia are composed of a green-black or blue-black serpentine-like
mass. Fragments of rock which are found in the karroo formation, such
as sandstone, shale, and diabase, are to be found in the blue ground.
There are also other rocks in the shape of boulders of greater or less
size, which are not known in the karroo formation, and have doubtless
come from a much greater depth than the karroo beds, possibly from
rocks upon which these beds lie. The mass of blue ground consists of
olivine more or less changed by oxidation, with the following
minerals: chromic diallage, bronzite, pyrope containing chromium,
flesh-colored zircons (locally called Dutch
boart),cyanite,biotite,chrome, titanium, and magnetic iron, and also
small crystals of perofskite.
In
the Jagersfontein blue ground corundum is said to have been found. This
was for a time held to be cordierite. The existence of small crystals
of tourmaline and rutile is also reported. Professor J. G. Lawn,
Kimberley School of Mines, reports that he discovered rubies and
sapphires of inferior quality in the Frank Smith mine near Kimberley.
Iron pyrites and barytes are found in the deposit resulting from
washing the blue ground. The pyrites come from the country rocks, and
become mixed with the diamond-bearing ground during the process of
mining. The barytes is a secondary formation of small veins in the blue
ground, or at its junction with the country rock. Beautiful crystals of
doubly refracting or Iceland spar are occasionally found also near the
junction of the blue ground and the rock.
