124 THE DIAMOND MINES OF SOUTH AFRICA
In
Professor Lewis's discussion of the genesis of the diamond in 1886, he
designated the blue ground variously as " dunite porphyry," " Saxonite
porphyry," and " dia'mantiferous perido-tite." His application of the
term " Kimberlite," now generally accepted by geologists, first appears
in his paper of the following year, 1887, at the British Association
meeting at Manchester.1 Dr. Stelzner thought this name
should be adopted as concisely covering " a porphyritic volcanic
peridotite of basaltic structure."
In
the mass of diamond-bearing blue ground in De Beers mine there is a
curious dyke of igneous rock which extends from the southeast part of
the mine around the east and north sides, and is lost in the unexplored
poor blue ground of the west. Owing to its taking a serpentine course
across the mine, it has received the local name of " snake." The upper
end of this snake is at or near the surface, and the body extends down
to the lowest workings. It does not adhere to the blue ground, and is
very easily separated from it. It stands like a vein, nearly vertical,
varying in thickness from two to seven feet. No diamonds have been
found in it, yet Dr. Stelzner's investigations show that its
composition is substantially the same as the surrounding breccia. It
was difficult to obtain slides of the blue ground for microscopical
observations and comparison, but after many trials Dr. Stelzner
succeeded in getting a few sections which revealed these interesting
facts : —
"
The main body of the blue ground is entirely analogous to the snake
rock, naturally more decomposed, but in essential points the
microscopic features of blue ground and snake (not taking into
consideration the numerous little slate fragments in the blue) are in
an extraordinary degree alike. It therefore impresses upon one's mind
that the "snake" is a younger eruptive formation coming from the same
volcanic source as the blue ground." 2
1
"The Matrix of the Diamond," Henry Carvill Lewis, M.A., F.G.S.,
Professor of Mineralogy in the Academy of Natural Sciences,
Philadelphia, U.S.A., at meeting of British Association at Manchester,
August and September, 1887.
2 Letter of Dr. Stelzner addressed to Gardner F. Williams.
