The
basement rocks of the region are micaceous schists, quartz-ite,
granitic, and basic igneous gneisses intruded by granites, diabases,
and gabbros. They are older than "Permo-carboniferous" and are probably
of pre-Cambrian age. On this basement series "Jura-triassic" sandstones
and shales were deposited. These sandstones and shales now form a
blanket over the central basin of Kongo River. The deeper streams have
cut through them to the basement rocks, and it is in such a stretch of
Kasai River that diamonds have been found. The diamonds may have
originated from one of the basic igneous rocks of the older series, or
have formed pebbles in the sandstones. It is thought to be more
probable they will be found in basic intrusive masses cutting
"Jura-triassic" sandstone. It is interesting to note that these beds
are the stratigraphic equivalent of the youngest sedimentary rocks cut
by the kimberlite of South Africa.
GERMAN SOUTHWEST AFRICA.
According to Consul General T. St. John Gaffney,1
the diamond fields in German Southwest Africa have not come up to
expectations after five years of development. The dividends paid by the
Colonial Mining Co., the Kolmanskop Society, and the German Diamond
Society have fallen off considerably and the Luderitzbucht combined
companies paid no dividends in 1910. Difficulties reported are the
exhaustion of the rich supply of the surface diamonds, requiring more
costly mining methods, a shortage of labor, and an export tax of 33-1/3
per cent of the gross value of the diamonds.
NOTES ON DIAMOND.
Larger diamonds of South Africa.—L. J. Spencer2
has given a list of 26 of the larger diamonds of South Africa, with
their weights, the number of stones cut from many of them, etc.
Twenty-five of these diamonds weighed over 100 carats each, and nine of
them yielded single cut gems of over 100 carats weight. Of 13 of the
diamonds the yield of cut gems ranged from 34-1/4 to 59 per cent. Among
the diamonds mentioned are the Cullinan, Excelsior, Jubilee, Imperial,
Tiffany yellow, and Star of South Africa. Errors and uncertainties
concerning the weight of some of the stones are corrected and adjusted,
thus: The Excelsior diamond is shown to have weighed 969-1/2 carats,
the Jubilee 634 carats, and the Imperial approximately 456-4/5 carats.
Metric carat.—In
the article mentioned Spencer calls attention to the difficulty of
getting the correct weight of many gems, especially of those of
international interest, when a standard weight of measure is not used
and when the word "carat" means somethmg different in different
countries. The simplicity of the metric carat of 200 milligrams as a
substitute for the variable carat weight of many countries is made
evident. The metric carat is 5.304 milligrams lighter than the English
carat. It has been legalized in Bulgaria, Denmark, France, Germany,
Holland, Japan, Norway, Portugal, Roumania, Spain, Sweden, and
Switzerland. The metric carat became official in Germany8 on April 1, 1911, and should be adopted by all countries for simplicity and convenience.
