666 
Lewis never visited Kimberley. I have done 
so, and from field observations reached the same 
conclusion as he did from the study of collec- 
tions. Unfortunately my collection disappeared 
in transit, but a few notes may tend to throw a 
little additional light on the subject of the 
genesis of the diamond. 
The De Beers mine and the Kimberley mine 
are each extensively developed, the former to 
1,200 feet and the latter to 1,600 feet. As is 
well known, the mines are in volcanic necks. 
I gathered some evidence that these necks are 
not drilled through the rocks, but are local en- 
largements of persistent fissures. The adjacent 
rocks are in a nearly horizontal position and in- 
clude quartzites, shales, sheets of melaphyre 
and of basalt. The quartzites are also cut by 
dikes of basic eruptives. The only bituminous 
rock in the district is the black shale, which is 
about 180 feet thick at the De Beers and 250 
feetatthe Kimberley. At each mine the under 
surface of this shale is only at the very moderate 
depth of about 350 feet from grass. In theupper 
part of the mines the kimberlite carried very 
numerous fragments of the shale. Below the 
shale stratum, however, the shale horses or ‘float- 
ing reef’ diminished in number very greatly, 
so that at over a thousand feet of depth only 
occasional fragments of shale are to be found. 
Those which I saw seemed totally unaltered. 
The edges of the fragments were sharp and 
there was no macroscopical evidence of any loss 
of carbon. Much of the lava is brecciated, but 
much also is solid with gradations between 
the two varieties. There is one well marked 
dike in the De Beers. Itis knownas the Snake, 
from its meandering course. It was determined 
by Stelzner as pikrite porphyry and as substan- 
tially the same rock as the blue ground itself. 
The ‘Snake,’ however, contains no diamonds. 
The diamonds are distributed throughout the 
necks, excepting that they never occur either in 
horses (whether of shale or of other rock) or in 
the snake dike. They are found in what appears 
to be massive lava as well as in the breccia, but 
there are portions of the mass which are too 
poor in diamonds to pay for extraction. The 
diamonds are not grouped about, or especially 
associated with, extraneous fragments. On the 
other hand, they are curiously grouped in ver- 
SCIENCE. 
[N. 8S. Vou. VI. No. 148, 
tical pipes within the necks. The existence of 
these pipes was pointed out by Mr. M. Chaper 
and they were discussed by Mr. A. Moule. 
The diamonds from the several pipes seem to 
differ not only in abundance, but also in tint or 
physical character, so that with care a discrim- 
ination is possible. The pipes apparently rep- 
resent successive eruptions. 
It is important to note that the contents of 
blue ground in diamonds remains very nearly 
constant at different levels. The De Beers 
mine is now some 850 feet below the bottom of 
the shale, the Kimberley about 1,250 feet, and 
the walls of the necks or of the deposits con- 
tain no sensible amount of carbon over these 
intervals of depth, yet the average number of 
carats per ‘load’ of 16 cubic feet (about 1,600 
pounds) does not vary to an important extent. 
The yield per load for the year ending June 30, 
1896 (viz., 0.91), was almost exactly the same 
as for the year ending June 30, 1892 (viz., 0.92). 
In 1893 it was somewhat higher, in 1894 and 
1895 somewhat lower. Now, to imagine that 
the carbon has come from the shale and per- 
meated the lava downwards, in opposition to 
the flow of melted rock, to such depths as 1,000 
feet, without great diminution of quantity, is 
almost impossible. One can, indeed, fancy still 
more deeply seated shale beds, but for this hy- 
pothesis there is no evidence at all. 
While the diamond appears to be an original 
constituent of the kimberlite and occurs in 
South Africa in this manner at many localities, 
it may be classed as accessory, rather than an 
essential mineral, if there is any real distinc- 
tion between these groups. Some of the rock 
is rich, some too poor to be worked, and some 
seems to be absolutely barren in diamonds. So 
also there are basaltic rocks poor in olivine 
and passing over into varieties substantially 
barren so far as this mineral is concerned. 
Similarly granites are sometimes rich in zircon 
and sometimes poor. Even so-called essential 
constituents, such as the quartz of granite, like- 
wise vary in quantity, as, for example, when a 
granite passes over a syenite. ! 
Lewis seems to have confounded two phe- 
nomena in discussing fractured diamonds. 
There are diamonds usually of a brownish tint 
which, after being exposed to the air for some 
