THE NATURAL FORMATION OF THE DIAMOND
An hypothesis is of little value if it only elucidates half a
problem. Let us see how far we can follow out the ferric hypothesis
to explain the volcanic pipes. In the first place we must remember
these so-called volcanic vents are admittedly not filled with the
eruptive rocks, scoriaceous fragments, etc., constituting the
ordinary contents of volcanic ducts.
Certain artificial diamonds present the appearance of an elongated
drop. I have seen diamonds which have exactly the appearance of
drops of liquid separated in a pasty condition and crystallised
on cooling. Diamonds are sometimes found with little appearance
of crystallisation, but with rounded forms similar to those
which a liquid might assume if kept in the midst of another
liquid with which it would not mix. Other drops of liquid carbon
retained for sufficient time above their melting-point would
coalesce with adjacent drops, and on slow cooling would separate
in the form of large perfect crystals. Two drops, joining after
incipient crystallisation, might assume the not uncommon form of
interpenetrating twin crystals.
Many circumstances point to the conclusion that the diamond of
the chemist and the diamond of the mine are strangely akin as to
origin. It is evident that the diamond has not been formed _in
situ_ in the blue ground. The genesis must have taken place at vast
depths under enormous pressure. The explosion of large diamonds
on coming to the surface shows extreme tension. More diamonds are
found in fragments and splinters than in perfect crystals; and it
is noteworthy that although these splinters and fragments must be
derived from the breaking up of a large crystal, yet in only one
instance have pieces been found which could be fitted together, and
these occurred at different levels. Does not this fact point to the
conclusion that the blue ground is not their true matrix? Nature
does not make fragments of crystals. As the edges of the crystals
are still sharp and unabraded, the _locus_ of formation cannot have
been very distant from the present sites. There were probably many
sites of crystallisation differing in place and time, or we should
not see such distinctive characters in the gems from different
mines, nor indeed in the diamonds from different parts of the same
mine.
I start with the reasonable supposition that at a sufficient
depth[9] there were masses of molten iron at great pressure and
high temperature, holding carbon in solution, ready to crystallise
out on cooling. Far back in time the cooling from above caused
cracks in superjacent strata through which water[10] found its way.
On reaching the incandescent iron the water would be converted
into gas, and this gas would rapidly disintegrate and erode the
channels through which it passed, grooving a passage more and more
vertical in the necessity to find the quickest vent to the surface.
But steam in the presence of molten or even red-hot iron liberates
large volumes of hydrogen gas, together with less quantities of
hydrocarbons[11] of all kinds--liquid, gaseous, and solid. Erosion
commenced by steam would be continued by the other gases; it would
be easy for pipes, large as any found in South Africa, to be
scored out in this manner.
Sir Andrew Noble has shown that when the screw stopper of his
steel cylinders in which gunpowder explodes under pressure is not
absolutely perfect, gas escapes with a rush so overpowering and a
temperature so high as to score a wide channel in the metal. To
illustrate my argument Sir Andrew Noble has been kind enough to try
a special experiment. Through a cylinder of granite he drilled a
hole 0·2 inch diameter, the size of a small vent. This was made the
stopper of an explosion chamber, in which a quantity of cordite was
fired, the gases escaping through the granite vent. The pressure
was about 1500 atmospheres and the whole time of escape was less
than half a second. The erosion produced by the escaping gases and
by the heat of friction scored out a channel more than half an
inch diameter and melted the granite along the course. If steel
and granite are thus vulnerable at comparatively moderate gaseous
pressure, it is easy to imagine the destructive upburst of hydrogen
and water-gas, grooving for itself a channel in the diabase and
quartzite, tearing fragments from resisting rocks, covering the
country with debris, and finally, at the subsidence of the great
rush, filling the self-made pipe with a water-borne magma in which
rocks, minerals, iron oxide, shale, petroleum, and diamonds are
violently churned in a veritable witch’s cauldron! As the heat
abated the water vapour would gradually give place to hot water,
which, forced through the magma, would change some of the mineral
fragments into the existing forms of to-day.
Each outbreak would form a dome-shaped hill; the eroding agency of
water and ice would plane these eminences until all traces of the
original pipes were lost.
Actions such as I have described need not have taken place
simultaneously. As there must have been many molten masses
of iron with variable contents of carbon, different kinds of
colouring matter, solidifying with varying degrees of rapidity,
and coming in contact with water at intervals throughout long
periods of geological time--so must there have been many outbursts
and upheavals, giving rise to pipes containing diamonds. And
these diamonds, by sparseness of distribution, crystalline
character, difference of tint, purity of colour, varying hardness,
brittleness, and state of tension, have the story of their origin
impressed upon them, engraved by natural forces--a story which
future generations of scientific men may be able to interpret with
greater precision than is possible to-day.