GEOLOGY OF THE MINE AND SURROUNDINGS.—SECTION OF REEF STRATA.—SURFACE
SOIL.—CALCAREOUS TUFA.—LIGHT COLORED SHALES.—BLACK CARBONIFEROUS
SHALE.—LIMONITE. LANDSLIPS.—BURNING REEF.—SULPHUR VAPORS.—NATIVES AFRAID
TO WORK.—COAL PLANTS.—FIRE AND CHOKE-DAMP.—IGNEOUS ROCKS.—CONTRACTION
AND EXPANSION OF MINE.—STRATA OF MINE ITSELF.—RED SAND.—TUFA.—YELLOW
GROUND.—BLUE GROUND.—RICH AND POOR CLAIMS.—REMARKABLE BOULDERS.—GREASY
SLIPS.—MESSRS. MASKELYNE AND FLIGHT’S OBSERVATIONS.
Any work dealing with Kimberley would be incomplete that did not treat
of the geology of the mine. For many years I have had the pleasure of an
intimate acquaintance with Mr. George J. Lee, who has made the
mineralogy of the Kimberley mine his special study, and who has been, as
I have already mentioned, in Kimberley since the mine opened. To his
kindness I am indebted for most of the information and for the drawings
in the two following chapters.
In the first of these I propose to give an account of the geological
formation, not only inside, but also as revealed by shafts sunk by the
Kimberley mining board and the Central and French companies, outside the
mine.
The depth of the lowest working, either within or without the mine, is
now about 550 feet, whilst that of the deepest trial shaft is 620 feet.
A section of the strata in the Mining Board shaft, about 700 feet from
the north side of the mine, I will now describe, which strata, I may
tell my readers, vary very little at any point in the vicinity, so far
at least as is shown by the examination of the other shafts.
The surface soil, which has an average depth of six feet, is a bright
red ferruginous sand, composed of somewhat fine rounded grains of
quartz. Next a thin layer of calcareous tufa is found in some places,
but not invariably; then follows a layer of laminated trap or blue whin,
evidently an intrusive rock varying in thickness and depth in different
localities. This layer, which is of a very decomposed and friable
character, runs down diagonally in some parts, in broad sheets through
the blue shale. A very even layer of light colored shales, on an average
twenty-five feet thick and very soapy to the touch, comes next. These
shales are of various colors, as pale bluish white, olive, yellow, gray,
etc., and the laminæ are often thickly marked with different designs
formed by decomposed iron, probably pyrites. Some of these markings are
very delicate and beautiful, often resembling minute ferns or _algæ_,
and in this bed various fossils have been found.
Beneath these we have a vast layer of blackish, or neutral tint,
carboniferous shale, containing four or five seams of “iron band” (bog
iron ore, or limonite) from one inch to one foot in thickness. The
nodules forming these “bands” are very full of cavities, and are of
different colors, yellow, red, and blue, with a dull appearance and
rather soft. In some specimens the colors are beautifully bright,
especially in the cavities. Many fossils have been found in this shale
as well at all depths.
Very many nodules of a similar shale, but harder, varying in size from
an inch to one or two feet in diameter, are found imbedded in this rock,
or “main reef” as the miners term it, in contradistinction to “floating
reef,” an erratic rock which will be mentioned when we come to describe
the diamondiferous ground. These nodules often assume the forms of casts
of shells, such as those of oysters, mussels, etc., and give the idea
that the main body of the reef was formed from the grinding down of an
older rock, of which these nodules are the remains, an assumption which
there is much further internal evidence to show in all probability to be
correct.
It may be as well here to mention that all the above shales, after
drying by exposure to the air, disintegrate and form a fine friable
mould, or even mud after much rain has fallen. This property of becoming
friable after exposure is the cause of the frequency of “land slips”
falling into the mine. When a fall or slip does take place, the bulk of
such mass is usually in very small pieces, not in large solid blocks,
which would be the case if the reef were not of a nature so extremely
rotten and easily decomposed.
When these shales have been much exposed to the weather and the rain has
penetrated to some depth, or when after a fall of rain upon friable reef
this is covered up by another slip, a very interesting chemical process
is set up, viz.: the decomposition of the iron pyrites which is very
plentifully distributed throughout the black shales. This decomposition
sets up such an intense combustion that the shale _débris_ becomes red
hot. This combustion I have known to last for months, even years, and
the sulphurous acid gas evolved to be so plentiful that in damp weather
or before and after sunrise it could be seen for many miles. To
strangers looking into the mine this “burning reef” is a very startling
sight, and many visitors of a superstitious turn of mind have often
precipitately left in affright, on being jokingly told by miners when
the latter were questioned about the heat, smoke and pungent sulphur
smells—that the “Old Gentleman” was trying to break loose, or that they
had come so near the “Old Gentleman” that he was now showing his anger.
This and similar stories have often sufficed to scare away simple
farmers from the place.
On many parts of the reef the surface becomes heated to such an extent
that articles placed upon it are almost at once destroyed. Laborers
cannot work upon it at all without thick boots, and these even are
destroyed in a few hours, while in many cases natives cannot by any
inducement be persuaded to go near.
The sudden combustion of coal in ships is caused by the very same
agents, moisture and insufficient ventilation. These set up chemical
change in the iron pyrites contained in the coal, and thus cause the
explosions or fires at sea of which we frequently hear. This process of
nature for reducing the iron pyrites is daily being imitated by the
smelters in Spain and other countries, in the preliminary process for
the reduction of the sulphur in copper ores, or copper pyrites.
Some of the crystals of iron pyrites found in the reef shale are so very
beautiful and perfect that they have been set in pins and other
ornaments.
After the black shale reef has been ignited by the above chemical means,
and has cooled, it is as red as an ordinary clay flower-pot. It may be
that the promoters of a brick company that was started here for the
purpose of manufacturing bricks from the shales of the Kimberley mine,
took their idea from the appearance of the shales when thus burnt by
nature. The bricks made from reef, after being pounded and ground by
heavy steam machinery, were latterly turned out of a very good quality,
but the company collapsed, probably because they could not sell enough
bricks to make it pay, as the high price of fuel rendered it necessary
to charge £11, 10s. per 1,000, a nearly prohibitive price for ordinary
building work.[25]
I may here mention that several narrow streaks of coal varying from an
inch up to one of eight inches in thickness, discovered in the French
shaft, have been found in the black shale. This coal partakes of the
nature both of anthracitic and bituminous coal, the former quality
predominating, but I may say, that even if it were found in quantity, it
would only be suitable for specially constructed furnaces, as it is very
refractory.
No “structure” or organic remains have been discovered in any specimen
of coal found in the reef of which I am aware, but many coal plants have
been found in the reef itself, and specimens have been deposited in
various museums.
Some years ago the olive-colored shales of the reef at the southeast of
the mine very frequently cracked, making great fissures at the surface.
These often remained open for months and even years, until the whole
mass of reef which was thus detached from the main body, gradually slid
down to a solid bearing. In doing so, when the moving mass was heavy
enough, the pressure upon the particles of rock within the fissure was
so great that a fine, smooth and shiny face or a light gray color was
made, entering many feet, with streaks or grooves running down,
glistening and lustrous, the prismatic colors of which could then be
seen for weeks together if viewed from a suitable position. When seen to
advantage the sight was really charming, and could never be forgotten by
those who had the good fortune to view it. But the first shower of rain
would disintegrate the striated face of the rock, and by this means
destroy the beautiful play of light described.
In the drives and deep workings in the black shale the miners are
troubled both with fire-damp and choke-damp, and many accidents, more or
less serious, have occurred therefrom through their ignorance.
The black shale is water bearing, and wells sunk in it to a depth of
eighty feet or more yield in most cases a good supply.
I have, I think, for the present done with the sedimentary formations,
and come now to the igneous rocks below them.
The first rock under the black shale gone through, as shown in the
diagram, is a light gray volcanic rock, three feet thick, with a
specific gravity of 2.815. The body of the rock is of a light bluish
gray color, and contains beside other minerals nodules of quartz, agates
and jasper, from the size of a mustard seed to that of a walnut.
Next is found eight feet of a compact augitic or hornblendic rock, of a
deeper gray color and very tough, then a seamy rock resembling basaltic
trap, which was only worked to a depth of four feet in the shaft shown
in the section, but is of unknown depth, for, since the Mining Board
shaft was abandoned, others have been sunk, one by the French company,
and another to a depth of 620 feet by the Central company, and they both
are still in this same rock.
This rock contains very numerous nodules of quartz (amygdaloidal), some
of which are split into flakes, and others completely fractured. Many of
these are white in color and semi-opaque; others have a red skin
(jasper), with transparent white quartz within, while others are
entirely red. There are also many small, dead grain-like nodules of a
white color, as well as ribbon and other agates. In some of the deeper
portions of this rock are many small fissures, presenting somewhat the
appearance of having been caused by shrinkage; these, in most cases, are
distinct from each other, and are filled with white crystallized
calcite, or carbonate of lime. Some of the layers of this rock vary in
color, and a very compact fine-grained specimen of a reddish color
contains amongst other minerals rhombic crystallized carbonate of lime,
white; very brilliant iron pyrites, and a mineral looking very much like
galena, but harder, of a dark steel gray color, very brilliant, and
easily separated into small cubes and laminæ. This is probably specular
iron ore. Another mineral has also been found in small quantities,
running in veins in the stone, which upon the top appears as having been
fused and run, like tin or lead. It is sectile, and of the color of pale
bell metal. Very many forms of crystallized carbonate of lime, including
“dog-tooth” spar of a pale yellow color are found both in the hard and
soft reef, and zeolites of various colors and species, bristling upon
and within cavities of the rock. The reef as a whole is fairly even all
around the mine, but at the southeast it is very much contorted in the
upper layers.
The wall formed by the hard rock around the mine is very compact and
smooth, and runs inward in some places as much as 30° or 40°, but in
others very much less. This naturally causes a cutting out of the claims
or a contraction of the mine, but it is said that the blue ground of the
Kimberley Central company is now gaining again, and this state of things
may also take place with other claims in the Kimberley mine at different
depths. (See illustration.)
I think this is a fairly full and accurate description of the geological
strata outside of the Kimberley mine. I shall now pass on and give a
description of the formation of the interior of the mine.
When the place was first prospected there was little to distinguish it
from the surrounding country, but in the eyes of the experts of those
days the slightest difference was enough to urge them to seek,
“fossick,” or prospect for diamonds, that is to say, scratch or dig up
the surface, sieve and sort it, and sink small trial shafts, when if
diamonds or good indications were not discovered at a moderate depth,
the place was abandoned, and the prospector tried or “fossicked”
elsewhere. A digger would pitch upon what appeared to him to be a likely
spot, when, if after passing the red sand and the lime or calcareous
tufa, he came to shale, he abandoned his “prospect” as useless; but if,
on the other hand, after going below the lime he came to “yellow
ground,” a substance something like greenish compact wood ashes, he
would continue his work for some time, in the full expectation of being
rewarded in the end by a good find. But in many cases this desired
result was not attained, although the digger had every encouragement to
persevere by finding garnets of various kinds, the pyrope garnet,
usually called a ruby, especially giving him encouragement to proceed
with his work; epidote, pisolites, talc ilmenite, called by the diggers
carbon, iron pyrites, ice spar, zircon, and various other minerals;
still after all his perseverance, and after finding all these
indications, and sinking to a depth very often of twenty or thirty feet,
not a single diamond would be found, he would abandon the place, when
possibly after a time it would be tried again by other parties of
diggers, but with no better result.
The surface, as already stated in the description of the reef, was a
red, sandy soil of an almost uniform depth of six feet, followed by a
layer of calcareous tufa and one of yellow diamondiferous soil,
averaging in thickness sixty-five feet.
In the red sand diamonds were frequently found, especially when it was
mixed with nodules of calcareous tufa which had been thrown out by the
ant-bear (_Myrmecophaga jubata_) in making its enormous burrow, but the
distribution of the diamonds which were picked up on the surface of a
large tract of country was mostly attributed to birds. Coming next to
the tufa bed proper, this was of a thickness from two to eight feet, but
was not a compact homogeneous mass, but composed of honeycombed nodules
and masses impacted together, which required much labor to break out.
Some diggers smashed up all the nodules thus broken out with sledge
hammers, or the sides of their picks, and sometimes, but very rarely,
they were rewarded by finding diamonds.
[Illustration: Kimberley Diamond Mine, Griqualand West:——South
Africa.—1885]
When the yellow ground was arrived at, which, as already stated, was of
an average depth of sixty-five feet, and contained many nodules of
calcareous tufa of all sizes, the color was of a pale yellowish green,
but when it became dry it was somewhat of the color of bath-brick, very
friable, and appeared much like fine wood-ash pressed together. It could
be broken up to a fine powder with very little beating either with
sticks or shovels. Most of this yellow ground was sorted dry, as washing
was not practiced at the dry diggings at that time.
Some few feet, perhaps eight or ten, before the yellow ground joined the
blue, a very gradual change of color took place from yellow to a
lightish green, then light greenish blue, gradually darkening to blue;
but still a line of demarcation of the two kinds of ground could easily
be made out.
The top of the blue bed was not a level surface, but full of heavings
and billows like a choppy cross sea. From the crest of a rise to the
bottom of a hollow was about six feet; and the distance from crest to
crest from thirty feet to sixty or more.
At this junction there was found in many instances a thin layer of
porous soil holding water in the hollows or wave bottoms. This proved to
be merely catch water, as after a short time it was invariably removed
in the ordinary course of work, which would not have been the case had
it been derived from perennial springs, yet this at the time created
considerable alarm. All or most of the water that now finds its way into
the mine, is through the main reef.
To turn again to the blue ground, it is mixed with small rounded stones
of basalt and small angular fragments of carboniferous shale, as well as
with many other minerals which I shall mention further on.
The blue ground is rather hard and tough when wet, but easily broken
when it becomes dry, when the same characteristics which the black reef
presents here show themselves in pulverizing when again wetted, much
after the manner of quicklime, but differing from it in not developing
heat. This property is made use of by the miners to release the
diamonds.
Miles of country may be seen covered with blue lumps spread out to a
thickness of from one to two feet, awaiting rain, or for the purpose of
being watered by means of carts and water-hose, after which process it
is rolled and harrowed by means similar to those used in dressing
ploughed fields. The diggers used to employ gangs of natives to beat and
break up this diamondiferous ground with picks and wooden beaters, yet
the dusky native found time with eagle eye to watch his chance to steal.
In this way it was thought the majority of the largest stones were lost
to their rightful owners—but of this in another chapter.
The specific gravity of blue ground from claim No. 132 at about 170 feet
from the surface was 2.268, air being 60° Fahr., and barometric pressure
25.83 inches. A cubic foot of wet blue ground would therefore weigh
141.34 lbs. The density varies slightly in different parts of the same
claim and in different localities of the same mine.
The greatest depth to which the blue has been worked in the open is 420
feet, and underground by means of shafts and drives about 620 feet. The
deepest trial shaft, as I have already stated, is 620 feet, and still no
change is found in the character of the ground, except that it is a
little denser and harder to work, and in most cases is also richer in
diamonds.
It is a curious fact that when a claim was first opened out and found to
be rich in diamonds, it generally remained so right down, and in like
manner poor claims remained poor.
But bad or poor layers for a time changed this rule, so far as rich
claims were concerned, for after some feet had been passed, say ten to
thirty feet, a change for the better would invariably take place, so
that in the long run a close estimate of what a claim would yield could
be made. At the present time claims which were enormously rich at the
surface, notably those now forming the Northeast company, were then and
are now amongst the richest claims in the mine. I may also note that a
belt existed around the mine adjoining the reef, from one-quarter to
half a claim in width, which was invariably poor.
Poor surface claims often became changed to rich ones owing to the
vicinity of a wall of floating reef, and it was often found that after
months, even years, had been spent in working down a claim unprofitably,
it would suddenly alter to a payable or probably a rich one, for at a
varying depth alongside the floating reef, or after the reef had been
removed, I have many a time known a change for the better to take place,
and the owner, who had been almost reduced to the verge of bankruptcy,
to be soon set upon his legs again.
Some of the claims at the west of the mine never paid the expense of
working at the surface, nor even in the yellow ground below, and remain
absolutely worthless to this day, although the blue ground has been
reached, worked, and tested. About five claims in the very centre of the
mine were proverbially poor, vast quantities of erratic boulders being
mixed up with, and in some cases almost displacing, the yellow and blue
ground. Some of these fragments of rock were of immense size, weighing
thousands of tons, and at the same time were so solid that they had to
be blasted to pieces. They consisted for the most part of dark and light
colored shales, whinstone or basaltic boulders, and large masses of
fine-grained micaceous sandstone, containing fragments of coal and
remains of a fossil reptile. Much lignite in large stems and branches
was also found amongst the boulders and in the blue. When this floating
reef was removed, many feet of unprofitable blue had also to be removed
before payable ground was reached, but when all the poor ground was
taken away, the junction of which with the richer ground was very
perceptible by a change of color, the remaining blue did not differ in
average return of diamonds from the rich claims surrounding it.
Several remarkable detached boulders were found. One in No. 3 road
south, of such a large size that it covered the two claims of a
well-known digger named Olsen, Nos. 136 and 166. It was a pudding-stone,
its chief constituent being grayish white crystals and fragments of
feldspar, transparent and translucent, concreted together with carbonate
of lime. This “erratic” contained also nodules of quartz, crystals of
iron pyrites, and many small cavities filled with diamondiferous soil of
a light brownish gray color.
Again in Nos. 8 and 9 roads south, there was an isolated basaltic
boulder, nearly round, of gigantic proportions. It almost filled up
claims 432 and 433 in road No. 8, and claims 462 and 463 in road No. 9,
and measured in diameter 70 feet, 35 feet in the yellow ground and the
same in the blue below. It was much decomposed, and large flakes or
layers frequently fell away into neighboring claims, causing many
lawsuits. The claim-holders burrowed round this piece of rock as long as
they possibly could, but at last they had to face the expense of
breaking it up and removing it. The upper portion of this stone in the
yellow ground had a yellow, and the lower portion in the blue a blue
tint, showing that the same cause which affected the coloring of the
diamondiferous ground also affected the imbedded “erratic.”
The floating reef proper in the Kimberley mine extended from the
southeast to the northwest; but there was also a large patch of floating
reef running north and south in No. 8 road.
The width of this reef varied from five feet to forty feet, and the
average depth was about 200 feet. It was formed of a light olive-gray
colored laminated shale, with a few rounded stones and angular pieces of
basaltic rock, as well as occasional fragments of fine grained
sandstone. These walls of shale were for the most part compact and
unbroken, with a few straggling pieces of various sizes in the immediate
vicinity. This rock was totally different from any forming the walls of
the mine, and no similar shale has been recognized to my knowledge in
any part of the country.
At the southwest of the mine, in ground at that time belonging to
Messrs. Lewis and Marks, there was a large floating reef composed of
disintegrated igneous rock, resembling dolerite, which commenced near
the main reef, and which jutted out to a distance of 120 feet in a
northwesterly direction; this was a different rock from that forming the
walls of the mine or any discoverable in the adjacent country.
Veins or seams of crystallized carbonate of lime frequently run through
the blue ground in all directions, in sheets more or less broad, and
varying in thickness from a mere trace to two or three inches. The
calcite is covered with a coating of a grayish white substance, very
soapy to the touch, resembling steatite. Some of this lime contained
cavities, having a fine deposit of iron pyrites which exhibited the most
brilliant iridescence conceivable. One specimen which I saw was so
beautiful that it was thought even worthy of presentation to royalty,
and the owner gave it to Captain Harrell (formerly Cis-Molappo
commissioner) for the purpose, but through some blundering or other it
never reached its destination. Eventually, I believe, Captain Harrell,
with the consent of the donor, caused it to be deposited in the South
Kensington Museum.
When Sir Henry Barkly was here in 1872 this was shown him, and he
confessed it to be unique, in his opinion, and the prettiest specimen of
the kind he had ever seen.
When a wall or block of blue ground is dressed down and left standing
with such a vein—or _greasy slip_, as it is termed—in it, it becomes
highly dangerous, as all above the vein (at times an immense mass) is
liable to come down without a moment’s warning.
In 1874 an account of the chemical and optical properties of the yellow
and blue ground, and the contained minerals was given in London to the
members of one of the learned associations there by Prof. N.
Story-Maskelyne, F. R. S., keeper, and the late Dr. W. Flight,
assistant, of the mineral department, British Museum, and although
rather technical in character will no doubt be interesting to those who
would like to make themselves acquainted with the formation of the
diamond mines. Extracts from this account will show the nature of the
rock commonly but erroneously called the matrix of the diamond, and also
furnish a list of minerals found in the samples furnished to the above
mentioned scientists. Alluding to the yellow and blue these gentlemen
say:
“Ground mass, pulverulent, soapy, light yellowish in the upper, and
of an olive-green to bluish gray color in the lower regions of the
excavations, is a hydrated bronzite. Through it is disseminated a
considerable amount of vermiculite.... In this ground mass fragments
of shale and a micaceous looking mineral—sometimes an important
constituent.... It is a mineral of the vermiculite group....
Ferriferous eustatite (bronzite), prismatic crystals of a bright
green color not infrequent, of the size of canary seeds—colored
brilliant green by nickel.... Hornblendic mineral accidental—closely
resembling smaragdite garnet. Ilmenite.[26]... Diallage much
altered, opaline silica, sometimes resembling hornstone. Calcite.
Bronzite, small bright green crystals, with something of an emerald
tint, prismatic; angle 87° 20′. Hydrated bronzite; drab (or pale
buff tint) much broken up, and cemented by calcite in bar-like
forms, resembling feldspar.... Through it is disseminated a
considerable amount of vermiculite—vaalite—surface of a fine bluish
green like that of clinochlore, giving color to the mass. Hexagonal
prisms 60° and 120°, resembling halite. Garnet zircon, brownish
white. Hornblende crystals, with the appearance of smaragdite.
Grossular garnet. Brilliant little black tourmalines.
Smaragdite:—brilliant grayish green fragments of crystal, angle 125°
15′ equal to hornblende type. Olivine, steatite; variety of
vermiculite, or a ferriferous eustatite or bronzite. Transparent
striated mineral, of fibrous irregular outline, augitic cleavage;
pale brown, in some lights a violet tinge. Between the fibres minute
bars of brown vaalite, but tolerably free from calcite; occasionally
associated with a yellow wax-like substance, probably opal. This
striated mineral may be regarded as made up of
Bronzite 43.850
Hydrated bronzite 24.017
Opaline silica 30.895
Alumina .970
Chromium oxide .251
——————
99.983
“The base of the rock[27] consists of the same ingredients as that
described above, the mass of it being the hydrated bronzite. The rock
is further very full of fragments of the shale, which has been
altered, but still contains carbon; indeed the character of the rock
is almost that of a breccia, in which these masses of shale are
cemented by hydrated bronzite containing the vaalite and the bright
green bronzite, with ilmenite and the other minerals associated with
it.... The several minerals composing the rocks, exhibit this
undoubtedly once igneous rock in the light of a bronzite rock,
converted into a magnesium silicate, which has the chemical character
of a hydrated bronzite.... The steatite like magma in which the other
minerals and shale fragments are contained may have originated in an
augitic mineral, but this is not very probable. The alterations that
have ensued from the shattering of the eustatite rock, at a period
subsequent to its becoming solidified, having aided in effecting the
hydration that has so largely changed it from an eustatite rock into a
mixture of eustatite with a hydrated eustatite, a combination which,
both in its composition and structure, recalls vividly to the mind the
similar mixture of the former mineral with the so-called pseudophite
in which it occurs at Zdar, in Moravia.”
From the foregoing description a fair idea may be arrived at of both the
formation and appearance of this mine, which may fairly lay claim to be
one of the wonders of the world. In the next chapter I will give the
principal theories concerning the origin of this mine.
[Illustration]