IN olden times little was known about the diamond beyond the superficial facts that it was hard, brilliant, and crystallized in. a certain definite form. In India, where it was first found and used as a jewel, imagination usually answered the questions of the curious, and if the answers were adopted by those in authority, they were universally received, for rulers did not tolerate differences of opinion. So it was that diamonds were believed to be the gift of heaven, crystallized in the earth by thunderbolts. The wise men of the day dutifully adduced as proof, the assertion that diamonds were abundant in mines where there were also thunder-bolts.
As in these days, but to a greater degree, people received the statements emanating from high places with-out question, for it is easier to believe than to think, and so it was that for centuries of bookless, newspaperless years, these statements satisfied a world which had not yet learned to trouble itself much about the antecedents of things.
Generation repeated to generation the explanation, and when the gem began to drift from the old world of the Orient to the younger Occident, the same old story went with it, and was received with the respectful credulity to which such a grave and ancient source was entitled.
It is difficult and sometimes bitter, for a people or an individual, in age, to discard the imaginations of youth.
But after four or five thousand years, the growing light of knowledge acquired about other things, fell from a thousand lamps kindled about it, upon the diamond, and as the glamour which had enveloped it was dissipated, the need came to fill the place of the going fable with facts, for they only could bear the light. The prominence and preciousness of the stone attracted attention, but its value hindered experiments, so that there was little definite knowledge of the composition of it even, prior to the seventeenth century.
During the last two or three centuries, scientists have sought by careful research and costly experiments, to learn how Nature succeeds in isolating one of her elements in such a beautiful and enduring form. But while men have learned to measure the stars, and have conceived an idea of infinity; to harness electricity to wheels and engines and transmit thought on its ethereal waves; while they have filled their archives with a myriad discoveries of light, heat, force, and the whole kaleidoscope of Nature; established the natural rights of man and placed the compass of his mental horizon in the heavens among the gods ; while this and more has been accomplished, all they have learned of the crystallization of carbon is, that it can be done by heat and pressure, and in a very small way to do it.
The Hindus believe to this day that rock crystal is transformed by lightning to diamond. This is a poetical fancy, but it may have some foundation in fact, for the power of electricity over the elements is great, and it is possible that under certain conditions, it could crystallize carbon as it can separate the component gases of water. Some have thought that diamonds grow. The Hindus noticed that they were often found, after heavy rains, in ground that had been carefully searched many times before. The rains undoubtedly washed away the clay which hid them from former searchers, but the finders said ” No, they have grown since we looked last.” There are men to-day, not ignorant or imaginative, who think it possible that diamonds grow by the slow precipitation of infinitesimal crystals to a nucleus.
Shrewd guesses have been made in the past, however, for Boetius De Boot, in the early part of the seventeenth century, arrived at the conclusion that diamonds would burn. Probably about that time there was considerable speculation and some experimenting, in the endeavor to determine the nature of the diamond. Robert Boyle, about 167o, showed that part of one subjected to a high temperature, was “dissipated in acrid vapors.” The in-complete combustion was probably due to a lack of oxygen. In 1694 Florentine academicians succeeded in burning one in the presence of Cosmo III, Grand Duke of Tuscany, by exposing it to solar heat concentrated by a powerful burning-glass. The Emperor Francis I burned diamonds in 1751 in Vienna, by placing them in a smelting furnace for twenty-four hours. Twenty years later M. Macquer again demonstrated the combustibility of the diamond by burning a large one completely.
By these and other experiments, it was learned that the diamond was made of some combustible material, but what that material was, remained a matter of conjecture. It should be remembered here, that combustion as generally understood, is simply the rapid oxidation of the elements of things, accompanied by light and heat of which we are sensible. For instance, if a piece of coal is heated to the degree at which carbon combines with oxygen, the carbon leaves the coal, combines with the oxygen of the air, escapes with it in the form of gas, and we say the coal is burned. There were some at this time who still disputed the combustibility of the diamond: among others, M. Mitouard, a jeweler. In the presence of Lavoisier, the chemist, he took three diamonds and packing them in charcoal in an earthen pipe-bowl, fired them. Upon cooling, the diamonds were found unharmed. Knowing that they could be burned, Lavoisier was not satisfied, and after studying the mat-ter, arrived at the conclusion that the powdered charcoal, by taking up all the oxygen of the air at the combining heat, had prevented any from reaching the diamonds to produce combustion. He further determined the fact that the product of the combustion of a diamond was carbonic acid gas. By experiments, Sir Humphry Davy proved in 1814 that the gem was practically pure carbon. In a practical way Sir George Mackenzie and others did the same, by converting iron into steel by the addition of powdered diamonds, steel being simply carbonized iron. Mr. Smithson Tennant went further and showed that the carbon dioxide produced by combustion, corresponded to the oxygen actually consumed, or in other words, the carbon dioxide evolved, equaled the weight of the diamond plus the oxygen used to consume it and form the composite gas.
Thus knowledge of the diamond was gradually acquired, until the fact that it was simply pure carbon was established beyond question or doubt. But it was a form of carbon only. Graphite, the other form in which it is found in Nature, to sight and touch distinctly different, is nevertheless chemically the same. Though it requires less heat, it combines with oxygen in the same way, the resulting carbon dioxide showing that the graphitic carbon and the oxygen consumed in uniting, exist without appreciable loss in the gas. In comparative tests it has been shown that the diamond burns more easily than foliated graphite, but compact graphite succumbs more readily to heat than the diamond.
Some experimenters claim that upon oxidation, the diamond leaves no residue whatever. Streeter says that in experiments made by Professor Pepper under his observation with about one hundred small stones, a very small amount of bluish ash remained.
When oxygen is supplied, diamonds burn slowly at about the temperature given as that of molten silver. If air is excluded they withstand the heat at which pig-iron melts, but at the temperature at which bar-iron melts, while retaining their form, they become coated with graphite. M. Moissan, using his electric furnace, found that the graphite resulting from the partial burning of diamonds, assumed irregular crystalline forms.
From the various experiments made by a number of scientists, it appears that diamonds at a very high temperature without access of oxygen swell up and are converted into graphite. In a current of air they gradually become smaller and finally disappear. If the supply of oxygen is insufficient for perfect combustion, they become coated with graphitic carbon and burn slowly. At a very high temperature in oxygen, the edges of the sharp angles are first rounded, the crystals split, lose their transparency and luster, and are eventually entirely consumed. During the process of combustion, successive black spots appear on the surface of the crystal and disappear. It also gives out bright red sparks. If the process is suspended, the diamond at once ceases to burn and shows a leaden surface. The inference is, that the heat first transforms the carbon of the surface to the graphitic form, which then combines with the oxygen and passes off as carbon dioxide, or carbonic acid gas.
Many interesting illustrations of the chemistry of diamonds have been given by scientists in their experiments. It has been shown that if one is sufficiently heated and then plunged into liquid oxygen, it burns brightly, and the carbonic acid formed by the combustion, becomes in the low temperature of the condensed oxygen, a solid which appears like snow. The gas from a burning diamond passed through clear limewater will cause it to become milky, and finally, an insoluble compound, calcium carbonate, will be thrown down. By filling a flask with oxygen and limewater, and placing within it a diamond held by a coil of platinum wire joined to the wires of a galvanic battery passing through the stopper, the entire process can be seen upon turning on the cur-rent; the platinum wire will become white hot, the diamond will burn, and the carbon dioxide created, will act upon the calcium hydroxide of lime. At an extremely high temperature, M. Moissan succeeded in volatilizing carbon.
Having settled definitely the question of the composition of the diamond, scientists next turned their attention to the methods or method by which Nature created the compact and beautiful crystal, and incidentally to enquire how and from what source she obtained the necessary carbon. As to the means by which the transformation was effected, they have succeeded so far that they can make the crystals in microscopic size, and thereby illustrate in a general way the larger methods of Nature; but whence she gathered the supply of material for her furnaces is still an open question.
The process of making diamonds as described by Sir William Crookes is to select pure iron free from sulphur, silicon, phosphorus, etc., and pack it in a carbon crucible with pure charcoal from sugar. This must be put into the body of an electric furnace. After heating for a few minutes to a temperature above 4,000 deg. C., at which heat the iron melts and volatilizes, the current is stopped, and the crucible plunged into cold water and held there until it sinks below a red heat.
The outer layer of iron, solidified by the sudden cooling, holds the molten interior in a rigid enclosure. The inner liquid expands as it solidifies, thus creating an enormous pressure, under the stress of which the dissolved carbon separates out in microscopic crystals which though small are veritable diamonds.
Crookes places the theoretical melting point of carbon at 4,400 deg. C. absolute, and the melting pressure as 16.6 atmospheres. He found what he believed to be diamonds, in residue obtained by exploding cordite in closed steel cylinders. This meant a pressure of 8,000 atmospheres and a temperature of about 5,400 deg. absolute.
Prof. Moissan first crystallized carbon artificially.
His method, which has been followed by other chemists, is almost identical with that of Sir William Crookes, except that he plunged the carbon-saturated iron into molten lead, to act as a binder for the expansion by cooling of the interior mass.
Carbon at a high temperature will seize on and combine with oxygen if it exists in any compound, air or what not, with which it comes in contact. It volatilizes at the ordinary pressure at about 3,600 deg. C. and passes from a solid to a gaseous state without liquefying, but as with other bodies of similar action, the addition of sufficient pressure at the necessary temperature is thought to produce liquefaction and with cooling, crystallization. The difficulties, therefore, which scientists had to contend with were, first, to secure the enormous temperature necessary to volatilize the carbon. This was obtained by the development of the electrical furnace. Second, to hold the carbon inert, and prevent its escape by combining with oxygen and flying off as carbonic acid gas. As it was known that molten iron will dissolve carbon, and that any excess of carbon beyond that which the iron can hold will separate on cooling in the form of kish, which are crystalline graphite plates, iron filings were used to enclose the charcoal, and the whole was packed in a carbon crucible. The problem of pressure was solved as described, by the expansion of a cooling interior mass within the rigid enclosure of a suddenly cooled exterior shell.
From these experiments the most generally accepted hypothesis has been advanced, that diamonds are a form of carbon produced by heat and pressure, but how Nature obtained the carbon, held it inert from its affinities, and subjected it to the necessary forces, still keeps the world guessing.
The largest diamond made artificially was less than one millimeter across. Moissan several times obtained as many as ten to fifteen from a single ingot, of which the largest was 0.75 mm. long, the octahedra being 0.2 mm. With the transparent pieces obtained by artificial process are some that are black and some amorphous. Many are shattered, as if they had burst in pieces when released from pressure. Others break and splinter, weeks and even months after they are liberated, the fissures being covered with minute cubes. This tendency to explode occurs among the Kimberley diamonds, where it is not uncommon for one, on being released from the matrix, to burst asunder, especially when warmed by handling or carrying it on the person. Large stones are more apt to do this than smaller ones. It is said that in the old times of individual claims in Africa, miners would encourage responsible men to handle and carry large crystals just mined, thereby transferring the liability of loss at a critical period. It is also reported that it was a common practice in ship-ping large stones to England, to embed them in raw potatoes as a safeguard. Later and careful observation has shown that the stones which explode in this manner are always pale brown or smoky.
The fact that some diamonds taken from the African mines, burst after being released from the matrix, as artificial ones do, is accepted by many as evidence that they were formed under great pressure. Moissan claimed that the form of the carbon depends upon the amount of pressure existing at the temperature which permits transformation.
Some argue that these explosions are due to gas held under great pressure in the interior of the crystal. Mr. Williams declares that to be an argument against the theory of formation in an igneous magma at high temperature. Broken diamonds are frequently found in the diamondiferous pipes of South Africa. Though the cause of the fracture is unknown, it has been ascribed to the volcanic action by which the diamond-bearing clay was forced through intervening strata to the surface.
Many theories have been advanced as to the source from which Nature obtained the carbon. Newton and later eminent scientists believed it to be of vegetable origin, some basing their conclusions mainly on the microscopic study of the residual ash. Crookes on the other hand asserts that iron is the chief constituent of the ash, and uses that as an argument in favor of deep-seated masses of molten iron saturated with carbon, a larger process of the method employed in the laboratory by himself and Moissan. In opposition to this Mr. Williams states that many exhaustive tests which he has made with all kinds of diamonds for iron, metallic or oxidized, with powerful magnetic apparatus, indicated either an entire absence of iron, or infinitesimal traces only. Inasmuch, however, as the crystallization appears to depend largely on the complete segregation of the carbon from that with which it was previously combined, this argument against the theory of Crookes does not appear forcible. As science has made diamonds from saturated molten iron, Nature may certainly have used the same means, though the indications are that it could not have been the only method.
Liebig, Dana and others concluded that diamond is the product of the gradual decay of organic matter under influences at present unknown. The former op-posed the theory of high temperature because under such the carbon would not have crystallized, but would have separated as a black powder. The experiments of Crookes and Moissan contravert this, as they did crystallize carbon under high temperature, though they employed another agency in conjunction which appears not to have entered into Liebig’s calculations, i. e., pressure.
The theory advanced by the late Prof. Carvill Lewis, that the carbon was derived from carbonaceous shales decomposed by the action of an igneous magma forced through them by volcanic action, is considered disproved by the fact that there are no carbonaceous shales in the pipes near Pretoria, though they contain many diamonds. Such shales do overlay the lower strata surrounding most diamond pipes, and as the volcanic filling of the Pretoria pipes may have come from foreign sources, the theory is tenable.
Some have thought that diamond may have been formed from anthracite, possibly without passing from a solid state.
Eclogite deep in the earth was suggested by Professor Bonney as the possible matrix of the diamond, but Mr. Williams answers that eclogite is found in all the Kimberley mines and is thrown out in quantities as waste rock, and that he had over twenty tons of it crushed and carefully examined, without finding a diamond. The idea seems to have originated with the observation that eclogite bowlders were found with rock of the blue-ground type in Africa and the diamond region of New South Wales. Dr. F. W. Voit is reported, however, to say that both graphite and diamond have been found in the eclogite concretions of the Roberts-Victor mine.
Carbonic acid liquefied and held under great pressure deep in the earth, has been suggested as a probable origin of the diamond. The idea apparently is that the liquefied gas coming in contact with some form of carbon preexisting, the carbon would be dissolved, and by the slow evaporation of carbonic acid, the remaining carbon would crystallize. If, however, by upheaval there was a sudden relief from pressure, a quick evaporation would precipitate the carbon in the compact form of carbonado.
Similar to this is the theory advocated by several eminent men, that pure carbon was separated by electricity from carbonic acid surrounded by reducing agents. Other chemists have thought that diamond may have been formed by the gradual decomposition of gaseous hydrocarbons, whereby the hydrogen escaping through fissures, by oxidation was converted into water, part of the carbon into carbonic acid, and the remaining carbon left in a free state, crystallized. It is said black diamond was obtained by Rousseau by subjecting acetylene to electric furnace heat.
It is reported that Dr. Burton of Cambridge has succeeded in crystallizing carbon by means which do not include very high temperature and great pressure. His method is founded on the idea that diamonds are simply a denser form of charcoal. He used an alloy of lead and metallic calcium to hold charcoal in solution. To separate the calcium he introduced steam into the fused mass, whereby part of the carbon crystallized. It is said that if the alloy is in a state of ignition when the steam is introduced, graphite crystals are formed, but if at a lower temperature, diamond crystals. The crystals obtained by Dr. Burton are said to possess an unusually high power of refraction. These experiments have strengthened the belief of some that Nature used some solvent for carbon, as yet unknown, which by evaporation left part of the carbon in the crystallized form, as the crystals of other minerals are.
Hasslinger and others claimed to have obtained microscopic diamonds from carbon dissolved in molten silicates, which crystallized as the mass cooled.
The conditions under which diamonds were found prior to the African discoveries afforded no clue to their origin. In Africa it is evident that they are of subterranean origin, though a full consideration of the conditions there suggests the possibility that diamonds were not always produced by exactly the same methods, or if so, that they were crystallized under somewhat varying conditions and were forced to the surface in material which, if the original matrix, has since passed through a process of alteration.
As scientific experiments have demonstrated that the various forms of crystallized carbon can be produced artificially by a combination of heat and pressure, and we find in Nature that they come from volcanic sources, also that they exist, in form identical with the terrestrial crystals, in meteorites, which are fused masses, heat and pressure appear to have been present in the laboratory of Nature during their production, though the experiments of Dr. Burton suggest that the degree needful depends on conditions. Some are inclined to think that in the presence of favorable accessories, pressure only is necessary.
The Kimberley mines of South Africa lie in a cluster within a radius of a few miles. These mines, together with others in what was the Orange Free State and elsewhere, come to the surface in a great plateau extending from the Transvaal to the Bokkeveldt mountains at the Cape of Good Hope. The plateau varies in elevation from 2,700 to 6,000 feet above sea level, being 4,000 feet above, where the four principal mines are situated at Kimberley.
Until the discovery of diamonds in Africa, in what is believed to be the matrix in which they were formed, there were few hints of its origin in the circumstances of the diamond’s lodgment. It was found always in deposits left by the waters, and the beds in which it lay always showed the alterations of age and exposure. The gem, unscathed, rested in the decomposed fragments of the matrix that ages back had bound it. That the mountains were its original home is evident, for the diamondiferous deposits are on high plateaus, on the sides of the mountains, in the beds of old mountain water-courses, on the hillside banks and in the beds of the new streams, and sometimes far away in the plains below, where the mountain torrents have rolled them. And the crystals hold a record of the long, slow journey. In the mountains, their corners are sharp and clear, but as they get farther from home, they become more and more worn and rounded. Up in the hilltops, the big crystals, wedged in the crevices of the rocks and in the corners among the bowlders, resisted the drive of torrents which carried off the smaller ones with the sand, and held fast, each in turn, near or far, finding at last an anchorage where it could await the coming of man. So long have they lain, that in some places the débris of succeeding ages has buried them many feet deep from the surface. At every diamond deposit the world over, the signs all point to the headwaters of the rivers in the mountains, but there the clue fails, for the rocks beneath and the sky above are silent.
With the discovery of the African diamond chimneys came the conviction that Nature’s laboratory for the crystallization of carbon was deep down in the earth, from which place she belched the product forth to the surface, to be weathered and washed and scattered hither and thither over the face of the earth, as successive cataclysms broke up the shielding walls of rock and exposed their precious contents to the surface elements.
This advance of knowledge gave rise to many new theories, occasioned many and varied experiments, and suggested not a few pertinent questions, most of which yet remain unanswered. Among these queries are several which bar a solution of the problem: I, Whence and in what form did Nature draw the supply of car-bon? 2. How did she crystallize it? 3. Is the kimberlite of Africa the material in which the carbon was crystallized and is that material necessary to its crystallization? Following these comes the question, ” How were these vertical shafts or wells of Africa formed and filled with the diamondiferous earth? Before considering these questions let us review the conditions and circumstances attending the occurrence of diamonds,
In India, diamonds are found on a plateau, four to six or seven hundred feet high, in thin alluvial deposits at or near the surface of the earth. There are two distinct deposits, forming strata in the Upper Vindyan series of the north, and in the Lower Vindyan section (Silurian) of the south.
It is also believed that diamonds exist in the older Paleozoic rocks in the Himalayas, and it is thought that the diamonds of the Mahanadi river have been washed down by the headwaters higher up. The diamonds are always accompanied by pebbles of a siliceous and ferruginous nature, and a variety of others, among them occasionally, corundum. The deposits in which they occur are so altered from their original form that they afford no clue as to the exact nature of the matrix in which the diamond was crystallized. That the matrix was formed long ages ago, then disintegrated and scattered over the earth, is about all we know of the origin of the diamond which was released from its bonds and strewn over the earth, in India during the ages succeeding.
In Brazil the sources of the diamond are extensive elevated plateaus the faces of which are broken up into abrupt, rugged hills and gorges, from whence the diamonds with their decomposed matrix have been carried from level to level, as the mountain torrents wore their channels, through the ages, many being carried by the rivers having their headwaters in the mountains, down to the plains below. Wherever diamonds are found they have come evidently from high places, but in Africa only have they been discovered in their elevation, un-scattered by the waters.
The diamond chimneys of Africa are huge dykes or chasms, penetrating vertically the strata of the country known as the Karoo formation, to unknown depths, and filled evidently from below with a material quite unlike any of the strata which wall the pipes. These walls are the edges of the horizontal layers which form the crust of the earth in that section. In the Kimberley district, under a varying surface deposit of several feet of red clay and an underlying bed of calcareous tufa 5 to 20 feet thick, which covers the pipes and the surrounding strata alike, the layers consist of about 50 feet of pale shales of a grayish color, under which is about 275 feet of black bituminous shales. Beneath this are several hundred feet of melaphyr, about the same thickness as the black shale, and under that, is quartzite and olivine-rock. There are slight variations from this order owing to faults and intrusions, as for instance in the strata about the Dutoitspan mine, in which case there is a layer of quartzite above the melaphyr and 63 feet of diorite between them, but shale, melaphyr, quartzite, and granite or gneiss, is the usual arrangement of the Karoo formation.
The contents of these chimneys are in all cases similar. There are some small variations of little importance, but the general character of the contents of all the chimneys is the same, and in each chimney, except for an alteration of color and consistency in the upper part of the material filling it, due to weathering, the contents are precisely the same as far as they have been followed down. The ” blue ground,” as the diamondiferous material is called, at the 2,500 foot levels, is the same as that 1,000 feet down, and both are the same as the yellow ground which was found near the surface, except that exposure to the weather there had oxidized and turned it to a yellow color, instead of the greenish-blue it is below.
In most cases, the upper part of the contents of these chimneys formed small rounded hills or kopjes, ten or more feet above the surrounding level. The filling of the Wesselton only showed a depression. The diamondiferous material of the chimneys is quite unlike the surrounding reef. Without affecting the surrounding strata in any way, it usually fills the dykes to the walls, though there are intervals, in places, between the walls and the contents, and in these hollows are numerous calcite crystals. Nor do the walls show any signs of abrasion or heat, though the edges of the shales were bent upward slightly, as if by pressure from below.
The diamondiferous rock is a greenish-blue mineral, like dried mud with numerous inclusions. It carries many fragments of the surrounding reef, pieces of the shales being very noticeable. These foreign inclusions vary in size from very small pieces to one so large that it is called ” the island.” This is a block of olivine-basalt in the De Beers mine, having an area of nearly 3,000 square feet and penetrating to a great depth. Some inclusions must have been brought up from great depths, as they differ from any of the strata which compose the reef. Large blocks of gray sandstone, found at a depth of 250 feet, resemble the sandstone which in other localities forms part of the middle Karoo formation, and may be here an underlying stratum at great depth. These foreign inclusions, differing entirely in nature from the diamondiferous material with which they are mixed, are called ” floating reef ” to distinguish them from the walls of the funnels which are termed simply “reef.” The inclusions which differ from the reef are called “exotic fragments.”
The bowlders of floating reef, though occasionally rounded, usually have sharp corners and edges, showing no signs of attrition. They were more abundant in the upper levels of the pipes, but are found in irregular quantities at all depths. In places, the carbonaceous shales were met in such quantities that fire-damp, similar to the dangerous gases of the coal mines, was en-countered.
The diamondiferous material filling the pipes has been variously termed ” serpentine breccia,” ” volcanic tuff or agglomerate ” and later, the name “kimberlite ” was given to it by Prof. Henry Carvill Lewis, and as that is most generally used, reference to it will be made under that name. The kimberlite itself though comparatively soft, is harder in some places than in others. It takes twice as long to weather the De Beers kimberlite as it does that from the Kimberley, and much of the Premier kimberlite needs no weathering, but goes direct from the mine to the washers. It is somewhat soapy to the touch and it can be scratched with the finger-nail, but it has a quality which makes it difficult to work with the pick. It separates easily under an edge tool, how-ever. The various analyses made, agree in the main, the differences being unimportant. One from the Kimberley mine by Prof. Maskelyne and Dr. Flight gave :
Silica (SO2) 39.732
Alumina (A203) 2.309
Ferrous Oxide (FeO) 9.690
Magnesia (MgO) 24.419
Lime (CaO) 10.162
Carbon dioxide (CO2) 6.556
Water (H20) 7.547
100.415
Two of kimberlite from Africa by Prof. H. Carvill Lewis of which I was the least decomposed rock with few shale enclosures and II, the more decomposed rock with many shale enclosures (diamondiferous), are as follows :
I II
Silica, SO2 (with some TiO2) 33.00 34.80
Ferrous Oxide, FeO (including Al203) 12.00 14.40
Magnesia, MgO 32.38 30.76
Lime, CaO 9.60 2.70
Sodium monoxide, Na2O 0.67 1.40
Carbon dioxide, CO2 7.05 5.55
Water, H2O (Carbonaceous matter) 6.00 10.60
100.70 100.21
There are no horizontal layers in the kimberlite nor are there any beds of foreign rock in it, the floating reef being distributed throughout very irregularly, but there are very small vertical crevices in the kimberlite, filled with a foreign mineral resembling talc, which divide the kimberlite into vertical columns. These columns differ slightly from each other in color, composition and contained minerals, though each is the same in character throughout, and all are in general alike. The most important difference is that some of these columns are much richer in diamonds than others. The western end of both the Kimberley and De Beers mines were very poor, the richest part of the latter being in the center. Fifteen of these kimberlite columns have been observed in the Kimberley mine.
From the nature of the kimberlite, and the condition of the reef surrounding, it is evident that the dykes were not made by a volcanic eruption which forced the kimberlite through opposing strata of the earth’s crust, but either existed prior to the filling, as open chasms, or the earth’s crust was rent apart and the cavity simultaneously filled. A local volcanic upheaval of sufficient force to break a large funnel through thousands of feet of the earth’s strata, would not stop placidly when it reached the surface, but would have scattered evidence of its eruption far and wide. No such evidence exists around the diamond chimneys. Nothing has been discovered in the neighborhood of the mines, suggestive of kimberlite. The Karoo strata are over-laid in places by basalt, and everywhere by the red clay and calcareous tufa, neither of which could be altered kimberlite, and in these deposits are no diamonds nor the minerals which accompany the diamond.
Having these facts in mind, it appears possible that in some past age there was a tremendous derangement of the earth’s crust extending from the Bokkeveldt mountains at the Cape of Good Hope, far to the north, so extensive in area, and by a force so evenly distributed, that the strata of the plateau within the boundary walls maintained their natural horizontal trend in general, and which by the spreading of its surface, rent it in places and occasioned the huge funnel-like chasms now known as the diamond chimneys.
It is noticeable that all diamond fields of importance are within 300 north and south of the equator. They are situated, therefore, where vegetation is or has been extremely luxurious. Some of these sections in this age, elevated and denuded of soil, are almost barren, though the surface of the Karoos in South Africa, consisting chiefly of ferruginous reddish sands and clays which bake hard in time of drought, rests on a slaty rock which retains the rain water and keeps alive the bulbous and other alkali plants until the wet season transforms the country, with tropical rapidity, to oceans of blossoms. A large part of the South African plateau lying within the hills of the west coast, the Bokkeveldts in the South, and the Drakenberg mountains which skirt it on the east coast and turning westward form a northern interior frontier in the Transvaal south of the Limpopo river, probably held at one time lacustrine basins interspersed with great stretches of the rankest vegetation, which deposited during the ages immense stores of carbonaceous material. If by any means, vertical fissures were opened in such an area of the earth’s surface, there would be a great in-pouring of this material into the cavities, sufficient one might think reasonably, to supply an abundance of the carbon necessary to pro-duce the very small proportion diamonds constitute of the mass contained in the diamond chimneys.
As stated, this high plateau of the diamond-bearing part of South Africa appears to have been raised to its present elevation, from whatever cause or by what-ever means, either by one uplift, or by a gradual exercise of force which did not break up and distort the trend of the strata. In Brazil and elsewhere, the strata in which the diamondiferous deposits occur are broken and often folded. In many places they are raised to a sharp angle, and occasionally set up vertically, but in South Africa the strata lie in their natural horizontal position, undisturbed apparently except for these vertical dykes which make a clean boring through the even, natural formation. Igneous intrusions exist in places among the strata, and a stratum of basalt caps the shale about some of the mines, but they are quite independent of the diamondiferous contents of the chimneys, and do not appear to have had any influence upon the kimberlite, or to have been acted upon by it.
It is evident that these chimneys are not the vents of sudden, local, igneous, volcanic, eruption. Not only is the crater formation absent, but there has been no over-flow nor scattering of ashes or lava about the mouth of any one of them. The contents have apparently been raised to, or a little above, the surface of the surrounding land by a series of uplifts, or forced upward by the subsidence of the entire plateau. Nor do the edges of the surrounding strata forming the walls of the chimneys, show any sign of igneous action. The face of the quartzite stratum is even and unaltered; the highly in-flammable black shale, though bent upwards at the edges as if by pressure from below and the expansion of the contents of the chimney, carry no signs of firing, and the horizontal trend of the strata is undisturbed. The composition of the kimberlite breccia also suggests the idea that it was not solidified from a molten condition. It contains large quantities of the black shale, and the diamonds are said to be most plentiful where the shale inclusions are most abundant.
Nevertheless, Henry Carvill Lewis, in ” The Matrix of the Diamond,” edited by Prof. T. G. Bonney, says:
” That the rock was a true igneous lava, and not a mud or ash, is indicated by the following facts :
1. The minerals and their associations are those characteristic of eruptive ultra-basic rocks.
2. The porphyritic crystals are idiomorphic as in volcanic rocks.
3. The corrosion cavities in the porphyritic crystals are due to solution by the hot magma.
4. The character of the bronzite and diopside is similar to that in meteorites and eruptive rocks, but not in metamorphic or plutonic rocks.
5. The occurrence of a ground-mass and of traces of glass.
6. The traces of a second generation of minerals (pyroxene?) in the ground-mass.
7. The occurrence of fragmentary enclosures of the ad-joining rock and of deep-seated rocks, and the evidence of alteration by heat which these enclosures exhibit.
8. The traces of a fluidal structure shown on polished specimens.
9. The identity of the rock with one in Kentucky, which is a true eruptive dyke, and with others in the Vaal river, which also form dykes.
Undoubtedly the filling of these vertical dykes came from below. It is therefore an eruptive rock. It also appears from exhaustive examinations of its composition by Professor Lewis and others, that a part of the material at least has resulted from a molten condition. It does not appear possible, however, that some of the inclusions could have entered it while in that state, and in appearance it bears no resemblance now to the lava of volcanoes; its constituency suggests a dried and hardened mud. It has been said that the sharp edges of the diamond crystals found in the kimberlite would be impossible had they been formed in a molten mass, but as Moissan produced such diamond crystals, though small, from charcoal confined in fused iron, and diamond will not burn without a free supply of oxygen, the argument appears invalid.
Geologists assert that the center of the earth is solid, but that between the crust and that solid center, lies a mass of molten material. ‘ They also claim to have in-dubitable evidence that the earth’s bulk is gradually shrinking, while at the same time by astronomical forces. it assumes a somewhat elliptical form at the equator. In the process of shrinking, the uneven thickness and strength of the crust would produce uneven results. Some weaker parts of the area would settle lower, toward the center of gravity, leaving other stronger parts elevated above the general level, and they would become, thereby, mountain ranges where the buckling occurred, and high plateaus, if the area was large, within the mountainous border lines of greatest strain, marking the junction of the weaker sinking portions of the crust and the thicker and more stable part.
It seems reasonable to suppose that some such occurrence took place during past ages in South Africa, whereby the earth’s crust seaward, east, west, and south from the mountains surrounding the diamond plateau, sank, leaving the plateau at an elevation, with undisturbed horizontal strata, except for occasional vertical rents in it extending probably to the underlying magma. This hypothesis seems more probable than that of a deep explosive or expansive force sufficiently extensive and simultaneous in its action to lift such a tremendous area with little or no derangement of the strata within its mountainous borders. It would also account for the absence of eruptions of a volcanic nature, and the steady pressure of the crust upon a molten interior, would ex-plain the intrusion of igneous material, in places between the regular order of the strata, where the strain of rearrangement had left interstices.
There is always a period of strain before things as they are, break to a rearrangement. There must be a climax of power to produce results. The storm gathers before it bursts into thunder. For some time, when a volcano is in action, the internal pressure of gases must gather force before it is sufficient to burst asunder the old walls and cap of lava which held them pent up within. Then force with gathered impetus bursts forth and runs riot, and finding all the weaker spots in its path, vents itself there. It was probably so in the sinking of the earth’s crust around the diamond plateau. When the earth, seaward of what are now the mountains, sank, and the earth’s crust buckling made the mountains, the plateau, with rumble and roar, cracked in places to its foundations, and here and there over its wide face, the diamond chimneys were opened up.
These conditions being obtained by the pressure of gravity or weight from without the earth toward the center, effectual only over an area outside the boundaries of the plateau, the opening of these funnels to the interior would not necessarily produce violent eruptions of the molten material underlying, even if they penetrated the crust to it. Such eruptions arise from chemical reactions which change existing combinations into others requiring more space, as heat transforms water into steam. These being absent, the molten material would simply ooze into the funnels, and rise with the settling pressure of the crust of the plateau.
Another important factor would be introduced by the rending of the earth’s crust. Immense quantities of surface material, including probably great volumes of water, would pour in, dislodging and carrying with it fragments of the earth’s strata, from the surface down, which had been broken or loosened when the break occurred. At first this material would be assimilated on reaching the interior heat, but gases would be generated, steam evolved and a great cauldron of magma permeated with superheated steam, established. Huge bubbles would lift this mass in columns toward the surface; explosions would rend and dislodge protrusions of the reef about the walls of the chimney, and break up deep lying strata into fragments which would also be mixed and lifted with the mass. Probably very deep connections with similar funnels in the neighborhood would be established.
As the upper mass cooled, the fragments of the surrounding strata carried or falling into the cauldron would, in the inclusion, hold their original form and be recognized later, as the inclusions of the kimberlite are today.
Upon the character of this surface supply of material, the presence of diamonds in the agglomerate probably depends. There has been and is a general supposition that diamonds are always associated with kimberlite, but that the latter does not necessarily contain diamonds is demonstrated by the fact that it occurs in various places, notably New York State and Kentucky, without any, and in Arkansas though some diamonds have been found in a large body of it there, it is doubtful if that contains any considerable quantity. It is evident, therefore, that if the elements contained in kimberlite, under certain conditions, are requisite for the crystallization of car-bon, the presence of carbon and its crystallization have nothing to do with the peculiar formation of kimberlite. The South African chimneys are also traversed by dykes of kimberlite which contain few if any diamonds. It is the breccia, or more decomposed kimberlite containing the shale enclosures, which is diamondiferous. This black shale in the stratum surrounding the chimneys is combustible, but the fragments in the breccia have lost their sulphur and carbonaceous matter. Few diamonds are found in purely igneous or metamorphic rocks, though Henry Carvill Lewis referring to kimberlite says, ” Certain resemblances can be traced to the ground-mass of sundry decomposed basaltic or other basal rocks.” Sir H. E. Roscoe found on treating ” blue ground ” with hot water, ” an aromatic hydrocarbon could be extracted, and by digesting it with ether and allowing the solution to evaporate, this hydrocarbon was separated and found to be crystalline, strongly aromatic, volatile, burning with a smoky flame and melting at 50° C.”
These facts remind one again of the probable surface conditions existing at the time of the opening of the diamond chimneys through the earth’s crust. It is noticeable too, that beyond the trace of hydrocarbon in the ground-mass, and the carbon in the calcite, which is a decomposed product, all the carbon which entered into the original mass and remained, appears to have been segregated as diamond, though a considerable amount escaped probably as carbon dioxide.
The inference that the supply of carbon came from the surface seems justified also by the fact that the yield of diamonds in the chimneys was greatest in the upper levels. It is true that the surface yield of some of them was less than at a depth of several hundred feet, and that in one or two cases where the yield has been small from the beginning, the percentage continues very even, but generally there is a steady decline in the percentage of yield as the workings are carried to greater depths.
The sinking of the earth’s crust outside the borders of the diamond plateau and the natural gravitation of the plateau itself, would establish a steady pressure upon the underlying molten material and force the magma up the vertical fissures and into and through the surface material draining into them. This would result in the heating of the surface supply, the cooling of the magma, and the amalgamation of both. The pressure, however, would not be constant. Occasional slips in the readjustment of the earth’s crust would suddenly force columns of the cooling agglomerate upward, and this raising process would be repeated until the mass had become sufficiently solidified to resist the pressure from below. In this manner it is conceivable that the dykes could have been filled as we find them, by successive upheavals of separate columns.
Reviewing facts and inferences that may be fairly drawn from them, it seems probable that the diamond plateau of South Africa was left at an elevation by the shrinkage of the earth’s crust surrounding it.
Vertical rents, in the plateau were made in the process, into which a magma of ultra-basic rock exuded from the interior, and a mass of hydrocarbonated material poured from the surface, forming an agglomerate having the characteristic of an altered eruptive rock, yet differing from any other lava known.
Owing to the precipitation of carbonaceous surface material into the magma confined in the depths of the vertical fissures, processes ensued which segregated the carbon in the mass and crystallized it as diamond.
The cooling and cooled mass was raised in the chimneys by successive uplifts, occasioned by the generation of gases within the mass and the settlings of the earth’s crust.
The yield of diamond will decrease as the rock passes from an agglomerate of igneous lava and surface material, into the underlying eruptive rock which was not reached by the surface admixture.
Inasmuch as it is the brecciated kimberlite only which contains the diamonds, and the breccia though somewhat altered, has not been fully amalgamated with the ground-mass, the kimberlite was not in a state of ignition when the diamonds were crystallized.
The chemical reactions whereby the carbon was crystallized, remains a subject for speculation and the experiments of scientists, but it appears probable that it was accomplished in the African diamond chimneys by the passage of superheated steam through an agglomerate of magma while being cooled by carbonaceous material and water poured into it from above. That the crystallization of carbon as diamond does not depend absolutely upon the geologic structure, during crystallization, of the matrix in which it occurs, appears evident from the fact that diamonds have been found in eclogite, itacolumite and an igneous rock. Professor Bonney found ten small diamonds embedded in a bowlder of eclogite from one of the Newlands pipes in Griqualand West. It is reported also that they are found occasionally in the Roberts-Victor mine in the same matrix. In Brazil though usually found in drift, they occur to a limited extent in the itacolumite, thought to be the original matrix, and which by decomposition furnished the diamondiferous quartz pebble drift. Some geologists think that the Semri sandstone of India was the matrix there, because many fragments of it are found with the diamonds in the quartzose conglomerate which is the diamondiferous material of some parts of India. A diamond was found embedded in horn-blende diabase at Oakey creek near Inverell, Australia.
The sparse occurrence of diamond crystals in unaltered igneous rock, and their abundance in the kimberlite breccia, suggests that crystallization occurred during the metamorphosis by hydration of an igneous magma composed of favorable reducing chemical constituents. That the crystallization of carbon can occur under in-tense heat and pressure has been demonstrated by Professor Moissan, but that the heat and pressure was applied in the same manner in the diamond chimneys appears doubtful, for in them, the quantity of diamonds de-creases with the approach of the diamondiferous material to the source of heat, and the associate minerals are chiefly silica and magnesia. A natural solvent for carbon with sufficient heat to cause the necessary chemical reactions, and pressure, is probably Nature’s method of crystallizing carbon.