CHAPTER X.
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.