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.