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.