Inquiry into the Origin of Mud Bushes in De Beers Mine. 57 



While the rainfall theory has little support from these statistics, 

 its insufficiency is quite demonstrated when the actual rainfall and 

 mud-rush dates are put side by side. During the whole three years 

 there were twenty-six instances of wet weather in which the rain 

 exceeded 5 inch per day. Six of these were followed within eight 

 days by a mud rush, and twenty were not. It is to be noted, more- 

 over, that this ratio is obtained after favouring the rainfall theory to 

 the utmost ; for example, if the mud should come within nine or 

 ten days after the rain, or if the rain should not quite total - 5 inch, 

 it is still counted in its favour. Again taking the date of each mud 

 rush and putting it into one column or another, according as it was 

 or was not preceded by a rainfall of not less than *5 inch, we get 

 nine instances in favour of the theory and forty-three against it ; and 

 this also after favouring the theory in every reasonable way. It may 

 further be added that the heaviest rainfall of the whole period, i.e., 

 nearly 4 -J- inches in the twenty-four hours ending 8 p.m. December 

 30, 1896 (of which nearly 3 inches fell in two hours) has not been 

 followed by a mud rush. 



But, it may be objected, is it not still possible that a mud rush is 

 due directly to a heavy downpour of rain, even though nothing of 

 the sort takes place within any specified period of time ? Might not 

 the water lie dormant until its chance came ? To understand quite 

 clearly all that can be said in favour of this argument necessitates 

 an acquaintance with the rudiments of diamond-mining. But for 

 the purpose in hand Fig. 5, representing a section of the mine 

 through the main shaft, may prove sufficient — in essentials at any 

 rate. The De Beer's Mine (like every true diamond mine) consists 

 of a pipe, roughly circular in plan, extending for some unknown dis- 

 tance vertically downwards into the earth. At first it was excavated 

 at the top, the whole of the diamond-bearing ground being taken 

 right away, until such a depth was reached that the sides threatened 

 to collapse upon the quarrymen. Then underground working was 

 resorted to, so that what had hitherto been " diggings " became a 

 mine. 



As the ground from these underground workings is removed that 

 from above eventually falls in, and hence it is necessary to start 

 fresh levels in succession at greater depths in advance of the 

 continuous crushing process. In the long run, for every load 

 removed from below, an equal amount must settle down from 

 above, and thus the depth of the old open quarry would go on 

 continually increasing were it not that the boundary rock, commonly 

 called " reef," is concurrently breaking away from the sides and 

 falling into the hole. There is no doubt that the grinding of the 



