236 The Parent-rock of the Diamond in South Africa. 



Mr. Trubenbach, also show that the boulders are really waterworn. 

 Besides two unworn pieces of pyrite and a rough bit of eclogite, about 

 three-quarters of an inch in diameter, the pyroxenic constituent of 

 which was a bright emerald-green (? smaragdite), I find part of a sub- 

 angular fragment of chrome-diopside associated with two or three 

 flakes of the usual mica, a well rounded garnet fully 06 inch across, 

 and half a well worn pebble of eclogite, about one inch long and half an 

 inch thick. The rounded waterworn look of the great majority of 

 the smaller constituents (chiefly garnets and pyroxenes), about the size 

 of hemp seed, is very obvious. I had suspected some of the grains in 

 washings from the De Beers Mine to have been similarly treated ; but 

 here it is indubitable, indeed many of the dark green specimens are so 

 smooth outside that they could only be identified after fracture. The 

 ordinary diopside can, however, be recognised, with some of a clearer 

 and brighter green. Most of the garnets are pyropes, but a few re- 

 semble essonite. I find also some grains of iron oxide and of vein 

 quartz. Thus, the presence of waterworn fragments, large and small, 

 in considerable abundance, shows the "blue ground" to be a true 

 breccia, produced by the destruction of various rocks (some of them 

 crystalline, others sedimentary, but occasionally including waterworn 

 boulders of the former)— i.e., a result of shattering explosions, followed 

 by solfataric action. Hence the name Kimberlite must disappear from 

 the list of the peridotites, and even from petrological literature, unless 

 it be retained for this remarkable type of breccia. 



Boulders, such as we have described, might be expected to occur at 

 the base of the sedimentary series, in proximity to a crystalline floor. 

 The Karoo beds in South Africa, as is well known, are underlain in 

 many places by a coarse conglomerate of considerable thickness and 

 great extent, called the Dwyka conglomerate, which is supposed to be 

 Permian or Permo-carboniferous in age. It crops out from beneath 

 the Karoo beds at no great distance from the diamond-bearing district, 

 and very probably extends beneath it. If this deposit has supplied 

 the boulders, the date of the genesis of the diamond is carried back^ 

 at the very least, to Palaeozoic ages, and possibly to a still earlier era 

 in the earth's history. 



from which these boulders were taken (at various depths, from nearly 100 to about 

 300 feet) cannot be any alluvial deposit, but is the typical "blue ground," 

 practically identical with that in the Kimberley mines. 



