196 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Chaper to the conclusion that each of these vertical columns is the product of a distinct 

 eruption. Since the columns are similar in general character and differ only in minute 

 details, he considers that they have heen formed by a series of eruptions of the same type ; 

 in short, that each diamantiferous deposit or pipe is the product of a long-continued period 

 of volcanic activity. Thus, according to Chaper's view, the pipe of the Kimberley mine, 

 in which fifteen columns have been observed, is the result of fifteen successive eruptions. 

 Further observations in this direction are, however, desirable. 



From the considerations brought forward above, it seems very probable that the South 

 African diamonds were formed in a deep-seated crystalline rock which became fragmented 

 and erupted to the surface by the action of volcanic forces ; and moreover, that the greater 

 part of this ejected, fragmentary material fell back again into, and filled up the vent or 

 crater produced by the eruption. PVom the nature of the minerals which accompany the 

 diamonds in the volcanic tuff", it is perhaps possible to draw some conclusions as to the 

 character of the rock in which the diamonds were formed. Almost all the minerals, which 

 are constituents of the rocks generally known as olivine-rocks, and which are widely 

 distributed in the earth's crust, are found amongst the minerals associated with the diamond 

 in " blue ground." It is therefore highly probable that the original mother-rock of Cape 

 diamonds was an olivine-rock, situated at a great depth below the earth's surface and 

 containing as constituents biotite (represented by the altered mica, vaalite), enstatite 

 (bronzite), garnet, and all the other minerals already mentioned, including of course the 

 diamond. Such a rock, which would be similar to a Iherzolite in composition, has indeed, 

 though in a somewhat different sense, been named kimberlite, and from this, the " blue 

 ground " filling the pipes has been referred to as a kimberlite-breccia orakimberlite-tuft". 

 This kimberlite-breccia or tuff, at least as far down as it has been reached by mining 

 operations, has undergone great alteration, its originally predominant constituent olivine 

 being almost completely altered to serpentine, so that very little of it is now to be seen. 

 This more or less complete alteration of olivine to serpentine in an olivine-rock is not at all 

 unusual, being a matter of common observation in all parts of the world. The other 

 constituents of the original rock have undergone less alteration and are in a more or less 

 fresh condition. At greater depths the decomposition of the olivine has been less complete, 

 and here may be found traces of kimberlite still unaltered, in which the olivine retains more 

 or less completely its original character. 



The foregoing pages have been devoted to the consideration of the manner in which 

 the diamantiferous rock-mass actually occurs in the pipes. The question which naturally 

 follows, namely how the diamond itself was formed in its original mother-rock, the 

 kimberlite, will be treated generally below. We must first, however, notice certain other 

 theories as to the formation of the pipes, which are more or less opposed to that of Cohen, 

 as set forth above. According to the theory first promulgated by the late Professor H. 

 Carvill Lewis, and which has some substantial support, the "blue ground " is not fragmentary 

 material or tuff^, but was forced up from below into the pipe as a molten mass which 

 consolidated on cooling. According to this view, therefore, the " blue 'ground " is an 

 ordinary igneous rock, which solidified in the situation in which it is now found, and it 

 was with this supposed origin in his mind that the name kimberlite was^proposed for the 

 diamantiferous rock by Carvill Lewis. This rock, which was originally an olivine-rock, is 

 supposed to have subsequently undergone the same alteration processes as described 

 above. 



There appears to be a similar, though very sparing occurrence, of diamonds near the 

 village of Carratraca in the province of Malaga in Spain. According to the statement of 



