496 Mr. William CrooJces [June 11, 



The country round Kimberley is remarkable for its ferruginous 

 character, and iron-saturated soil is popularly regarded as one of 

 the indications of the near presence of diamonds. Certain artificial 

 diamonds present the appearance of an elongated drop. From 

 Kimberley I have with me diamonds which have exactly the appear- 

 ance of drops of liquid separated in a pasty condition and crystallised 

 on cooling (Fig. 28). At Kimberley and in other parts of the 

 world, diamonds have been found with little appearance of crystal, 

 lisation, but with rounded forms similar to those which a liquid 

 might assume if kept in the midst of another liquid with which it 

 would not mix. Other drops of liquid carbon retained above their 

 melting-point for sufficient time would coalesce with adjacent drops, 

 and on slow cooling would separate in the form of large perfect 

 crystals. Two drops, joining after incipient crystallisation, would 

 assume the not uncommon form of interpenetrating twin crystals. 

 Illustrations of these forms from Kimberley are here to-night. 

 Other modified circumstances would produce diamonds presenting 

 a confused mass of boarty crystals, rounded and amorphous masses, 

 or a hard black form of carbonado. 



Again, diamond crystals are almost invariably perfect on all sides. 

 They show no irregular side or face by which they were attached 

 to a support, as do artificial crystals of chemical salts; another 

 proof that the diamond must have crystallised from a dense liquid. 



When raised the diamond is in a state of enormous strain, as 

 I have already shown by means of polarised light. Some diamonds 

 exhibit cavities which the same test proves to contain gas at 

 considerable pressure. 



The ash left after burning a diamond invariably contains iron as 

 its chief constituent ; and the most common colours of diamonds, when 

 not perfectly pellucid, show various shades of brown and yellow, from 

 tbe palest "off colour" to almost black. These variations accord 

 with the theory that the diamond has separated from molten iron, 

 and also explain how it happens that stones from different mines, 

 and even from different parts of the same mine, differ from each 

 other. Along with carbon, molten iron dissolves other bodies 

 which possess tinctorial powers. One batch of iron may contain an 

 impurity colouring the stones blue, another lot would tend towards 

 the formation of pink stones, another of green, and so on. Traces of 

 cobalt, nickel, chromium and manganese, metals present in the blue 

 ground, might produce all these colours. 



A hypothesis, however, is of little value if it only elucidates 

 one-half of a problem. Let us see how far we can follow out the 

 ferric hypothesis to explain the volcanic pipes. In the first place we 

 must remember these so-called volcanic vents are admittedly not 

 filled with the eruptive rocks, scoriaceous fragments, &c., constituting 

 the ordinary contents of volcanic ducts. At Kimberley the pipes are 

 filled with geological plum pudding of heterogeneous character — 

 agreeing, however, in one particular. The appearance of shale and 



