Thomson. — The Gem Gravels of Kakanui. 493 



There is no trace of any gem of value in either the " gem 

 sands " or the breccia, although the garnets and green augites 

 would make pretty stones when cut. There are many points of 

 geological interest, however, about the breccia which make a 

 more complete examination of interest. In the first place, the 

 presence of granulite and gneiss point to the existence, at no 

 great depth, of such rocks as form the Wanaka formation, of 

 the west coast of Otago. 



The fact that so many of the minerals have a rounded form 

 and high polish wants explanation. 



The similarity, both in occurrence and composition, of the 

 holocrystalline rocks to those of Kimberley at once raises the 

 interesting question, Is it probable that diamonds will be found 

 at Kakanui ? This similarity is probably the reason that Pro- 

 fessor Ulrich considered there was great probability that gems 

 would be found at Kakanui. It will be of interest to discuss 

 this similarity. Both occurrences are volcanic breccias ; both 

 are calcareous, the Kimberley breccias being now only seen 

 in pipes or the necks of old craters, the Kakanui being well 

 stratified and originally submarine : there must, however, be 

 necks, and probably one occurs on the South Head. Both con- 

 sist mostly of ultra-basic minerals and rocks. At Kimberley 

 no volcanic rock is present, at any rate in a recognisable state ; 

 at Kakanui there is a limburgite or feldspar-free basalt, whose 

 explosion may be assumed to have caused the eruption. 



The amount of alteration that the Kimberley breccia has 

 undergone has almost disguised its original character, and it is 

 sometimes taken as a rock species. It has been shown by Bonney, 

 however, that this alteration has had nothing to do with the 

 formation of diamonds. Both breccias contain rounded rock- 

 fragments. 



The following isolated minerals are recorded from Kimber- 

 ley : Enstatite,*f topaz,* chrome-diopside,* diallage,* iron- 

 pyrites,* ilmenite,*f olivine,f smaragdite,f chrome-diopside 

 ("omphacite" of some authors), f a brown mica,f garnet (mostly 

 pyrope, but more than one variety observed), f magnetite, *j* 

 chromite.f 



The following rock - fragments are recorded : Garnet-green 

 pyroxene (omphacite) rock = eclogite,* bright-green pyroxene, 

 zoisite purple-garnet rock,* pyrope-chrome diopside (olivine) 

 rock = eclogite, f garnet-diopside-mica-enstatite rock = enstatite 

 eclogite,f garnet-bastite rock,! olivine-bastite rock = saxonite,f 

 garnet - enstatite - chrome - diopside - olivine rock = granatiferous 



* Bonney, " Geological Magazine," 1891, pp. 413, 414. 

 t Ibid., 1899, p. 309. 



