494 Transactions. 



lherzolite,* olivine-bastite-green-augite-pyrope rock = enstatite 

 eulvsite,* pegmatitic hornblende gabbro. 



The Kakanui breccia, then, contains six rock - forming 

 minerals also found at Kimberley — viz., olivine, diopside, di- 

 allage, garnet (two varieties), smaragdite, and spinel (c/. chro- 

 mite). Both contain lherzolites and eulysites, and each con- 

 tains two granatiferous peridotites — viz., eulvsite and eclogite 

 at Kimberley, and eulysite and garnet-pyroxene as Kakanui. 



The diamond at Kimberley is found included in the eclogite 

 and in the pyrope. Dr. Bonney considers the garnet very im- 

 portant : "As the ordinary varieties of the latter mineral 

 [garnet] seem to be produced at a high temperature, the associa- 

 tion [of diamond with it] may be significant." 



The origin of the diamond at Kimberley has been traced to 

 a pyrope in an eclogite. The diamond is included in the pyrope 

 as well-formed crystals. Its ultimate origin is obscure, as the 

 parent rock has so high a percentage of oxygen that, accord- 

 ing to Dr. Bonney, it is difficult to understand how so small 

 an amount of carbon escaped oxidation. It seems as if the 

 diamond formed in some more basic magma, and had been 

 either taken up by the eclogite magma, or the magma containing 

 it had mixed with one or more acid to form the eclogite magma. 

 In any case, under the present amount of knowledge, the presence 

 of diamond in an eclogite cannot be taken as a sign that diamond 

 occurs only in eclogite, or that all similar eclogites contain dia- 

 monds. Many eclogites occur elsewhere without diamonds. 



Under these circumstances, to establish a resemblance be- 

 tween the breccias at Kakanui and at Kimberley will give a 

 possibility that diamonds occur at the former place, but not a 

 very high degree of probability. The presence of two distinct 

 garnetiferous peridotites in the former and of several in the 

 latter creates a great similarity, and shows a similarity of con- 

 ditions — magmas with little alkalis and crystallizing at a, high 

 temperature. Such a similarity is of sufficient interest to justify 

 a thorough examination of the New Zealand locality, both 

 from a theoretical and practical point of view. 



The occurrences resemble one another also in that both are 

 volcanic breccias. This can only bear on the subject in that, 

 as diamonds would only be likely to occur in ultra-basic rocks, 

 and as such rocks generally occur at a great depth in the crust, 

 there would be greater probability of their occurrence in a rock 

 formed from deep-seated eruptions than in any other way. 



In regard to the other point of similarity, the occurrence in 

 both breccias of rounded fragments, Dr. Bonney has proved to 



Bonney, "Geological Magazine," 1900, pp. 476-70. 



