26— THE PETROGRAPHY OF THE ROCKS SURROUND- 

 ING THE DIAMOND-PIPES OF THE KIMBERLEY 

 DISTRICT. 



By Robert Heron Rastall, M.A., F.G.S. 



Fellow of Christ's College, Cambridge. 



(Contributed by Mr. A. F. Williams, General Manager, De Beers 

 Cons. Mines, Ltd., Kimberley, S.A.) 



I.— INTRODUCTION. 



The manner of occurrence of the diamond bearing rocks of the 

 Kimberley district has been so often described that it is unnecessary 

 to do more than refer very briefly to this part of the subject. As is 

 well known, the diamonds occur in a peculiar brecciated rock, the so- 

 called "Blue Ground," which fills a number of vertical pipes or 

 necks, of a somewhat cylindrical form. The mode of origin of these 

 pipes has given rise to a great deal of controversy, but it is now 

 generally agreed that they are of volcanic origin, although the precise 

 type of vulcanicity which gave rise to them is still undecided. The 

 literature of the diamonds themselves and of the rock which contains 

 them is an extensive one. The whole subject is treated exhaustively 

 in Mr. Gardner Williams's great monograph on the " Diamond Mines 

 of South Africa," and in a section contributed by the same authority 

 to " Science in South Africa," the official handbook of the South 

 African meeting of the British Association in 1905. 



The object of the present paper is rather to give a short petro- 

 graphical description of the rocks which surround the diamond- 

 bearing pipes, and, in particular, of the rock types which have been 

 exposed in the deep shafts at the Kimberley and De Beers Mines. 

 The former has now reached a depth of 2520 feet, and the latter 

 2040 feet. The Bultfontein, Dutoitspan and Wesselton Mines are 

 of much less depth, not exceeding 750 feet. 



The specimens on which the following descriptions are based 

 have been selected from two collections presented bv the De Beers 

 Co. in 1905 to the Mineralogical Departments of the Universities of 

 Oxford and Cambridge. The specimens preserved at Cambridge 

 were placed in my hands for determination bv Professor Lewis, and 

 Mr. Hutchinson, and I have to thank Professor Miers, of Oxford, for 

 kindly permitting me to examine certain of the Oxford Series unre- 

 presented in the Cambridge collection. A few of the specimens were 

 collected by Mr. A. Hutchinson, Demonstrator of Mineralogy in the 

 University of Cambridge, during the course of a visit to Kimberley 

 in the autumn of 1905, when facilities for examining the De Beers 

 Mine were afforded him by the Company. In what follows, indivi- 

 dual specimens will be referred to by means of the numbers attached 

 to them by the De Beers Company. 



Although so much has been written on the diamonds themselves 

 and the rock in which they are contafned, there are very few refer- 

 ences in the literature to the petrographical character of the rocks 



