part 2] THE GEOLOGY OF THE MARBLE DELTA. 119 



7. The Geology of the Marble Delta (Natal). By 

 Alexander Logie du Toit, B.A., D.Sc, F.G.S. (Read 

 February 5th, 1911).) 



[Plate IX - Microscope-Sectioxs.] 



Contents. 



Page 



I. Introduction 119 



II. The Marbles 121 



III. The Granite and Gneiss 123 



IV. Reaction -Phenomena along Dykes 124 



V. The Chondrodite-Marbles 129 



VI. The Calcitization of Dolomitic Limestone 132 



VII. Exomorphic and Endomorphic Changes 133 



VIII. Summary 136 



I. Introduction. 



The crystalline dolomitic marbles near Port Shepstone have 

 formed the subject of no less than three communications to this 

 Society. 



That by C. L. Griesbach, 1 written nearly half a century ago, 

 gives us the first glimpses of this interesting occurrence ; the next, 

 by David Draper 2 was fuller, but lacking in most of the important 

 details; the third, by Dr. F. H. Hatch & Mr. R. H. Eastall 3 is a 

 particularly valuable contribution, especially from the petro- 

 graphical standpoint: the intrusive relationship of the granite is 

 placed beyond dispute, and the operation of the action known 

 as dedolomitization is fully demonstrated. 



Consequently, it is not inappropriate that the principal con- 

 clusions arising out of the geological survey of the area (carried out 

 in 191o-10) should be communicated through the same channel, 

 especially as they confirm and amplify the expressions of Hatch & 

 Rastall as to the nature of the metamorphic actions involved. 



The sole point of disagreement concerns the 'boulder' of 

 alkali-granite, which, from the evidence available, can only have 

 formed part of an intrusive tongue in the marble, while the other 

 reputed instances of 'inclusions' must be placed in the same 

 category. 



The object of the present paper is the description of this and 

 certain other cases, where there has been an actual transfer of 

 material from the intrusion across the contact to the rock invaded, 

 a rather infrequent phenomenon, and one termed 'contact - 

 metasomatism' by .1. Barrell, and 'pneurnatolytic contact- 

 metamor ph ism' by V". ML Groldschmidt ; the action termed 

 'calcitization' is also described and discussed, possibly for the 

 first time. 



1 Q. J. G.S. vol. xxvii(1871) p. 50. - Ibid. vol. li (1895) p. '51. 



3 Ibid. vol. lxvi (1910) p. 507. 



