160 F. L. Stilhtrll : 



fifteen yards lon<r. Tlie outcrop, which does not reveal the 

 extent of the material, is distinctly granite deconiposed i?! situ, 

 for there has been no resorting of the material. A rounded 

 patch in the face contains much finer-grained material than the 

 average, and represents a basic secretion in the original granite. 



The material consists of unaltered, angular quartz with white 

 mica set in white kaolin. The white mica and kaolin represent 

 the secondary products of the original mica and felspar in the 

 granite, while the quartz has remained unaltered. It is a 

 question Avhich cannot be decided on the available evidence, in 

 what manner the change has been brought about. Such occur- 

 rences may be ascribed to subaerial agencies. Surface water 

 and atmospheric carbon dioxide are capable of converting the 

 alkalies present in the granite into carbonates which would be 

 leached out, leaving a resultant product, chiefly kaolin and 

 quartz. With regard to this it may be noted that the occurrence 

 is very close to the contact of the basalt, and the conditions 

 would be favourable to an excess of drainage along this junction. 

 An alternative view is to regard the origin of the kaolin as due 

 to pneumatolytic action of the emanations from the mass of 

 cooling granite in its final stages of consolidation. This is the 

 proved origin of kaolin deposits in Cornwall (2). The chief 

 kaolinising agents in this case are sho^^^l by F. H. Butler (2) 

 to be water and carbon dioxide. Fluorine and boron vapours 

 often accompany them, and may result in tourmaline, fluorspar 

 and topaz. Such minerals would most likely be concentrated 

 along veins. No veins have been observed in the present case, 

 and very little evidence is to be had for or against either theory. 

 The pure white face and the abundance of white mica might lead 

 one to lean towards the theory of a deep-seated origin. 



Similar occurrence is found along the border of the granite 

 outcrop at Bulla,i four or five miles to the north-west, and is the 

 subject of a brief report by E. J. Dunn (3). The outcrop is of 

 much greater extent, and the rock is spoken of as soft and 

 crumbling, and consisting of ordinary quartz granules and kaolin, 

 the latter replacing the felspars. He says it has been carted 



1 Since writing- the al>ove, a discussion of the Bulla material has been published by 'Mr. 

 R. W. Arniitage, B. 8c. Vict. Nat., vol. xxviii., July, 1911. 



