Yol. 65.] EEATTJEES OE THE CAKPALLA CHINA-CLAY PIT. 159 



that the principal agents in the kaolinization of the granite have 

 been acid solutions or vapours containing fluorine, boron, and, to a 

 less degree, chlorine. 



The sequence of events at Carpalla appears, therefore, to have 

 been somewhat as follows : — 



(1) Deposition of fine-grained sediments (killas) in, presumably, Lower 



Devonian times. 



(2) Intrusion of granite. 



(3) Alteration of killas into tourmaline-schist, by development of tourmaline 



between the laminse. Simultaneous kaolinization locally of the 

 felspars of the granite. 



(4) Faulting-down of schists during a period of 'reaction' or local subsidence. 



(5) Denudation of surface, so as to remove nearly the whole of the schist 



resting upon the granite. 



(6) Formation of growan subsoil or ' head, and of surface-soil or ' meat- 



earth.' 



EXPLANATION OF PLATE VII. 



View of Carpalla china-clay pit, from a photograph taken by Mr. J. H. Coath, 

 of Liskeard, showing the overburden of tourmaline-schist resting on 

 china-clay rock. 



Discussion. 



Mr. Barrow congratulated the Author on having proved the 

 continuity of the kaolinization of the granite under a killas-roof. 

 The faulting-down of this roof in the manner shown by the diagram 

 was not uncommon ; for instance, at Darley Vein, near the Cheese- 

 wring, the roof had been dropped to a depth of about 2000 feet. 

 The dispute as to the nature of true ' kaolinization ' of granite 

 was largely due to the misapplication of the word to alkali-felspars 

 that had undergone normal decomposition ; these, in most cases, 

 were not kaolinized at all, the mineral produced being white mica. 

 The speaker had examined a large area of granitic rocks in Scotland, 

 and had seen no serious trace of true kaolinization anywhere. 

 During ordinary decomposition the brown mica became altered 

 to chlorite, and if the decomposition-products were washed and 

 examined, that mineral was easily found. But no such chlorite was 

 produced during kaolinization ; the process was essentially similar 

 to the alteration of granite to greisen, in which, as Mr. Scrivenor 

 had shown (in the pages of the Society's Journal), all the unstable 

 iron-bearing material was used up in the formation of schorl — a 

 stable mineral. Decomposition usually resulted in the brown 

 staining of the granitic material, as the chlorite was formed ; 

 while kaolinization resulted in the production of a snow-white clay 

 (kaolin) disseminated through, or containing the other minerals 

 still left, such as quartz, schorl, white mica, etc. The characteristic 

 product of the decomposition, chlorite, was what was known as 

 a low-temperature mineral ; while the characteristic product of the 

 kaolinization was schorl, a high-temperature mineral, and the process 

 was clearly a metamorphic one. 



