160 GEOLOGICAL FEATURES OF CARPALLA CHINA-CLAT PIT. [May 1909, 



Dr. J. S. Elett said that, since first he saw the geological relation- 

 ships of the china-clay rock to the granite in the field, he had 

 always been strongly of opinion that the kaolinization of the Cornish 

 granites was in no sense the result of ordinary atmospheric decom- 

 position. This opinion had recently been confirmed by a microscopic 

 study of the St. Austell granite and its various modifications. Three 

 types of pneumatolytic alteration would be recognized : — tourmalini- 

 zation, greisening, and kaolinization. In the first was seen the 

 formation of tourmaline, in the second of white mica, and in the 

 third of kaolin from the original minerals of the granite. Boric 

 acid was certainly one of the principal agents concerned in the pro- 

 duction of schorl-rock ; while fluorine was active in the greisening 

 process. The greisens of the St. Austell granite were often exces- 

 sively rich in topaz, a fluoriferous mineral. In the china-clay rock 

 topaz and fluorspar frequently occurred in large quantity ; but it 

 seemed very probable that another gas had also been present, and this 

 the speaker thought was probably carbonic acid. Igneous magmas 

 contained that gas in solution, and it was not unreasonable to look 

 for products of its action on the felspar. Slides of the kaolin-rock 

 showed that dense masses of kaolin invaded the felspar along its 

 cleavages and other planes of weakness. The felspar was often 

 completely honeycombed by the process of kaolinization, only small 

 isolated fragments being left, surrounded on all sides by finely 

 crystalline kaolin with some quartz and muscovite. The appearance 

 in the microscopic slides was very different from anything that 

 the speaker had ever seen, in granites which had been exposed only 

 to the ordinary agencies of weathering. 



Mr. T. Crook asked the Author upon what evidence he relied for 

 the inference that chlorine had played any considerable part in the 

 pneumatolysis of the Cornish granite. He would like to know 

 whether chlorine had been proved to exist in any minerals which 

 could be safely regarded as direct products of pneumatolytic action 

 in this particular case ; and if so, to what extent. 



Dr. J. W. Evans drew attention to the evidence afforded by the 

 kaolin of the presence of a considerable amount of water-vapour 

 among the gases by which the pneumatolytic action was effected. 

 This strongly confirmed the views of those who believed that water of 

 intra-telluric origin ('juvenile water ') played a very important part 

 in the Earth's history. 



The President (Prof. Sollas) remarked that there seemed to 

 be a general opinion in favour of the formation of kaolin-stocks 

 by pneumatolytic action ; and the Author's observations certainly 

 supported this view. The consequences were very far-reaching : the 

 existence of juvenile waters was indicated, but these, on escaping 

 from the granite, must be richly charged with sodium-salts, probably 

 with sodium-chloride ; and he had lately been led to believe that the 

 presence of sodium-chloride in inland waters, such as the rivers of 

 Bohemia, where ancient salt-deposits were absent, was to be accounted 

 for in some such way. At the same time, it did not follow that 

 kaolinization was not also produced by subaerial agency : in the 



