Discussion

Abstract

DR. E. A. VINCENT asked whether the author would elaborate on his suggestion that the finer grain of the rocks late in his time-sequence might be due to the concentration of volatiles causing an attenuation of the crystallization range of the minerals. Could the author explain how this was brought about ? Dr. S. I. T omkeieff said that he would like to suggest a different hypothesis in explanation of kaolinite genesis in the St. Austell granite, namely : a biphyletic hypothesis, according to which an exogenetic phase was superimposed on the endogenetic phase. Many years ago, G. Hickling in his study of the Cornish china-clay deposits (1908, Trans. Inst. M. E. 36 , 23) suggested that muscovite was an intermediate product of the transformation of feldspar into kaolinite. The transformation of mica into kaolinite was a well-known sedimentary process. Numerous German kaolinite deposits justified the “moor water” theory, according to which kaolinite and other clay minerals were formed through the action of ground-waters charged with humic substances. The widespread kaolinite deposits of the western Ukraine occurred in granites and other feldspar-bearing rocks of the Pre-Cambrian foundation overlain by Tertiary deposits often containing seams of brown coal. According to I. I. Ginzburg, who studied these deposits (1912, Ann. St. Petersburg Polytechn. Inst 17 , and 1915, ibid 22 ), they were of the nature of residual formations and, according to the evidence, were formed through leaching of underlying rocks by ground-waters in the lower zone of weathering. Dr. Tomkeieff added that he