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. Tomkeieff 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. Inst17, and 1915, ibid22), 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