A Micrometric Study of the St. Austell Granite (Cornwall)

Abstract

The St. Austell granite-mass has attracted considerable attention in the past, because of the economic importance of its mineral deposits and the petrological problems that arise out of the wonderful display of pneumatolytic activity manifested in the district. The principal references to the literature are given in the footnote, and since these contain full bibliographies, it is unnecessary to compile another for the purpose of this paper. Two reasons induced me to undertake this work. In the first place, it appeared desirable to develop and apply to a coarse-grained igneous rock-mass quantitative microscopic methods. Secondly, it seemed probable that much light could be thrown upon the problems of the St. Austell granite by quantitative and qualitative data obtained from a larger number of representative slides than has been available hitherto. A preliminary investigation in the field soon revealed that the district presented some disadvantages in the application of the method, owing to the occasional occurrence of very coarse porphyritic structure in the granite; but, at the same time, it was evident that the results would outweigh a little uncertainty arising from this difficulty. The present paper deals solely with the granite, and all post-consolidation and metamorphic changes are excluded from its scope. The aim has been to ascertain what evidence exists of differentiation and variation in the original rock. Certain pneumatolytic effects have been generally recognized as inextricably bound up with stages in the intrusion and crystallization of the original magma, and such, of course, come within the purview of