546 ME. W. A. EICHARDSOX : A MICBOMETEtC [vol. lxxix, 



20. 



A Miceometeic Study of the St. Austell Gbanite (Corx- 

 wall). By William Alfred Richardsox, B.Sc.(Eng.), 

 M.Sc, F.G-.S. (Read December 20th, 1922.) 



Contents. 



I. Introduction 546 



II. Mineral Composition of the Granite 547 



III. The Minerals and their Distribution 553 



IV. Correlations among the Minerals 562 



V. Structure and Grain 564 



VI. The Rock-Types and their Distribution 566 



VDZ. History of the Intrusion-Phenomena '. 573 



I. Ixteoductiox. 



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, 1 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 investiga- 

 tion 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 metainorphic changes are excluded from its 

 scope. The aim has been to ascertain what evidence exists of 

 differentiation and variation in the original rock. Certain pneuma- 

 tolytic 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 

 the work. In the choice of material in the field care was taken to 



1 ' The Geology of the Country around Bodmin & St. Austell ' (Explanation 

 of Sheet 347) Mem. Geol. Surv. 1909 ; J. A. Howe, ' A Handbook to the 

 Collection of Kaolin, China-Clay, & China-Stone in the Museum of Practical 

 Geology ' Mem. Geol. Surv. 1914 ; T. C. F. Hall, ' The Petrology of the 

 St. Austell Granite ' Proo. Geol. Assoc, vol. xxv (1914) pp. 180-92. 



