[ 1135 | 

 OXXII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 640.] 



December 21st, 1921.— Mr. E. D. Oldham, F.E.S., President, 

 in the Chair. 



HPHE following communications were read : — 



1. ' The Nature and Origin of the Pliocene Deposits of the 

 County of Cornwall and their bearing on the Pliocene G-eography 

 of the South- West of England.' By Henry Brewer Milner, M.A., 

 D.I.C., F.G.S. 



The author discusses the petrography of those Tertiary deposits 

 of Cornwall which, by reason of similar mode of occurrence and 

 lithological resemblance to the well-known fossiliferous beds of 

 St.. Erth, have been provisionally assigned to the Pliocene Period. 

 Such deposits occur at St. Agnes, St. Erth, Lelant Downs, 

 Polcrebo, and St. Keverne ; except those of St. Erth, all are 

 unfossiliferous. 



By petrographic methods of correlation, using St. Erth as a 

 type-locality, the relative age of the St. Agnes and St. Keverne 

 beds is established. At Lelant Downs no deposit occurs in situ, 

 but scattered blocks of an indurated ferruginous grit are found at 

 an elevation corresponding to that of the St. Erth beds on the 

 opposite side of the valley, and a potential relationship is accord- 

 ingly admitted. The Polcrebo gravels differ from the other 

 deposits, and are referred to a later period. 



The average composition of the St. Agnes, St. Erth, and 

 St. Keverne deposits is shown to be substantially the same, the 

 important ' heavy ' minerals common to all being magnetite, 

 ilmenite, tourmaline, staurolite, epidote, zircon, anatase, rutile, 

 topaz, andalusite, and cassiterite ; garnet was found only at 

 St. Erth, kyanite at St. Agnes and St. Erth. On this basis, 

 correlation of the deposits is effected by considering (1) the 

 frequency of occurrence of individual species ; (2) their persistence 

 or distribution; and (3) the constancy of crystallographical, 

 physical, and optical properties of grains of the same mineral, 

 wherever met with. 



The source of the material is essentially local, although kyanite 

 and staurolite are ' foreign ' species ; the former, from its striking 

 Lower Greensand habit and absence from St. Keverne, is thought 

 to be derived from the north-east, the latter from a land- 

 mass lying on the south, probably united to Cornwall and 



