348 ME. H. B. MiLXEE o:s' THE [vol. Ixxviii, 



12. The Xatuee and Oeigix of tlie Plioce>'e Deposits of the 



Cor>'TT of CoEywALL, and tlieir Beaeixg on tlie Pliocene 



G-eogeapht of tlie South-West of Englax'd. Bj HE>'Er 



Beewee Milxee, M.A., D.I.C., F.Gr.S., Lecturer in Oil 



Technology at the Eoval School of Mines. (Eead December 



21st, 1921.) 



Contents. 



Page 



I. Introduction 348 



n. Mode of Occurrence of the Deposits 349 



TTT. Methods of Investigation 353 



lY. Lithological Character of the Deposits 354 



V. Petrography of the Sediments 357 



VI. Comparison and Correlation of the Deposits 364 



Vil. Source of Origin of the Deposits 366 - 



Vin. Pliocene Geography of the South- West of England. 367 



IX. Summary and Conclusions 3 73 



I. Inteoducttox. 



The present paper deals more pai*ticularly with the petrography of 

 the younger Tertiary deposits of Western Cornwall Avhich, by 

 reason of their mode of occm-rence and lithological similarity to 

 the well-known fossihferous beds of St. Erth, have been generally 

 assigned to the Pliocene age. These deposits occur as isolated 

 patches of detrital material resting in eroded hollows in Palaeozoic 

 or older rocks in the sonth- western part of the county, and are 

 located at St. Agnes, -1 miles due south-west of PeiTanporth ; at 

 St. Erth and Canons Town in the Hayle-Eiver Yalley ; at Crousa 

 Common, a mile and a half south-west of St. Keverne (Lizard 

 district) ; and at Polcrebo, near Crowan, 1 miles south of Cam- 

 borne, where an outcrop of gravels of alleged Pliocene age is found. 

 With the exception of the St. Erth Beds, no fossils have hitherto 

 been discovered in these deposits, and theu' sti-atigraphical position 

 has been inferred entirely from general geological and topographical 

 considerations. 



The object of this paper is not only to establish the relative 

 ages of these deposits by petrographic methods, but to demonstrate 

 the importance of this mode of attack in problems of stratigra- 

 phical correlation where palseontological evidence is in some cases 

 lacking. In addition, the mineralogical factors concerned are used 

 as a guide to the determination of the origin of these sediments 

 and, in conjunction ^ith topographic data, to restore the geo- 

 graphy of this part of Britain in early Pliocene times, in so far as 

 the evidence at om- disposal allows. 



