part 2] TERTIARY OUTLIERS OF THE WEST OF ENGLAND. 221 



(4) The Pliocene Deposits. 



So far as our present knowledge goes, the Pliocene deposits are- 

 confined to Cornwall. Cei"tain high-level gravels in Devon may 

 be Pliocene, but their age has not yet been established. 



The deposits occurring at St. Erth, Lelant, Crouza Down (St. 

 Keverne), and St. Agnes are now generally accepted as of Pliocene- 

 age (see p. 207). That at Polcrebo is doubtful. Dr. H. H. 

 Thomas has briefly recorded some of the minerals in the St. Agnes- 

 sands. 1 



The petrology of these deposits has been worked out in detail 

 recently by Mr. H. B. Milner. 3 It is not proposed here to do 

 more than summarize the characters that they display. 



The minerals which I have identified are indicated in the table 

 on p. 226. 



The variety of minerals in the Pliocene deposits is striking,, 

 although of those recorded, garnet, phlogopite, and epidote are- 

 rare. Tourmaline and topaz are exceedingly abundant ; and 

 andalusite (while plentiful) is subordinate to kyanite at St. Erth,. 

 but is more abundant elsewhere. The distribution of kyanite 

 varies somewhat. At St. Erth and on certain horizons at St. Agnes, 

 it is very plentiful. On other horizons at St. Agnes it is uncommon ; 

 at Crouza Down, where the minerals seem to be of more local origin,, 

 it appears to be absent. 



The absence or rarity of certain locally-occurring minerals (sucb 

 as pyroxenes, amphiboles, biotite, chlorite, epidote, and garnet) is 

 noteworthy. 



That the deposits are, on the whole, sorted more thoroughly 

 than those of Oligocene and Eocene age is shown by the 

 mechanical analyses graphically represented in fig. 7 (p. 222). 



It is obvious that the progressive restriction of drainage-area 

 and consequent impoverishment in the mineral assemblage that 

 was characteristic of the Cretaceous, Eocene, and Oligocene 

 deposits ceased, and indeed was actually reversed in Pliocene 

 times. The Pliocene deposits are marine and glauconitic ; they 

 mark a widespread submergence, as demonstrated by the fauna 

 of the blue clay at St. Erth, and they carry a rich assemblage of 

 minerals derived in all probability from various sources. Kyanite 

 and staurolite are again, on the whole, abundant. No older 

 sedimentary rocks in Cornwall could have yielded them ; to 

 account for their presence, recourse must be had once more to 

 the old Armorican land-mass. 



The absence or rarity of garnets in the Pliocene, as in the 

 other Tertiary and Cretaceous deposits of Devon and Cornwall, 

 requires explanation, especially in view of their abundance in the 

 Permian and Triassic rocks of Devon. Their absence may be due 

 to one (or both) of two causes : (1) decomposition of the mineral 



1 ' The Geology of the Country near Newquay (Sheet 346) ' Mem. Geol. 

 Surv. 1906, p. 62. 



- Q. J. G. S. vol. lxxviii (1922) pp. 348-77. 



