part 4] PLIOCEJiE DEPOSITS OE CORIS^WALL. 869 



lieid's estimate of 340 feet is probably under rather than over the 

 actual amount, particularly when the St, Agnes and St. Keverne 

 deposits are also considered.^ The difference in topographic level 

 of the St. Erth deposits compared with those of the other localities 

 can only be due to the former having been laid down in a pre- 

 existing hollow which, as subsidence progressed, became ultimately 

 modified by the sculpturing of the ' 400-foot ' feature : this implies 

 deposition of the St. Erth material in deeper water than in the 

 case of the other deposits, a belief justified both on palseontological 

 and on petrographical grounds, and by comparison with the 

 markedly shallow- water character of the St. Agnes and especially 

 the St. Keverne facies. Thus the St. Erth valley, from its initiation 

 as an early Tertiary river-course, became transformed at the close 

 of Miocene times into a strait separating the Land's End area from 

 that of the main Cornish land-mass ; and it is noteworthy that, 

 even Avith the present configuration of the land, a subsidence of 

 only about 150 feet would be reqviired to re-establish such condi- 

 tions. With a subsidence of 364 feet, as above postulated, the 

 geography of Western Cornwall assumes widely different aspects, 

 the peninsula becoming in fact a series of large and small islands, 

 a miniature West Indies. 



Under such conditions of submergence the present nature and 

 disposition of the Pliocene sediments is readily understood : at 

 St. Agnes, the Beacon, rising to a height of 628 feet, becomes 

 part of a large island, the northern and leeward shoreline of which 

 formed the upper limit of the submarine shelf upon which the 

 deposits were laid down. In the vicinity of St. Erth and Lelant, 

 much larger islands existed on the Avest and on the east, the former 

 comprising the Land's End granite and metamoi'phic area, the 

 latter comprising the petrographical ly similar uplands of Carn Brea, 

 Carn Menellis, and the Grodolphin Hills. At St. Keverne, the Carn- 

 Menellis mass is again the dominant background feature, and the 

 gently sloping Lizard platform on the south becomes the locus of 

 deposition of a mass of sand and gravel thence derived, a remnant 

 of which has surviA'^ed late Tertiary erosion and is preserved in the 

 form of the present outcrop on Crousa Common. Then, as regards 

 the Polcrebo deposits, in my opinion their occurrence at a height 

 of 480 feet above O.D. can only Avith extreme difficulty be recon- 

 ciled to this phase of geological history : if, in the absence of all 

 other evidence, they are to be regarded as having been formed 

 collaterally Avith the Pliocene deposits, hoAv can we explain their 

 present anomalous position above sea-level, and the absence of any 

 like feature, either lithological or topographical, elsewhere in the 

 county ? Their general character Avould seem to fit in far more 

 reasonably with Quaternary erosion than Avith any older period o£ 

 denudation. 



Hence, revicAving the chief features of these deposits of South- 

 western CoruAA^all in the light of the foregoing topographical data,. 



1 ' The Pliocene Deposits of Britain ' Mem. Geol. Surv. 1890, p. 65. 



