GEOLOGY 



The Pleistocene age, in which flourished several forms of mammalia 

 some of which are now extinct, together with the subsequent interval, 

 make up the Quaternary period. By far the larger portion of the 

 Quaternary deposits has been spread out on the submarine shelf that 

 fringes the coast, a tract which the mutability of events may eventually 

 convert into a future land surface. The preceding Tertiary period, by 

 reason of its greater antiquity, has been subjected to a far longer 

 experience of crustal oscillation, in which sufficient time has been 

 afforded for those more ancient accumulations, together with such 

 portions of the submarine shelf on which they were deposited, to be 

 gradually upheaved ; so that a considerable area of the Cornish platform 

 of to-day marks the site of the bed of the Tertiary seas, while the few 

 marine accumulations of those seas that have survived the long period of 

 denudation since their emergence from the ocean floor, yield unequivocal 

 testimony to the vast changes in the past in which the boundaries of land 

 and sea have taken part. 



The rigorous conditions of the glacial epoch were preceded by 

 periods of subtropical climate, which characterized the Miocene age. 

 The interval between these two extremes, in which climatal conditions 

 represented a temperate zone, constitutes the Pliocene period, the youngest 

 division of the Tertiary strata. Although these deposits present a very 

 general resemblance to the beds at present being formed on our 

 littoral, the shells which they enclose are not confined to species that now 

 inhabit our seas, but include forms which at the present time find their 

 habitat in the more northern and more southern seas. Not only do the 

 fossil remains reflect the transitional conditions which connected the 

 climatal extremes already alluded to, but a large proportion of the species 

 which flourished in the Pliocene seas have become extinct. 



The only deposits in Cornwall which can with certainty be referred 

 to the Pliocene period occur in the neighbourhood of St. Erth, occupying 

 a very small area, and probably owe their preservation to the protection 

 afforded them by their physical situation. 



The discovery of this small relic of the Pliocene shelf is very recent, 

 and was brought about by the deepening of a clay pit which revealed 

 shells in the subjacent clay bed. The deposits, which are covered by a 

 few feet of head, consist of brown, blue and mottled clays, loam, sand and 

 gravel, but the beds change very rapidly, so that adjoining sections 

 present a different sequence. They have yielded numerous species of 

 mollusca and other invertebrata, together with microscopic forms of life 

 represented by the foraminifera. 



The marine shells were first described by Mr. Whitley, and subse- 

 quently studied by Messrs. S. V. Wood, Robert Bell, and P. F. Kendall, 

 while Mr. Fortescue Millett has been engaged in the investigation of the 

 foraminifera. Finally the beds have been studied by Mr. Clement Reid, 

 who discovered another outlier of these Pliocene deposits on the ridge 

 north of Cannon's Town, at an elevation of 150 feet, in which the fos- 

 siliferous clays of St. Erth are missing. Mr. Reid considers that the 



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