A HISTORY OF CORNWALL 



fossiliferous clays were formed in a depth of water extending to 40 or 

 50 fathoms, an estimate confirmed by his recent discovery of the ancient 

 shore line of the Pliocene sea at a height of about 420 feet above the 

 present sea level. 



Another outlier, which has been referred by Sir Henry De la Beche 

 to the Tertiary period, had been previously described in 1832 by Mr. 

 John Hawkins and Dr. Boase. This deposit, which occurs at St. Agnes 

 Beacon, and reaches, according to the latter writer, a height of 375 feet, 

 consists of sands and clays which up to the present have not yielded 

 determinable fossil remains. Like the St. Erth beds they exhibit rapid 

 variation, and it is probable that they may also be the products of the 

 Pliocene sea which Mr. Reid has shown to have exceeded even that 

 elevation. Mr. Thomas Clark of Truro has recently found a shell frag- 

 ment in the clay of this deposit, but too imperfect for identification. 



On Crousa Downs an isolated patch of gravel, consisting of rounded 

 quartz pebbles, occupies, according to Sir H. De la Beche, an area of about 

 half a square mile, at a height of about 360 feet above the level of the 

 sea. The origin of this deposit is wrapped in obscurity, but its corres- 

 pondence in elevation to the sands and gravels of St. Agnes Beacon 

 suggests that it may also be of corresponding age. 



Notwithstanding the paucity of those Pliocene deposits which have 

 survived the denudation of the subsequent ages, sufficient have remained 

 to enable us to restore in imagination the physical features of the period 

 to which they relate. The seas then covered large portions of the present 

 land surface of Cornwall, and if we could restore the geography of the 

 Pliocene period we should see an archipelago where Cornwall now stands, 

 while the Isles of Scilly would lie beneath the waves. 



While the Tertiary history of Cornwall is obscure, our knowledge 

 being confined to those few isolated deposits all of which probably repre- 

 sent events in the more recent division known as Pliocene, it was preceded 

 by the stupendous gap which extends over the Mesozoic ages, during 

 which was accumulated the succession of Secondary deposits that constitutes 

 the geology of the greater part of England. In that interval were laid down 

 the older Tertiary deposits forming the London and Hampshire basins, the 

 foreign equivalents of which have been involved in the structure of the 

 European mountains, and the fauna and flora of which ushered in our 

 present species of animal and plant life. The great depression of Creta- 

 ceous times permitted the slow accumulation of our Chalk formation from 

 the tiny remains of foraminifera. The still older Jurassic system, with its 

 divisions of the Lias and Oolite, forms a broad band which crosses 

 England from sea to sea ; and yet earlier the older Mesozoic period evolved 

 the great formations of the Trias and Permian. 



Of the millions of years that occupied the building of these 

 formations which represent the incoming and extinction of many forms 



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