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 14
