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 A HISTORY OF SURREY hence it is possible that the stream may have flowed from the eastward across a tract extending over what is now the southern part of the North Sea. The prevalence of the freshwater conditions must have been of very long duration, since the thickness of the Weald Clay alone in Surrey is estimated to range between 600 and 1,000 feet, and to this must be added at least 600 or 700 feet more for the Hastings Beds. The Weald Clay is interstratified here and there with thin bands of sand and silt, with layers of limestone made up almost entirely of a fresh- water univalve shell of the genus Paludina, and with nodular bands of clay-ironstone. These harder strata generally give rise to slight ' features ' or elevations of the surface, but they are rarely sufficiently thick to have much effect upon the character of the soil, which is principally a heavy clay. The farming of these ' strong ' lands has been most severely affected by the depression in agriculture, with the result that the acreage under the plough has largely decreased. The ironstone of the Wealden Beds was at one time extensively dug and smelted, though not so largely in Surrey as in the neighbouring counties of Kent and Sussex. An Act of 23 Elizabeth (1581) to restrict the use of wood in these iron-works makes exemption of the woods of ' Christopher Darrell, gentleman, in the parish of Newdegate, within the weald of the countie of Surrie, which woods of the said Christopher have heeretofore beene, and be by him preserved and coppised for the use of his iron- works in these parts.' 1 Although the great thickness of these Wealden freshwater deposits implies the duration of the same conditions of deposition over the area for a very long period, this must not be taken to denote that the land remained for all the time at the same level. In fact we can only imagine such an accumulation taking place where there was gradual subsidence that kept pace with the rate of infilling of the basin. A similar balance of conditions seems to be established at the present day at the mouths of many large rivers, and it is supposed that the weight of the accumulated sediments causes a gradual downward movement of the tract upon which the mass is spread. However this may be, it is clear that towards the close of the Wealden episode the waters of the sea began to gain ground, so that in the uppermost portion of the Weald Clay in Surrey, as exhibited recently in an enlargement of the railway-cutting between Redhill and Earlswood, brackish-water shells make their appearance among the freshwater fossils.* A further stage in the depression submerged the whole of the Wealden area beneath the sea, and henceforward for a long period marine conditions alone prevailed, though at first land probably still existed not far distant to the northward and north-westward. The submergence below sea-level seems to have taken place rather suddenly, ' See Geol. Survey Memoir, < Geology of the Weald,' chap. xix. pp. 329-346, for infor- mation regarding this extinct industry. 6
 * See also ' Geology of the Weald,' pp. no, 114.