The Lower Greensand--our next deposit--was formed after a long interval, and, owing to earth movements which had taken place during that interval, it rests upon the older rocks in an irregular manner. It is a marine formation, and only occurs in patches, the largest of which extends from Uffington to near Faringdon. Its greatest thickness is about 60 feet, and it consists of sand with some ironstone and chert, pebble beds, and a calcareous sponge gravel. The sponge gravel, so-called from the number of fossil sponges it contains, is dug for garden paths and walks, and is exported to long distances. The fossil sponges in the gravel are abundant and beautifully preserved, and they seem to have lived on the spot. The ironstone was at one time worked near Faringdon. At New Lodge, in the parish of Winkfield, the Lower Greensand was reached in a boring at a depth of 1234 feet. A good supply of water was obtained, but it contains a large quantity of common salt. The Gault, the next formation, consists of grey clay in the lower part and of a silty marl in the upper part, with a total thickness of some 220 feet. It crosses the county as a band, from one to three miles in width, from Ashbury to the Thames between Abingdon and Wallingford. It is a marine formation, and does not give a water-supply. The Upper Greensand runs across the county as a narrow and irregular band about 90 feet thick, and consists of green sands and grey marl, with beds of stone in places. It is of marine origin, and provides a supply of excellent water, and consequently many villages stand upon or close to it. Ashbury, Childrey, Wantage, Hendred, and Harwell are examples. The Chalk. This is far the most important geological formation in Berkshire, for it occupies a large portion of the surface of the county, and in the eastern part, when not at the surface, it is to be found underground. It is a light-coloured limestone, usually soft and earthy, but in parts very hard. Its full thickness is over 700 feet, and being a porous rock, the rain which falls on its great surface sinks in and furnishes a water-supply over its whole area whether the chalk be at the surface or underground. It was deposited in a sea which not only covered our district but spread over much of Europe. There was, however, probably land to the west which included Cornwall, parts of Wales, and of Ireland. The upper part of the Berkshire Chalk contains many layers and nodules of flint. There is a long break in our geological record after the newest beds of the Chalk found in Berkshire had been deposited, for both the top of the Chalk and the bottom of the next series are wanting here, and in order to fill the interval we have to study rocks in other parts of England, in Belgium, and in Denmark. During this great interval in time the chalk sea retired, and much of Britain became land.