It will be well to say something here of the history of the flints as the chalk which contains them is gradually denuded away. Rain water containing carbonic dioxide has a great effect in eating away all limestone rocks, chalk included. A vast extent of chalk, which formerly covered much of England has thus disappeared. The arch of chalk connecting our two ranges of downs has been cut through, and from the top of the downs themselves a great thickness of chalk has been removed. The chalk in the downs above Ventnor and Bonchurch is nearly horizontal. It consists of Lower and Middle Chalk; and probably a small bit of the Upper occurs. But the top of St. Boniface Down is covered with a great mass of angular flint gravel, which must have come from the Upper Chalk. The gravel is of considerable thickness, perhaps 20 ft., and on the spurs of the down falls over to a lower level like a table-cloth. It is worked in many pits for road metal. This flint gravel represents the insoluble residue which has been left when the Chalk was dissolved away. On the top of the cliffs between Ventnor and Bonchurch, at a point called Highport, is a stratum of flint gravel carried down from the top of the down. The shore here is strewn with large flints fallen from the gravel. The substance of many of the flints has undergone a remarkable change. Instead of black or dull grey flint it has become translucent agate, of splendid orange and purple colours, or has been changed into clear translucent chalcedony. In the agate the forms of fossil sponges can often be beautifully seen. The colours are due to iron-charged water percolating into the flint in the gravel bed, but further structural changes have altered the form of the silica; chalcedony having a structure of close crystalline fibres, revealed by polarized light: when variously stained and coloured, it is usually called agate. Many of these flints, when cut through and polished, are of great beauty. The main force of the tides along these shores is from west to east; and so there is a continual passage of pebbles on the shore in that direction. The flints in Sandown Bay have in the main travelled round from here; and towards the Culvers small handy specimens of agates and chalcedonies rounded by the waves may be collected. [Illustration: _Photo by J. Milman Brown, Shanklin._] SCRATCHELL'S BAY--HIGHLY INCLINED CHALK STRATA The extensive downs in the centre of the Island are largely overspread with angular flint gravel similarly formed to that of St. Boniface. Of other beds of gravel, which have been washed down to a lower level by rivers or other agency we shall have more to say later.