50 GEOLOGY OF THE ISLE OF WIGHT 



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 in- 

 soluble residue which has been left when the Chalk was 

 dissolved away. 



On the top of the cliffs between Ventnor and Bon- 

 church, 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 crystal- 

 line 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, 



