40 GEOLOGY OF THE ISLE OF WIGHT 



impure or sandy flint. The bands of chert appear to have 

 been formed by an infiltration of silica into a sandstone, 

 forming a dense flinty rock, which, however, has a dull 

 appearance from the admixture of sand, instead of being 

 a black semi-transparent substance like flint. But where 

 did the silica come from ? In the depths of the sea many 

 sea creatures have skeletons and shells formed of silica 

 or flint, instead of carbonate of lime, which is the material 

 of ordinary shells and of corals. Many sponges, instead 

 of the horny skeleton we use in the washing sponge, have 

 a skeleton formed of a network of needles of silica, often 

 of beautiful forms. Some marine animalcules, the 

 Radiolaria, have skeletons of silica. And minute plants, 

 the Diatoms, have coverings of silica, which remain like 

 a little transparent box, when the tiny plant is dead. 

 Now, much of the chert is full of needles, or spicules, as 

 they are called, of sponges, and this points to the source 

 from which some at least of the silica was derived. To 

 form the chert much of the silica has been in some manner 

 dissolved, and deposited again in the interstices of sand- 

 stone strata. We shall have more to say of this process 

 when we come to speak of the origin of the flints in the 

 chalk. Sponges usually live in clear water of some depth ; 

 so all shows that the sea was becoming deeper when these 

 strata were being formed. 



Along the shore of the Undercliff, Upper Greensand 

 fossils may be found nicely weathered out. Very common 

 is a small curved bivalve shell, a kind of small oyster, 

 Exogyra conica, as are also serpulae, the tubes formed by 

 certain marine worms. Very pretty pectens (scallop 

 shells) are found in the sandstone. Many other shells, 

 Terebratulez, Trigonia, Panop&a, etc., occur, and several 

 species of ammonite and nautilus.* A frequent fossil is 



* Of Ammonites, Mortoniceras rostratum and Hopliles splendens 

 may be mentioned : and of Pectens, Neithea qttivqueeostata and 

 quadricostata, Syncyclonema orbicularis, and ASquipecten asper. 



