50 THE STORY OF THE EARTH. 



surfaces, Oil which the remains of coniferous forest 

 trees Stand erect, are seen in the Purbeck beds of 



the Isle of Portland and the Isle of Purbeck in 

 1 I irsetshire. Presh-water sediments, alternations 

 o\ sands and clays are found with numerous rep- 

 etitions in the Wealden beds of the Isle of Purbeck, 

 and the [sle of Wight; and they are associated 

 into a few deposits, fairly well defined into sands 

 and clays, in the Wealden strata of Kent and 

 Sussex. 



Recognition of the fresh-water origin of all 

 such rocks rests upon the presence in them of 

 animals which lived in fresh water. When these 

 are shells they are often matted together to form 

 layers of some thickness. The types or genera 

 are identical with those which live in every pond, 

 lake and stream on the surface of the country at 

 the present day. The bivalve shells are usually 

 species of Cyclas % or U?iio, or Anodonta* The uni- 

 valve shells are either the pond shells Planorbis % 

 Paludina and Limnaa % or Mich river shells asi\Y;/'- 

 //>/</, and the fresh-water limpet. 



There is probably no fresh-water limestone 

 from which tile seed-vessels of the plant Chat a 



arc absent Sometimes the presence of the sili- 



18 spirilla- of the fresh-water Sponge, SpOngilla % 



has resulted in fossils being mineralized with silica, 



as m the Purbeck beds, or the formation of sili- 

 II layers and concretions m fresh-w ater Mine- 

 rs, which may be compared to the veins and 



concretions of flint found in marine strata like the 



< ilk and Carboniferous Limestone. 



