THE PLIOCENE PERIOD. 335 



CHAPTER XX. 

 THE PLIOCENE PERIOD. 



The highest division of the Tertiary deposits is termed the 

 Pliocene formation, in accordance with the classification pro- 

 posed by Sir Charles Lyell. The Pliocene formations contain 

 from 45 to 95 per cent of existing species of Mollusca, the re- 

 mainder belonging to extinct species. They are divided by Sir 

 Charles Lyell into two divisions, the Older Pliocene and Newer 

 Pliocene. 



The Pliocene deposits of Britain occur in Suffolk, and are 

 known by the name of " Crags," this being a local term used 

 for certain shelly sands, which are employed in agriculture. 

 Two of these Crags are referable to the Older Pliocene, viz., 

 the White and Red Crags, and one belongs to the Newer 

 Pliocene, viz., the Norwich Crag. 



The White or Coralline Crag of Suffolk is the oldest of the 

 Pliocene deposits of Britain, and is an exceedingly local for- 

 mation occurring in but a single small area, and having a 

 maximum thickness of not more than 50 feet. It consists of 

 soft sands, with occasional intercalations of flaggy limestone. 

 Though of small extent and thickness, the Coralline Crag is of 

 importance from the number of fossils which it contains. The 

 name " Coralline " is a misnomer ; since there are few true 

 Corals, and the so-called " Corals " of the formation are really 

 Polyzoa, often of very singular forms. The shells of the Coral- 

 line Crag are mostly such as inhabit the seas of temperate 

 regions; but there occur some forms usually looked upon as 

 indicating a warm climate. 



The Upper or Red Crag of Suffolk like the Coralline Crag 

 has a limited geographical extent and a small thickness, 

 rarely exceeding 40 feet. It consists of quartzose sands, usu- 

 ally deep red or brown in color, and charged with numerous 

 fossils. 



Altogether more than 200 species of shells are known from 

 the Red Crag, of which 60 per cent are referable to existing 



