PLIOCENE AND POST-PLIOCENE PERIODS. 555 



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 colour, 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 

 species. The shells indicate upon the whole a temperate or 

 even cold climate, decidedly less warm than that indicated by 

 the organic remains of the Coralline Crag. It appears, there- 

 fore, that a gradual refrigeration was going on during the 

 Pliocene period, commencing in the Coralline Crag, becoming 

 intensified in the Red Crag, being still more severe in the 

 Norwich Crag, and finally culminating in the Arctic cold of the 

 Glacial period. 



Besides the Mollusca, the Red Crag contains the ear-bones 

 of Whales, the teeth of Sharks and Rays, and remains of the 

 Mastodon, Rhinoceros, and Tapir. 



The Newer Pliocene deposits are represented in Britain by 

 the Norwich Crag, a local formation occurring near Norwich. 

 It consists of incoherent sands, loams, and gravels, resting in 

 detached patches, from 2 to 20 feet in thickness, upon an 

 eroded surface of Chalk. The Norwich Crag contains a mix- 

 ture of marine, land, and fresh-water shells, with remains of 

 fishes and bones of mammals ; so that it must have been de- 

 posited as a local sea-deposit near the mouth of an ancient 

 river. It contains altogether more than 100 marine shells, of 

 which 89 per cent belong to existing species. Of the Mam- 



