Norwich Crag. 275 



our region, and their bones been scattered on the surface. 

 A partial submergence of the country took place, so 

 that Britain became for a time an island, and the 

 marine Crag beds were deposited over part of our 

 eastern area, the relics of which still remain in Norfolk 

 and Suffolk. Some of the mammalia survived this 

 partial submergence, and continued to inhabit the island 

 during Pliocene times, and getting associated with 

 varieties and new species, the bones of some of the ex- 

 tinct species may have been mingled with others then 

 living, and all were washed into the basement beds of 

 the above-named Crag formations during various oscil- 

 lations of level. 



The Mammaliferous or Nonvich Crag consists of 

 sand, gravel, and shells, generally only a few feet in 

 thickness, and which, in Norfolk, lie upon the Chalk. 

 From the nature of the fossils of the Norwich Crag, it 

 is believed to have accumulated near the mouth of a 

 river. It is never seen in contact with or overlying 

 either the Coralline or Eed Crag, and it is considered by 

 Mr. Prestwich to be of the same age with the Eed Crag, 

 having been accumulated in an area partly estuarine, and 

 separated from the purely marine area of the Red Crag 

 by an emerged district consisting of the Coralline Crag. 



In the Norwich Crag 139 species of marine Mollusca 

 are known, of which 87, or 56 per cent, are common 

 to the Coralline Crag, 137, or 88 per cent, to the Red 

 Crag, and 93J per cent, are still living. ' Comparing 

 the three Crags the proportions of extinct species of 

 marine Mollusca are, Coralline Crag 1 6 per cent. Red 

 Crag, 7*7 per cent, and Norwich Crag 6*5 per cent.' 

 (Prestwich). The latter contains about 20 species of 

 land and fresh-water shells, such as Helix, Planorbis, 

 Paludina (P. lenta, &c.\ Pupa, Limncea, Cyclas, Cy~ 



T 2 



