Ch. xin.] 



Crag. 



THE NORWICH CRAG. 



Fig. 144. 



London Clay. 



197 



Chalk. 



chalk is explained in the foregoing diagram. Both the White and 

 the Ked Crag, as we shall see in the sequel, belong to the Older 

 Pliocene period, whereas a more modern deposit, occurring in the 

 neighborhood of Norwich, is referable to the Newer Pliocene. It 

 consists of beds of incoherent sand, loam, and gravel, which are ex- 

 posed to view on both banks of the Yare near Norwich. As they 

 contain a mixture of marine, land, and freshwater shells, with ichthy- 

 olites and bones of mammalia, it is clear that these beds have been 

 accumulated at the bottom of a sea near the mouth of a river. They 

 form patches varying from 2 to 20 feet in thickness, resting on white 

 chalk, and are covered by a dense mass of stratified flint-gravel. The 

 surface of the chalk is often perforated to the depth of several inches 

 by the Pholas crispata, each fossil shell still remaining at the bottom 

 of its cylindrical cavity, now filled up with loose sand from the in- 

 cumbent crag. This species of Pholas still exists, and drills the rocks 

 between high and low water on the British coast. The most common 

 shells of these strata, such as Fusus striatus, F. antiquus, Turritella 

 communis, Cardium edule, and Cyprina islandica, are now abundant 

 in the British seas ; but with them are some extinct species, such as 

 Nucula Cobboldice (fig. 145), and Tellina obliqua (fig. 146). JSfatica 

 helicoides (fig. 147) is an example of a species formerly known only 



Fig. 145. 



Fig. 146. 



Fis. 147. 



Nucula Cobboldia;. 



Tellina obliqua. 



ITaiica helicoides, 

 Johnston. 



as fossil, but which has now been found living in our seas ; and I have 

 recently seen, in the British Museum, a living shell from Vancouver's 

 Island, so closely allied to N. Cobboldice, that it would be considered 

 by many as merely a marked variety of the same form. 



The Norwich Crag is seen resting on chalk in the sea cliff between 

 Weybourne and Cromer, and is found at many points to the westward 

 in the interior. The only place where beds containing the peculiar 

 shells of this formation have been found directly overlying the Red 

 Crag is at Chillesford, near Orford in Suffolk ; but we do not require 

 the evidence of direct superposition to prove that the Norwich is a 

 much newer deposit than the Red Crag, since the proportion of recent 

 to extinct species is so much greater in the Norwich beds, amounting, 



