498 PLIOCENE MOLLUSCA. 



"Amstelien," containing a somewhat more boreal fauna than do the Soaldisien 

 and Poederlien of Belgian geologists, are not known from Belgium but only to 

 the north of it; they may probably represent the Newbournian or Butleyan zones 

 of the English Crag. Nothing has been met with in Belgium or Holland that 

 I can identify with the Icenian of East Anglia. 



The history of these Red Crag deposits shows, therefore, a gradual but constant 

 change in their fauna, from one having an affinity with that of the Mediterranean 

 to one decidedly boreal, with a noteworthy percentage of distinctly arctic species. 



THE ICENIAN CRAG. 

 Norwich, Chillesford, Weybourne. 



NoKwioH Horizon. 



The beds I group under this name occupy a distinct and wide-spread area to 

 the north of that of the Red Crag which they are not known to overlap (see map. 

 Fig. 4). They never exhibit the highly inclined bedding characteristic of the latter, 

 and appear to have originated under different conditions, in an open and shallow 

 sea, possibly as the western edge of the great delta deposit of the Rhine, which 

 attains such proportions in the sub-soil of Holland. 



The Icenian beds come on suddenly, extending northwards more or less 

 continuously from Aldeburgh in Suffolk to the Norfolk coast, a distance of 40 

 miles in one direction, and covering an area from west to east of about 20 miles in 

 another (Fig. 4). Towards the west they are com})aratively thin, and near Norwich 

 finally disappear, while they thicken rapidly in an easterly direction, having been 

 found to reach, at South wold in Suffolk, a thickness of 150 feet, the mollusca 

 maintaining generally the same comparatively recent and shallow- water character 

 throughout. In East Anglia, as in Holland, subsidence seems to have gone on 

 pari, passu with the accumulation of sediment. 



The marine fauna of the Icenian Crag is of an increasingly impoverished 

 character, the total number of species i-eported from the Norwich zone being not 

 more than 150 in all, of which only about 40 are really abundant, most of them 

 still living in British seas,^ while the number obtained from the later Weybourne 

 zone is still less. By this time the greater part of the characteristic shells of the 

 earlier Crag horizons had disappeared. Specimens from the Norwich sections are 

 generally smaller as well as thinner and more fragile than those of the Red Crag, 

 some of them approaching in character those of a freshwater deposit. Gardimn 

 edule, for example, specimens of which from the Red Crag of Oakley are usually 



1 The best list of the Iceniau Mollusca of Brauierton is that puldished by Mr. Jas. Eeeve in the 

 Proc. Norwich Geol. Soc, vol. i, p. 69, 1879 (70 species iu all), the result of many years' labour. His 

 specimeus are in the Norwich Museum. 



