494 PLIOCENE MOLLUSOA. 



arctic shells like N. dpspeeta made tlieir appearance ; in this way, I take it, the 

 general character of the molluscan fauna of Red Crag times was gradually 

 changed, eventually becoming distinctly northern.^ 



The Red Crag beds, nowhere attaining any consideral)le thickness, their fossils 

 being with few exceptions the drifted and stratified shells of dead mollusca, seem to 

 have been deposited as beaches against the shore or as submarine banks in proximity 

 to it, in the shallow water of land-locked bays or inlets. The position of these inlets 

 was successively shifted towards the north as the northerly depression progressed, 

 and they were silted up one after another by masses of shelly sand. As far as the 

 evidence goes, the different fossiliferous zones of the Red Crag do not overlap, 

 occurring in horizontal rather than in vertical sequence. As we trace the north- 

 ward retreat of the Crag sea from Walton-on-Naze on the south to Weybourne on 

 the Norfolk coast, we find the various deposits to assume regularly a more recent 

 as well as a more l)oreal and eventually a sulvarctic character. By comparing the 

 percentages of the more prominent northern or southern mollusca on the one hand, 

 and of the recent or extinct species on the other, we are al)le to apply to the Crag 

 beds in different localities a test of their comparative age. The older and distinctively 

 southern fauna occurs towards the south of the Crag district, the northern and 

 more recent towards the north. The change is gradual, and, as just stated, there 

 is no overlapping. The Red Crag deposits represent therefore a succession of 

 shelly false-bedded strata having a sandy and not an organic mati-ix, as in the case 

 of the Coralline Crag, accumulated near the then western margin of the North Sea 

 as submarine, shallow-water banks or, with a more highly inclined stratification, 

 as beaches against the shore. 



No such masses of dead shells as those which, in inconceivable numbers, were 

 heaped against the western coast of the North Sea in Red Crag times, are now to 

 be found in that region. One may sometimes walk for a mile or two along the 

 beach, for instance at Yarmouth or Lowestoft, without finding a single specimen. 

 When we pass to Holland, however, we find deposits of shelly (lrhri>t, continuous 

 for many miles, which can only be compared in extent and importance with those 

 of the Crag.^ The difference seems to have been due to the fact that the cyclonic 

 disturbances from the Atlantic which now cross East Anglia are attended by a 

 prevalence of stormy gales from the west, owing to the centres of the cyclones 

 passing to the north of it. The effect of stormy weather attended by strong 

 winds is to agitate the water to a consideral)le de})th and to sweep the sea-bottom 

 where mollusca live, their shells being afterwards carried forwards towards the 



1 This view lias been more fully discussed iu the first volume of the present memoir, p. 156 ef seq. 

 lu the Scaldisien of Belgium only the sinistral form of Nepinnea occurs, as at Walton ; in the 

 Poederlien the dextral and northern shell N. despcda is found with it, as at Oakley. 



2 During an excursion to the Hook of Holland with my friend Dr. Lorir I was shown some steam 

 dredgers which I was told raised 100,000 tons of dead shells annually for the manufacture of lime. 



