PRE3TWICH CRAG-BEDS OF SUFFOLK AND NORFOLK. 355 



which they spread, but, in spreading, retained at the same time 

 evidences of the grouping they had when living. 



After the deposit of the lower division of the Eed Crag a subsidence 

 of some 10 or 12 feet took place, and a new line of cliff, on a higher 

 level, was formed at Sutton. Even the rise of the tides might almost 

 be determined ; for we know that the Pholas crispata, the borings 

 of which are spread over the upper shore and top of the lower cliff, 

 lives only between water-marks. The perforated space occupies a 

 zone about 5 or 6 feet deep (see fig. 21). 



The increased prevalence of oblique lamination in the upper part 

 of the lower Eed Crag leads to the supposition that the sea was 

 gradually silting up. To this, however, succeeded a movement of 

 subsidence causing the erosion, by fresh submarine currents, of the 

 surface of the lower division, and, as the subsidence gradually con- 

 tinued, to the accumulation of the upper division of the Eed Crag. 



From the circumstance of the lower j)art of these upper sands 

 being formed of the debris of the previously deposited lower Eed 

 Crag, the mineral character is very similar ; but in the ascending- 

 series, formed as the sea-bed gradually became more depressed, the 

 sands are finer, they lose their red colour, and they pass upwards 

 into fine micaceous sands and grey clays, in which the Testacea are 

 constantly found on the spot where they lived. 



These sands and laminated clays (the Chillesford series) covered 

 up and levelled the whole of the various beds beneath them, spread- 

 ing over them all through Suffolk and Norfolk. 



The difference of age which the distinctive characters of the Eed 

 and Coralline Crags might suggest does not hold when each is 

 viewed separately in relation to the fauna of the present period. 

 There must have been other causes than mere lapse of time and de- 

 scent to account for this variation in their respective faunas ; and 

 this probably was owing both to the variation of the level of the bed 

 of the sea, to which we have directed attention, and by which many 

 of the deeper-water MolluKSca must have been destroyed within our 

 area, and also to the inflax of more northern or north-easterly 

 currents, and the increased cold, indicated by the physical pheno- 

 mena, bringing in a more northern fauna. It is a question of tem- 

 perature rather than of time. 



The absence of essential difference of age between the two crags 

 is also apparent in their correlation with the two upper Crags of 

 Antwerp, with both of which they show a close relationship — the 

 lower one of the two being probably of the age of the Coralline, 

 and the upper one having a near analogy with the Eed Crag. 



Species corn- 

 Antwerp beds, mon to the Proportion of 

 number of Antwerp beds Eed-Crag 

 species. and the species. 

 Eed Crag. 

 Scaldisien j S^bfes jaunes 197 138 74 per cent. 



\ Sables gris .. . 187 122 65 „ 



Diestien ... Sables noirs.. 228 61 26 „ 

 Common to the Sables jaunes and gris 139, or 75 per cent. 



We will, however, consider more fully the relations of the Belgian 



