Vol. 56.] UK. F. W. HAEMER ON THE CRAG OF ESSEX. 



'25 



analysis of the molluscan fauna op the different horizons of the 

 Crag. (Characteristic and abundant species only.) 



Living only Northern 



Not Tcnotvn in distant and 



living. seas. Southern. Northern. Southern. 



per cent. per cent. per cent. per cent. per cent. 



Gedgravian 38 4 26 1 3 



Waltonian 36 4 20 5 35 



Newbournian... 32 5 16 11 36 



Butleyan 13 4 13 23 47 



Icenian 11 — 7 32 50 



Weybournian. 1 11 - — '33 56 



In calculating the various percentages, the more characteristic 

 species of each zone only are taken into account. It is not always 

 easy to draw the line between rare and abundant forms, and possibly 

 the details which might be tabulated by another observer would be 

 somewhat different. I have little doubt however that, in any case, 

 the general results would agree with those here given, and that the 

 foregoing statistics, though they must be regarded as approximate 

 only, justify the conclusions to be drawn from them. 



The so-called Forest-bed Series of the Cromer and Kessingland 

 coasts, shown by Mr. Clement Eeid to consist of alternations of fresh- 

 water and estuarine strata, maybe known as Cromerian. 2 These 

 beds are equally distinguished by the southern character of their 

 fauna from the underlying Weybourn Crag with its boreal mollusca, 

 on the one hand ; and from the Leda-myalis Sands, and the Arctic 

 Freshwater Bed with Saliw polaris and Betula nana, on the other. 

 These three groups of deposits indicate a distinct change in climatal 

 conditions — an interruption, for the time, of the gradual refrigera- 

 tion of the Crag Period, similar to that of the Interglacial episodes 

 of the Glacial Epoch. 3 In the case of the Forest-bed, as in that of 

 the Crag, it is necessary to count specimens rather than species. 

 Northern forms are very rare at the former horizon, Gulo and Ovibos, 

 for instance, being known from unique examples only, while the 

 remains of Southern mammals, such as Elephas meridionalis, are 

 exceedingly common. 4 



These Forest-bed fossils, always fragmentary, occurring either in 

 estuarine or fluviatile mud, and flood-gravel, although they do not 

 necessarily represent the mammalian fauna of Norfolk at the period 

 in question, but rather that of the Rhine Valley to the south, still 

 show clearly, I think, that the climate of North-western Europe was 



1 The above figures do not adequately represent the modern character of the 

 Weybournian fauna. If individuals could be counted rather than species, the 

 recent shells would form more than nine-tenths of the whole : the specimens 

 of Tellina balthica alone far outnumbering all the others put together. 



2 See Eenevier's ' Chronogr. geol.' 2nd ed. (1896) of his ' Tableau des Terrains 

 sedimentaires.' 



3 Prof. James Geikie believes that the Weybourn Crag represents the lowest 

 Boulder Clay of Southern Sweden : see his ' Great Ice Age ' 3rd ed. (1894) 

 p. 479, also pp. 336 et seqq. 



4 I did not give, in my paper on the ' Pliocene Deposits of Holland ' Quart. 

 Journ. Geol. Soc. vol. Hi (1896) p. 774, such weight to these considerations as 

 I now think they deserve. 



