706 MR. F. W. HABMEE ON THE CEAG OF ESSEX. [NOV. I9OO, 



The evidence which I have collected shows, however, that some of 

 these species are not equally common throughout, and that horizons 

 in the Crag may be established whereat certain forms, extinct or 

 southern for the most part, seem to have been dying out, or to have 

 disappeared ; while others, generally recent or northern, were appear- 

 ing for the first time, or becoming more abundant. Moreover, the 

 proportion between recent and extinct, and northern and southern 

 shells at different spots varies considerably, all the evidence pointing 

 consistently in the same direction. 



Speaking generally, the molluscan fauna of any Red or Norwich 

 Crag locality resembles most nearly that of the parishes immediately 

 adjoining it ; while from Walton-on-the-Naze, at the southern limit 

 of the district, to Weybourn, on the northern coast of Norfolk, the 

 Crag-beds assume a more recent and a more boreal character as we 

 trace them northward. 1 



That these deposits arrange themselves in horizontal rather than 

 in vertical sequence is shown by the fact that, so far as the evidence 

 goes, the more recent Red Crag strata are not underlain by those of 

 an earlier stage, nor do Norwich Crag beds ever rest upon Red Crag. 2 

 For example, the former were pierced in borings at Southwold 3 

 and Beccles, 4 and proved to be 147 feet and 80 feet thick re- 

 spectively at those places; but at neither was the latter met with, 

 the mollusca obtained being all of the usual Norwich Crag type. 

 Instances of the overlapping of beds belonging to the same division 

 of the Crag, but to a slightly different horizon, sometimes occur, 

 however, as at Walton and Beaumont, and at the Norwich Crag pit 

 on Bramerton Common. 



In Holland, the Pliocene beds, perhaps nearly 1000 feet thick, 

 which represent the ancient delta of the Rhine and its affluents, 

 include a vertical and apparently continuous succession of strata 

 from Diestian to Amstelian. In Belgium, on the contrary, deposits 

 originating nearer to the then existing margin of the North Sea, 

 and belonging to distinct and disconnected horizons, occupy, more 

 or less, different areas : periods of disturbance having been followed 

 by periods of repose, during which only deposition took place in 

 that region. 



Similarly, the Red Crag deposits of East Anglia do not form an 

 unbroken sequence, but may be referred to three (or perhaps to four) 

 principal stages, of which the faunas, although possibly not entirely 

 contemporaneous, are sufficiently distinct to justify their separate 

 classification. 5 



The term Red Crag, including, as I believe it does, beds differing 

 considerably in age, is vague, and when we attempt to correlate the 

 East Anglian deposits with those of other countries, inconvenient : 



1 The Coralline Crag and the estuarine Chillesford Beds are, however, for 

 obvious reasons, exceptions to this rule. 



2 See footnote, p. 721. 3 Mem. Geol. Surv. (1887) Southwold p. 79. 



4 Ibid. Norwich (1881) p. 156. 



5 The deposition of the Norwich Crag was, however, as I shall endeavour 

 to show farther on, of a similarly continuous character to that of the Dutch 

 deposits. 



