INTRODUCTTON. 490 



large, tliick and solid, arc smaller and rather delicate in the Tc(Miian Crag of 

 Bramerton, so nnicli so that it is not always easy to obtain them in perfect condition. 



In the first volume of this work (p. 11:3) T suggested that the impoverished 

 condition of many of the Tcenian fossils may have been due to a decrease in the 

 salinity of the Crag water — a feature reported by Swedish observers as met with in 

 the Baltic.^ It seems to me not unlikely that the advance of the Scandinavian ice 

 may have wholly or partially l)lockod the northern outlet of tlu; Icenian sea, which 

 may have still received volumes of fresh water (at least in summer) from the rivers 

 of central or southern Europe. 'The local presence of land and freshwater species 

 in these deposits was formerly supposed to indicate the existence of estuarine 

 conditions at this period.^ A more probable exi)lanation seems to be that such 

 fossils were introduced in places by streams into what was then becoming a land- 

 locked and increasingly brackish lake. 



The matrix of the Icenian Crag appears to have been to a considerable extent 

 of southern origin. It contains many pebbles of white quartz and much mica, as 

 do the Chillesford beds, next to be described, probably derived in both cases from 

 the Palaeozoic rocks of the Ardennes. The Rhine and its effluents seem to have 

 been a not unimportant factor in the later Pliocene history of East Anglia. 



When the Introduction to Wood's First Supplement to the Monograph of the 

 Crag Mollusca was written in 1872, his son and I were disposed to separate 

 a certain portion of the Icenian beds as newer than the rest, and as a fluvio-marine 

 deposit equivalent to that of the Churchyard-pit at Chillesford, but I now consider 

 this was a mistake. Further comparison of the faunas of these various exposures 

 has shown that the difference between their fossils is very slight, while the fauna of 

 the Churchyard-pit, so far as it goes, is, I think, very different. With the latter 

 I associate the many exposures of laminated clays to be dealt with in the next 

 paragraph, confining to these two the term " Chillesford beds." The conditions 

 under which they originated, if my explanation of the subject is correct, Avere of 

 an estuarine character, while the beds which I now group as those of the Norwich 

 horizon originated in a shallow and somewhat wide-spread and brackish sea. 



Among the characteristic species of the Norwich division the following may be 

 specially mentioned : 

 Nassa incrassata. Scala groenlandica. 



„ imsillina. Turbonilla iaternodiila. 



Purpura lapillus, recent variety. JAtforin<i littorea and varieties. 



Neptunea antiqiia. „ rvdis and varieties. 



Bela turricula. Bissoa semicostaia. 



Potamides tricinctus, var. icenica. Paludestrina minuta. 



Turritella terehra. Nucula Gohboldise. 



1 See also as to this, H. W. Shimer, Amer. Natur., vol. xlii, p. 473, 1908. 

 " Heuce the term " Fluvio-marine Crag," 



