Vol. 54.] AlfD THE CORALLIN^E CRAG. 30^ 



Chillesford respectively, still holds good, and further that there are 

 Ked Crag-beds which do not exactly correspond with those found 

 at any of these localities, which may therefore be conveniently 

 separated from them. The various Red Crag-beds are, in my 

 opinion, the marginal accumulations of a sea gradually retreating 

 northward and eastward, in consequence of the earth-movements 

 described in my paper on ' The Pliocene Deposits of Holland.' ^ 

 The molluscan fauna of the diiferent exposures of the Red Crag 

 assumes a more recent and a more boreal character as we trace them 

 successively in a north-easterly direction, and they arrange themselves 

 therefore in horizontal rather than in vertical sequence. 



I still consider that the Crag of Norfolk and of the northern part 

 of Suffolk is newer than any part of the Red Crag, except perhaps 

 that which occurs in the upper part of the section in the Stack- 

 yard pit at Chillesford ; and that the Weybourne and Belaugh beds, 

 containing Tellina halthica, mark a yet more recent horizon of the 

 Pliocene period. 



Early in Red Crag times, I believe, the communication which 

 had previously existed between the North Sea and the English 

 Channel was interrupted,^ and in consequence southern species of 

 mollusca became by degrees less abundant in the former, and 

 finally disappeared from it, while, coincidently, an invasion of the 

 Crag area by boreal and arctic forms took place. Hence, if we can 

 trace out the history of the gradual disappearance of the southern,, 

 and of the gradual arrival and increasing abundance of the northern 

 shells, and if we can ascertain the relative proportion of these different 

 groups at different localities, we shall have the material at hand for 

 correctly classifying the various beds of the Upper Crag. 



The old lists of the Crag mollusca as a rule did not distinguish 

 between the rare and the abundant species, and therefore the con- 

 clusions drawn from them are not wholly reliable. To attach the 

 same importance, for the purpose of analysis, to a species of which a 

 single specimen, or at the most a very few, may have been discovered 

 as the result of the labours of nearly a century, as to one of 

 which a hundred specimens may be easily obtained in the course of 

 a few minutes, is obviously misleading. The collections in our 

 museums are not so instructive as they would be if the efforts of 

 collectors were not so largely directed to the acquisition of rare 

 species and of perfect specimens. From a geological point of view, 

 it is the abundant rather than the rare species which are important, 

 and shells which are seldom found in collections, as it is difficult to 

 obtain them whole, are sometimes among the most characteristic of 

 the deposit in which they occur. In order to ascertain the age of 

 a bed or the character of its fauna, it is necessary to count speci- 

 mens, so to speak, rather than species. 



Much more attention than formerly has been paid of late years to 

 this point, but the Crag mollusca as a whole have not been so dealt 

 with. In the lists which I hope to publish with my next paper, I 



1 Quart. Journ. Geol. Soe. vol. lii (1896) p. 748. 



^ See also Prestwich, Quart. Journ. Greol. Soc. vol. xiv (1858) p. 331. 



