14 It. M. Brydone — Further Notes on the Trimmingham Chalk. 



In the recently published memoir of the Geological Survey on 

 the Upper Chalk of England Mr. Jukes-Browne has established 

 a zone of Ostrea lunata for the Trimmingham Chalk. It is clearly 

 desirable that a formal zone should be so created now that the fauna 

 is known to be so distinctive, in spite of the present impossibility 

 of identifying its upper or lower boundary, but the choice of 

 0. lunata as a zone-fossil, though natural, is unfortunate. 0. lunata 

 has two characteristics of an ideal zone-fossil in that it is, as far as 

 •we know, almost confined to the Trimmingham Chalk, and that in 

 that Chalk zone it always occurs abundantly, if at all. But it fails 

 to fulfil the most important requirement for a good zone-fossil in 

 that it is not distributed all through its so-called zone. There are 

 at least ten, and probably more, distinct horizons occurring in four 

 sequences at different points. The relations of the members of each 

 sequence to one another are quite clear ; the relations of the 

 sequences to one another are at present doubtful (except that 3 is 

 clearly identical with part of 4), but I believe that series 1 is the 

 uppermost and series 4 the lowest, and I often fancy that 

 series 2 is composed of the bottom bed of series 1 and the upper 

 beds of series 3. The series (in descending order in each case) 

 is as follows : — 



1 . Grey chalk Avitli abundance of small Ostrea vesiciilaris. 

 White chalk with 0. lunata. 



,, ■without 0. lunata. 



,, with 0. lunata. 



,, without 0. lunata. 



Grey chalk with 0. canalictilata, 0. inecquicostata, Tcrehrattda ohesa, 

 and Stegaster. 



2. Grey chalk with 0. eanaliculata, 0. inaquicostata, T. obesa, and a grit 



seam or bed at base. 

 White chalk with 0. hmata. 

 ,, without 0. lunata. 



3. White chalk with 0. lunata. 



,, without 0. lunata. 



Hard chalk, weathering very lumjiy. 



4. White chalk ■with 0. lunata. 



,, ■without 0. lunata. 



Hard chalk, weathering very lumpy. 



White chalk very much mottled by grey blue streaks. (It has only just 

 been disclosed below the hard chalk, and I cannot yet say anything 

 of its fauna except that it does not contain 0. lunata.) 



Now a remarkable thing about 0. lunata is that it occurs 

 exclusively (with the exception of one specimen) at the horizons 

 noted as characterised by it. It will be seen that there are, at most, 

 five of these, and at least nine without 0. hmata. It is true that 

 the horizons with 0. lunata are all of some thickness, and owing 

 to the vast profusion of beautiful specimens of this very striking 

 form -which they contain, and to their supplying the greater part 

 of the two bluffs — the only chalk always accessible — they figure 

 very prominently in one's impressions of the Trimmingham Chalk 

 as a whole. But it is safe to say that in more than half of the 

 total thickness of Chalk exposed 0. lunata is not to be found, and 



