its original condition the cretaceous ooze was, like that of 

 the Atlantic deep sea mud, filled with the spicular skeletons 

 of sponges. At the same time they furnish no evidence of 

 the hypothesis of iJr. W'allich that alternating periods are 

 established during which one of the two predominant animal 

 t\-pes (Foramenifera and Sponges) gradually overwhelms and 

 crushes out the other over indefinite local areas, the strata of 

 the chalk in the one case and the intercalated flint beds in 

 the other, being the issue of these contests, (op. cit. p. 72) 

 The contents both of the Irish and Horstead flints show that 

 the sponge spicules are equall)' as much intermingled with 

 foramenifera and other calcareous organisms as in the Atlan- 

 tic ooze, and that therefore both these -> animal t\'pes« flourished 

 contemporaneously without alternately choking the life out 

 of each other. 



These hollow flints throw no fresh light upon the causes 

 which have brought about the arrangement of the nodules in 

 definite layers but the perfect state of preservation ot their 

 contents show that in certain cases at least they were formed 

 before there was any great accumulation of overlying material, 

 which would have crushed the tender shells inclosed within. 

 But whatever may have been the forces which dissolved and 

 re-deposited the silica in its present form, it seems evident 

 that the delicate skeletons and spicules of sponges have been 

 the original source of the material itself. 



