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constituents, but has stopped short of destroying their forms or 

 dissolving them. A study of these fossils thus wonderfully 

 preserved enables us rightly to interpret the changes 

 which have taken place in the production of the chalk 

 and flints out of the cretaceous ooze. The foramenifera, 

 ostracoda, and other small organisms whose skeletons 

 are formed of carbonate of lime have been for the 

 most part so crushed and broken up by the mechanical 

 pressure to which they have been subjected, that though the 

 fragments are sufficient to indicate their former existence, it 

 is only here and there under special circumstances that their 

 skeletons are found complete in the chalk. A still more 

 complete change awaited the skeletons of the .sponges. These 

 structures, after the death of the animal, appear largel)' to 

 have fallen apart into their constituent .spicules, and though 

 composed of silica, which till latel)' was regarded as more 

 stable than carbonate of lime, they were so completely dis- 

 solved and removed from the chalk that an analysis of this 

 material in which they flourished, does not yield a trace of 

 siHca to show their former presence in it. The silica resulting 

 from the solution of these sponges became aggregated to form 

 the nodules of the flints or was deposited in the joints and 

 fissures which intersect the chalk strata in many places. In 

 the same proportion as the chalk indicates the remains of the 

 foramenifera, ostracoda, and other organisms with calcareous 

 skeletons, so also do the imbedded nodules and masses of 

 flints bear witness to the siliceous skeletons of the sponges 

 which contemporaneously existed with them. This may appear 

 a somewhat far-rtaching conclusion to arrive at from the pre- 

 sence of the.se sponge spicules in this single chalk flint but 

 evidence from other quarters corroborates and strengthens it. 

 Mr. Wright's investigations in the material inclosed in chalk 

 flints from various places in the North of Ireland prove that 

 there also siliceous .sponges abounded mingled with the shells 

 of foramenifera and other calcareous organisms, whilst in the 



