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 largely to 
have fallen apart into their constituent spicules, and though 
composed of silica, which till lately 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 
silica 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-reaching conclusion to arrive at from the pre- 
sence of these 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 
