147 
DEEP-SEA FORMATIONS. 
nite there is no doubt,' but the presence in the chalk of deep- 
sea brachiopods, cephalopods, and crustacea, tends to prove the 
deep-sea nature of the old white chalk. 
We should, however, bear in mind that at the present day the 
distinctions made to determine faunistic regions are often based 
on the presence or absence of certain species, and not of genera. 
Genera have a far wider range both geographically and bathy- 
metrically, and can only be used for such comparisons with the 
greatest care. It would certainly be impossible to find any- 
where to-day a littoral fauna with the facies of the characteristic 
chalk. 
It is difficult to institute a comparison of the globigerina 
ooze with the chalk proper, as from the length of time the 
latter has been exposed to the action of the atmosphere its com- 
position must have changed materially either by solution or 
metamorphosis.” 
The difference in the analysis of the chalk and of the globi- 
gerina ooze consists mainly in the fact that there is more car- 
bonate of lime and less alumina in the former; while the flint 
nodules in the chalk are replaced perhaps by the large amount 
of silica in the globigerina ooze. Coccoliths and coccospheres, 
similar to those detected by Wallich in the chalk, also occur in 
the deep soundings. 
Murray and Renard consider that “chalk must be regarded 
as having been laid down rather along the border of a continent 
1 This was discovered by Hébert. In 
the seas of the present day the shells of 
pteropods and globigerime are, as has 
been discovered by the * Challenger,” 
dissolved beyond a certain depth. Pter- 
opod ooze is not found in depths greater 
than fifteen hundred fathoms, and glo- 
bigerina ooze disappears in its turn at 
about twenty-five hundred fathoms. 
2 The solvent power of the ocean dur- 
ing some of the earlier geological depos- 
its seems to have been far less than in 
later times. What the cause is which 
has acted so much more vigorously in 
later periods, and eaused the disappear- 
ance of aragonitie organisms in the mod- 
ern liméstones, is not known. “Silica 
has been removed and segregated into 
flints from the white chalk. Other chalks 
have become eherty. Siliceous skeletons 
of sponges are replaced by calcite, calcite 
by silica, and aragonite has been entirely 
dissolved away.” Professor Dittmar sug- 
gests that the dissolving of shells may be 
due to the action of sea-water itself, as 
after a sufficient length of time it can take 
up an amount of lime additional to what 
it already.contains. But the carbonic acid 
present in sea-water undoubtedly plays 
some part in this dissolving process. Pter- 
opod shells, in all possible stages of 
decay and solution, form an important 
element in the deeper deposits of the Gulf 
of Mexico. 
