DEEP-SEA FORMATIONS. 147 



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 coccosjiheres, 

 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 



^ This was discovered by Hubert. In has been removed and segregated into 



the seas of the present day the shells of flints from the white chalk. Other chalks 



pteropods and globigerinae are, as has have become cherty. Siliceous skeletons 



been discovered by the " Challenger," of sponges are replaced by calcite, calcite 



dissolved beyond a certain depth. Pter- by silica, and aragdnite has been entirely 



opod ooze is not found in depths greater dissolv'ed away." Professor Dittmar sug- 



than fifteen hundred fathoms, and glo- gests that the dissolving of shells may be 



bigerina ooze disappears in its turn at due to the action of sea-water itself, as 



about twenty-five hundred fathoms. after a sufficient length of time it can take 



- The solvent power of the ocean dur- up an amount of lime additional to what 



ing some of the earlier geological depos- it already contains. But the carbonic acid 



its seems to have been far less than in present in sea-water undoubtedly plays 



later times. What the cause is which some part in this dissolving process. Pter- 



has acted so much more vigorously in opod shells, in all possible stages of 



later periods, and caused the disappear- decay and solution, form an important 



ance of aragonitic organisms in the mod- element in the deeper deposits of the Gulf 



ern limestones, is not known. " Silica of Mexico. 



