American and English Fossil Floras. 497 



moving some of its original constituents. Silioa has been dissolved 

 and re-precipitated as flints, oxide of iron has been segregated into 

 crystalline masses, manganese has assumed the form of dendritic 

 markings, siliceous sponge skeletons have been dissolved and re- 

 placed by calcite, and calcite shells bj'^ silica, while aragonite shells 

 have been entirely dissolved away. The analyses of fragments of 

 chalk without at least adding a proportion of flint and iron are mis- 

 leading. Mr. Wallace, who has been foremost in endeavouring to 

 explain away the resemblance of chalk to an oceanic deposit, relied 

 gi'eatly on the relatively smaller percentage of carbonate of lime 

 in the existing ooze ; but apart from the considerations mentioned 

 above. Mr. Murray has recently stated that the percentage varies 

 from 40 to 95 per cent, of carbonate of lime, so that the supposed 

 great difference in the constituents can no longer be relied upon. 

 The presence of volcanic ash in existing oceanic deposits, on which 

 some stress was laid, and its absence in chalk, merely indicates 

 that the great fissure eruptions which seem to have preceded crater 

 eruptions, were either more intermittent, or, as there is evidence to 

 show, were not accompanied by the ejection of quantities of ash. 



Dr. Gwj'n Jeffreys, indeed, formed the opinion that the Chalk was 

 a shallow-water deposit from an examination of its mollusca, but the 

 lists submitted to him were largely compiled from the Gi'ey Chalk 

 and Marls at the base of the true Chalk, and a peculiar band from 

 the base of the Chalk in Ireland. He was also biassed to some 

 extent by the absence of such deep-sea forms as Leda, Verticordia, 

 Necera, and the Bulla family, though these belong to the group of 

 mollusca with thin aragonite-tests which are unrepresented in true 

 Chalk, and have only left faintly-marked casts in the Chalk-Marl. 

 The widely-spread and representative mollusca in the White Chalk 

 of England belong to the genera Terebratula, Pecten, Armissinm, 

 Lima, and Spondylus, and all but the latter have already been 

 recorded from depths of over 1400 fathoms. When we reflect that 

 for one cast of the dredge in abyssal depths, a thousand have 

 perhaps been made in the littoral zone, we must hesitate to pro- 

 nounce definitely that any genus is without deep-sea species. More- 

 over, the Cretaceous temperature was very much warmer than at 

 present, and if, as believed by Prestwich, the Chalk sea did not 

 communicate with the Arctic Ocean, its abyssal depths would have 

 been much warmer, and thus contained a fauna that would have been 

 compelled to seek shallow water in order to obtain anything like a 

 corresponding temperature at the present day. In stating that they 

 are a tropical assemblage. Dr. Gwyn Jeffreys lends strong support 

 to this view. The Gault and Greensands representing the Blue and 

 Green muds of the Atlantic pass gradually into Chalk, as these do 

 into Globigerina ooze. As the Chalk overlies and overlaps them, 

 the change in sediment could not have been due to a shallowing of 

 the water, since its area would then have become more restricted, 

 instead of extended. As a change of such a nature is not brought 

 about without cause, we must conclude it to have resulted from 

 increased depth and distance from land. The alternative theory 



DECADE III. — VOL. I. —NO. XI. 32 



