122 
POPULAR SCIENCE REVIEW. 
extinct, and nothing therefore can be safely predicated con- 
cerning their habits. 
In the Grey Chalk near Folkestone dark impressions of nearly 
all the deep-sea Mollusca enumerated above are abundant ; and 
the Gault and a part of the Lower Greensand are full of their 
shells in perfect preservation. Their absence in England at 
least, from the Chalk, seems very clearly due rather to subse- 
quent destruction than to their never having been present. Of the 
Chalk genera that are preserved, Pecten , Amussium, Lima , Spondy- i 
lus, Anomia , and the Brachiopoda are represented by Dr. Gwyn j 
J effreys as having been dredged at from 1450 to 1750 fathoms and j 
upwards. As for the abundance of Ammonites showing, as Dr. S. 
P. Woodward once supposed, the water to have been as shallow 
as thirty fathoms, Mr. Wallace himself would be the first to 
repudiate such mere supposition, were it urged against the 
theory he seeks to establish. Were Nautilus and Spirula i 
shallow- water forms they would long since have been captured 
abundantly. The still existing shells of the Chalk itself are so 
few that little weight can be attached to them as an indication 
of depth, but in the lower Cretaceous deposits Mollusca abound, 
as already stated, and in perfect preservation ; and their facies, 
taken with the complete absence of shallow-water forms, 
implies, Dr. Gwyn Jeffreys believes, a depth of sea in the 
Gault period of somewhere about 1000 fathoms. Mr. Sorby, 
from quite other considerations, believed the Gault to be an 
altered red clay, similar in all essential respects to the red clay j 
now forming at the ocean-bottom. There seems thus to be abun- 
dant evidence, endorsed by our greatest authorities, that at least 
some of the Cretaceous deposits were deep-sea, while there is a 
total absence in them of anything necessarily indicating the 
proximity of land.* With regard to the Chalk itself, however, 
the facts are still somewhat contradictory, for it far overlaps 
the Gault and Grey Chalk in Devonshire, and rests upon green- 
sand ; yet although it thins out to the west it remains a per- 
fectly pure rock, without any apparent evidence of the upper 
part of the formation having gradually shallowed as the sea-bed 
became upheaved. 
The immensity of the gap, seldom adequately realized, 
between the true Cretaceous and the next overlying beds, 
implies an interval sufficient to have permitted the grandest 
changes in the distribution of land and water, and the gulf of 
the Atlantic, which stretched over the greater part of Europe, 
* No American or European so-called Cretaceous land-flora can be proved 
to be as old as our White Chalk. The few vegetable remains found in marine 
Cretaceous rocks are not incompatible with the deposits having taken place 
at a distance from shore. 
