532 
THE CRETACEOUS ROCKS OE BRITAIN. 
microscope. The careful investigations carried out by Dr. A. S. 
Hume and Mr. W. Hill (Chapter xxii. &xliii.) have shown decisively 
that there are great differences in the composition of “ chalk/’ 
according to the zonal age of the material. As these differences 
have become known, it has become evident that it is most 
unscientific to compare the Chalk as a whole to Globigerina ooze 
as a whole. Each stage of the Chalk and each part of such a stage 
must be considered separately, and more work is yet required in 
the investigation of the lithological differences presented by each 
zone in its extension from area to area. 
Again, there are several ways in which too limited a view of 
the analogies between chalks and modern oozes may he taken. 
It is obvious that an observer who bases his conclusions on the 
examination of chalks obtained from a single district will not 
have the requisite material for comparison. Thus, the Middle 
Chalk of the extreme north-eastern part of Erance may have 
been formed in comparatively shallow water, but it does not 
follow that chalk of the same age but of much purer character in 
the central and western parts of the Paris Basin was formed in 
water of the same depth. 
Again, we must not forget that the Cretaceous chalks are 
ancient deposits, that they have been raised above sea-level for 
a long time, and have been exposed to the action of percolating 
waters, and have been saturated with fresh water up to various 
levels at various times and places. There can be no doubt that 
some important changes have been effected in the minute structure 
and composition of the Cretaceous chalks since they were at the 
bottom of The sea, and the scope of these changes must be con¬ 
sidered, but we do not think they can be credited with the entire 
removal of Globigerina or of other Foraminifera. 
In this connection the Tertiarv chalk of Barbados becomes of 
the greatest importance, because it forms part of a series of beds 
which are unquestionably comparable with the Reel Clays, the 
Radiolarian ooze, and the Globigerina ooze of the modern oceans. 
x4t the same time, Barbadian chalk presents a very close resem¬ 
blance to English white chalk, not only in its general aspect 
but in its structure, and especially in the small proportion of 
perfect Foraminifera to the mass of the fine-grained powdery 
matrix. * 
In the same way with regard to modern deposits it ought to 
be remembered that Globigerina ooze is a general name for a class 
of deposits, and not for a single variety of nearly constant com¬ 
position. The exploration of the oceanic depths by the Chal¬ 
lenger and other expeditions have greatly enlarged our know¬ 
ledge, and have resulted in a useful general classification of marine 
*See Geology of Barbados, Quart. Journ. Geol. Soc., Vol. xviii., p. 179 
(1892). 
