33 
arise respecting the difference which may exist in other cir¬ 
cumstances necessary to constitute an identity of geological age. 
There is one point in the structure and formation of the beds 
essentially characteristic of the cretaceous as a rock system, 
and which distinguishes it in a marked manner from the 
Atlantic deposits. In the chalk there are bands and lines of 
flint, masses of amorphous (or rather as it has been called 
Crystalline) Silica. This Silica seems to have fllled up and taken 
the shape of any cavities existing in the beds. Often they are 
shapeless, but may and do occasionally assume the shape of 
cup-like sponges and often the shell of a sea urchin, or other 
remains form the mould of a flint. When chalk is dissolved by 
an acid, a small portion of silex is usually left, which is 
generally crystalline, and apparently of inorganic origin, being 
simply fragments of mineral matter. In the ocean ooze it is 
true that Silex in some abundance is found, hut with this wide 
difference, that it is of organic origin, and in the form of 
beautiful siliceous organisms, such as diatoms, spicules of 
sponges and Polycystina, all of which although occasionally 
present, do not exist in great abundance or enter largely into 
the constitution of white chalk. The disseminated silex found 
in the ooze is of volcanic origin, the mineralogical characters 
being readily recognizable to an experienced eye. This charac¬ 
ter seems to constitute a wide difference of conditions, the 
siliceous concretions of the chalk appear to have been derived 
from disseminated siliceous matter moulded into cavities or 
collected as it were round centres of crystallization, whilst in 
the sea mud the substance is found in distinct siliceous coated 
organic bodies. 
Another phase of the subject has to be considered. Chalk 
chiefly consists of Globigerina Cretacea asssociated in almost 
equal proportions with a minute Textularia and with coccoliths. 
The fossil Globigerina is probably but a variety of G. Bulloides; 
hence so far as this foraminifer is concerned, ancient and modern 
deposits may have been continuous. But in none of the 
modern Globigerina beds have I found anything resembling 
the fossil Cretaceous Textularia, the disappearance of which 
requires to be accounted for. I should feel much hesitation in 
