JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 395 
Agassiz, who has examined both Chalk and Coral-mud, has no hesi- 
tation in saying that " Chalk is certainly not derived from the disin- 
tegration of Coral-reefs."" 
The White Chalks agree with the Globigerina and Pteropod 
oozes in containing Coccoliths and calcareous spheres ; they differ 
from both in containing fewer perfect shells. The most highly 
calcareous Globigerina oozes of the present day are crowded with, 
and often almost entirely made up of, Globigerina shells, while the 
more purely calcareous Chalks consist mainly of fine calcareous dust, 
and the single calcareous spheres are always more abundant than the 
Globigerina?. This difference, however, may be due to the fact that 
we only possess samples of the highest layer of the modern oceanic 
ooze, this layer.naturally consisting largely of fresh Globigerina shells. 
In deeper layers the fine material resulting from the decay and dis- 
integration of the shells is likely to be more abundant, and such a 
deposit would greatly resemble Chalk. 
While, therefore, no exact modern counterpart of Cretaceous 
Chalk has yet been discovered, yet it is a fact that certain varieties of 
Globigerina ooze resemble Chalk more than any other modern deposit, 
and I think we may reasonably infer that this similarity implies 
the existence of similar physical conditions. 
* Three Cruises of the "Blake," vol. i., p. 148. 
