WHITE CHALK COMPARED WITH CALCAREOUS OOZE. 503 
deposits, but it is possible that in this excellent general view the 
local differences presented by each kind of deposit have not been 
sufficiently allowed for, and perhaps they are not yet sufficiently 
understood. It is useful to have a general name like “ Globigerina 
ooze,” but no one can peruse the Challenger Report on Deep 
Sea Deposits without realising that there are many varieties of such 
ooze. The investigators have always been careful to say that 
Globigerina ooze passes into red clay on the one hand and into 
Pteropod ooze or into green mud on the other ; but geologists 
would have been grateful for a more complete description of some 
of these gradations and varieties of deposit, and especially of those 
which occur in the lesser depths of the oceanic basins, or in such 
areas as the Caribbean Sea. 
Professor A. Agassiz has drawn attention to some of the differ¬ 
ences which the calcareous oozes exhibit in the Caribbean Sea and 
the Gulf of Mexico. * He speaks of them as modified Globigerina, 
and Pteropod oozes, and Sir J. Murray in his report on the samples 
submitted to him says : f “It should be remembered, however, that 
both in the size of the mineral particles and in the nature of a large 
number of the calcareous particles these deposits differ considerably 
from similar deposits found far away from land in the open 
ocean, and called also Pteropod and Globigerina oozes.” It does 
not appear that any samples from the central parts of the Caribbean 
sea were examined. 
The principal difference between the purer white chalks and the 
more calcareous oceanic oozes described in the Challenger Report 
on Deep Sea Deposits, consists in the relative proportions of 
perfect or nearly perfect Foramimfera to the mass of the deposit. 
In the modern oceanic deposits the shells of pelagic Foraminifera 
are very abundant; they generally make up about half the actual 
mass of the deposit, and in the case of many Globigerina oozes they 
form between TO and 80 per cent, of the mass. The following 
table has been compiled from statements given in the Chal¬ 
lenger Report: — 
— 
Proportion of Pelagic 
Foraminifera. 
Proportion of 
Foraminifera 
living on Bottom. 
F rom 
Average 
Average. 
In Globigerina ooze - 
25 to 80 .. 
53.1 
2.13 
In Pleropod ooze - 
30 to 75 .. 
47.15 
3.15 
In Coral mud 
10 to 56 .. 
31.27 
14.64 
In green mud 
1 to 35 .. 
14.59 
‘ 
2.94 
* Three Cruises of the Blake, Vol. i., p. 281, and map on p. 28G (1888). 
t Bull. Mus. Comp. Zool., U.S., Vol. xii., p. 45 (1885). 
4219. 
M M 2 
