NARRATIVE OF THE CRUISE. 
923 
zones, at depths where the action of the waves is not felt, and in areas to which the ter- 
rigenous materials are rarely transported, forming vast accumulations of Globigerina and 
other pelagic Foraminifera, Coccoliths, Rhabdoliths, shells of pelagic Molluscs, and remains 
of other organisms. These deposits may perhaps be called the sediments of median depths 
and of warmer zones, because they diminish in great depths and tend to disappear towards 
the poles. This fact is evidently in relation with the surface temperature of the ocean, 
and shows that pelagic Foraminifera and Molluscs live chiefly in the warm superficial 
waters of the sea, whence their dead shells fall to the bottom. Globigerina ooze is not 
found in enclosed seas nor in polar latitudes. In the southern hemisphere it has not 
been met with beyond the 50th parallel. In the Atlantic it is deposited upon the bottom 
at a very high latitude below the warm waters of the Gulf Stream, but is not observed 
under the cold descending polar current wdiich runs south in the same latitude. These 
facts are readily explained, wdien it is remembered that this ooze is formed chiefly by the 
shells of surface organisms, which require an elevated temperature and a wide expanse 
of sea. But as long as the conditions of the surface are the same, one would expect the 
deposits at the bottom also to remain the same ; such is not the case. Globigerina ooze 
is rarely found in the tropical zone at depths exceeding 2400 fathoms ; when depths 
of 3000 fathoms are explored in this zone of the Atlantic and Pacific, there is found an 
argillaceous deposit without, in many instances, any trace of calcareous organisms. In 
descending from the “submarine plateaux” to depths which exceed 2250 fathoms, the 
Globigerina ooze gradually disappears, passing into a greyish marl, and finally is wholly 
replaced by an argillaceous material which covers the bottom at all depths greater than 
2900 fathoms. 
The transition between the calcareous formations and the argillaceous ones takes place 
by almost insensible degrees. The thinner and more delicate shells disappear first. The 
thicker and larger shells lose little by little the sharpness of their contours, and appear to 
undergo a profound alteration ; they assume a brownish colour, and break up in 
proportion as the calcareous constituent disappears. The red clay predominates more 
and more as the calcareous element diminishes in the deposit. 
It has been noted that when the sounding rod brings up a graduated series of 
sediments from a declivity descending into deep water, among the calcareous shells those 
of the Pteropods and Heteropods disappear first in proportion as the depth increases. 
At depths less than 1400 fathoms in the tropics a Pteropod ooze is found with abundant 
remains of Heteropods and Pteropods; deeper soundings then give a Globigerina ooze 
without these Molluscan remains; and in still greater depths, as before mentioned, there is 
a red clay in which calcareous organisms are nearly, if not quite, absent. At first it would 
be expected that the Foraminiferal shells, being smaller, would disappear from a deposit 
before the Pteropod shells; but if it be remembered that the latter are very thin and 
delicate, and, for the quantity of carbonate of lime present, offer a larger surface to the 
