SUBMARINE DEPOSITS. 267 
pod ooze or a globigerina ooze is found in deeper water in the 
tropics than in temperate regions. 
The red clay occupies those portions of the ocean’s bed in 
the central parts of the ocean basins where the carbonate of 
lime surface shells have been almost or completely removed. 
The composition of this clay is rather varied. Mr. Murray has 
shown that it is most largely composed of fragmental voleanie 
material, which in very many cases has undergone profound 
alteration. He also points out the important part played by 
floating pumice-stone in the formation of oceanic deposits. 
After volcanic eruptions, such as that of Krakatoa, the sea is 
frequently covered for square miles in extent with floating pum- 
ice. These fragments are carried far and wide by ocean cur- 
rents, and are often arranged in long lines on the surface of 
the ocean ; they are knocked against each other by the action 
of the waves, and there results from this trituration a fine rain 
of dust, which falls to the bottom of the sea. 
The larger fragments themselves become water-logged after a 
time, and also fall to the bottom. These pumice-stones have 
been found in all the varieties of deposits, but they are espe- 
cially abundant in the red clay areas. At the foot of Misti, 
near Arequipa, Peru, the torrent which sweeps round its base 
carries during floods quantities of pumice, which are drifted 
out to sea and float for a time along the coast. Mr. Murray 
has also shown that these pumice-stones are occasionally thrown 
up on coral islets in immense numbers, and, there decomposing, 
form the red clay and red earth of these islands, the source 
of which was long a great mystery. During the “ Chalen- 
ger” expedition, pumice was continually taken in the tow-nets 
in all parts of the ocean’s surface. There is also abundant evi- 
dence, in many deposits and fragments, of showers of volcanic 
ashes, most probably derived from submarine eruptions. In 
the red clay areas these voleanie matters have usually under- 
gone decomposition into clay, and also have given rise to other 
secondary products, such as manganese nodules (Fig. 185) and 
zeolitie crystals. 
It must be remembered that all the bottom deposits merge 
into one another, and at times it is difficult to say whether a 
