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 volcanic 

 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 "Challen- 

 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 volcanic matters have usually under- 

 gone decomposition into clay, and also have given rise to other 

 secondary products, such as manganese nodules (Fig. 185) and 

 zeolitic crystals. 



It must be remembered that all the bottom deposits merge 

 into one another, and at times it is difficult to say whether a 



