10 LIFE IN THE SEA [CH. 



on page 9 represents it after the finer amorphous 

 sediment which makes up the greater part of its 

 bulk is washed away. The rounded shells of which 

 it is composed are those of pelagic Foraminifera, 

 protozoa which live mainly in the upper parts of 

 the sea, and which dying there sink down to the 

 bottom, where their shells accumulate to form the 

 deposit. How deep it is we have no opportunity of 

 knowing, but telegraph engineers believe that it forms 

 at the rate pf about an inch in ten years in some 

 places. It is the characteristic deposit of the North 

 Atlantic ocean floor, and only here and there the 

 deeper parts of the latter are covered with material 

 of volcanic origin. At depths over 2500 fathoms the 

 Globigerina ooze tends to disappear, for the lime 

 becomes dissolved by the sea water at these depths. 

 The deposit where the sea is about 3000 fathoms in 

 depth elsewhere than in the Atlantic consists of 

 either volcanic material or the mud called Radiolarian 

 ooze. This latter formation consists largely of the 

 skeletons of the pelagic protozoa known as Radiolaria. 

 It contains very little carbonate of lime, for the 

 skeletons of the Radiolaria are composed of silica. 

 We should also find the skeletons of these organisms 

 in the deposits in shallower parts of the ocean, but 

 they are not so evident because of the greater mass 

 of the limy shells of the Foraminifera. The siliceous 

 ^keleton of the Radiolarians is better able to withstand 



