32 THE WORLD BENEATH THE WAVES 



familiar chalk of soutM-eastern England.^' The shower 

 of shells may be, and often is, also falling where the 

 blue mud is being deposited, but there it is quite lost 

 in the infinitely more voluminous mass of river-borne 

 sediment ; so that it is only at a considerable distance 

 from land, and when all the river-borne material has 

 settled, that we get pure Globigerina-ooze. 



But Foraminifera are not the only animals whose 

 remains form, or help to form, oceanic deposits. The 

 surface waters of the open sea swarm with delicate 

 shell-secreting animals of other and far higher kinds. 

 Conspicuous among these, in certain latitudes, are the 

 sea-butterflies or Pteropoda — small mollusks not dis- 

 tantly related to the cuttle-fishes — whose shells resemble 

 little caps and purses of the finest glass. When they 

 die, their shells too fall to the bottom, and sometimes 

 in numbers sufficient to overwhelm the Foraminifera 

 ' shells, and to turn the Globigerina-ooze into Pteropod- 

 ooze. 



In our imaginary journey across the Bay of Bengal, 

 we should meet with nothing but the deposits men- 

 tioned above. For the first 150 miles from Madras 

 there would be blue and brown muds. These would 



Not all the Foraminifera whose shells are found in deep-sea 

 ooze live at the surface, though most of them do : some 

 undoubtedly live at the bottom. Again, not all Foraminifera are 

 minute : some of the species that inhabit shallow water in the 

 tropics are as big as a sixpence or a shilling, and there are certain 

 species among those, whose shells are formed of sand, that attain 

 the size of a crown-piece. 



