xi The Bed of the Oceans 205 



of carbonate of lime, varying in length from half an inch 

 downward. Innumerable forms of the simplest and smallest 

 of living creatures abound in the surface water. They are 

 most numerous in warm regions, and gradually disappear 

 toward the poles. One class of these is called Foramini- 

 fera, as they construct dense microscopic shells of carbonate 

 of lime pierced with innumerable little holes, through which 

 the soft substance of the animal projects during life. The 

 most common, is a kind called Globigerina, on account of 

 its globular form, the largest shells of which are about the 

 size of a small pin's head. It has been proved that an 

 animal requiring a shell of carbonate of lime can manu- 

 facture it out of any salt of lime, the carbonic acid coming 

 from the creature itself, hence all the lime of sea-water 

 ( 222) is available to be drawn upon. 4 The death of 

 countless millions of minute creatures produces a steady 

 though invisible snowfall of dead bodies falling from the 

 surface layers crowded with ever-renewed life, and gradually 

 subsiding through the cold still depths of water. This 

 takes place over every part of the hydrosphere, but within 

 reach of terrigenous deposits the shells are covered over 

 and buried in the rapidly increasing pile, of which they form 

 a small proportion. Deposits of organic remains are more 

 coherent and plastic than the muds, and have received the 

 general name of Ooze. Living creatures, such as sponges 

 which make skeletons of silica, calcareous sea-urchins, 

 crabs, and corals, exist on the bed of the ocean to all 

 depths, although they are incomparably more abundant 

 in the shallow water near shore. 



274. Pteropod Ooze is formed of the shells of all surface- 

 living organisms in tropical seas, and contains a consider- 

 able proportion of pteropods, whence its name. It is never 

 found below mean sphere level, but abounds on submarine 

 ridges rising to within 1000 fathoms of the surface. The 

 reason of this distribution appears to be that the delicate 

 shells of pteropods expose a very large surface to the sea- 

 water as they fall through it, and are dissolved away before 

 they reach the bottom when the depth is great. 



275. Globigerina Ooze. The small dense shells of the 



