40 THE FAUNA OF THE DEEP SEA 



It is supposed to extend over one-third of the earth's 

 surface. It is essentially a deep-sea deposit, and one 

 that is found in its typical condition at some con- 

 siderable distance from continental land. Like the 

 Globigerina ooze it is never found in enclosed seas. 

 To the touch it is plastic and greasy when fresh, but 

 it soon hardens into solid masses. When examined 

 with the microscope it is seen to be composed of ex- 

 tremely minute fragments rarely exceeding 0*05 mm. 

 in diameter. It contains a large amount of free silica 

 that is probably formed by the destruction of numer- 

 ous siliceous skeletons, and a small proportion of sili- 

 cate of alumina. It usually contains the remains of 

 diatoms, radiolaria, and sponge spicules, and occasion- 

 ally lumps of pumice stone, meteoric nodules, and, in 

 colder regions, stones and other materials dropped 

 by passing icebergs. 



In the great oceans, then, we find in the deepest 

 places red mud, or, where there is an abundant radio- 

 larian surface fauna, Radiolarian ooze ; in water that 

 is not deeper than about 2,000 fathoms, we find the 

 Globigerina ooze ; in shallower waters and in some 

 localities only Pteropod ooze. 



It must not be supposed that sharp limits can 

 anywhere be drawn between these different kinds of 



