206 The Realm of Nature CHAP. 



Globigerina can fall through a far greater depth than the 

 thin pteropods before they are dissolved. Globigerina ooze 

 accordingly covers a far greater part of the ocean bed. 

 It does not occur in enclosed seas, nor under the cold cur- 

 rents of the north-east Atlantic, nor in the Southern Ocean 

 south of 5 5 S. ; but otherwise it is practically universal within 

 certain limits of depth. Under the Gulf Stream its deposit 

 is carried far to the north, as the surface water of that current 

 swarms with globigerinse. The ooze is a white or pinkish 

 substance, which when dried is seen to have a fine granular 

 structure, due to the little round shells of which it is composed. 

 It varies in composition with the depth, that which has formed 

 in the deepest water containing only the stronger and denser 

 species, and the shells of these even being much corroded. 

 The percentage of carbonate of lime varies from 30 to over 

 80, sometimes reaching 95 ; and if the carbonate is dissolved 

 by a weak acid, the residue consists of a fine clayey sub- 

 stance mixed with the cases of diatoms and the spicules of 

 radiolarians. At depths exceeding 2500 fathoms, with rare 

 exceptions, none of this ooze occurs, the proportion of car- 

 bonate of lime in the deposit being reduced almost to the 

 vanishing point. Globigerina ooze borders the upper zone 

 of the Abysmal Area, and thins away toward the great 

 depths (see Fig. 41). 



276. Radiolarian and Diatom Oozes. The siliceous 

 skeletons of radiolarians and diatoms are present in small 

 amount in almost every deposit. Silica is not nearly so 

 soluble as carbonate of lime in sea-water ; hence when the 

 depth is greater than 2500 fathoms, and radiolarians abound 

 on the surface, their spicules form a large proportion of the 

 deposits reaching the bottom. The name of Radiolarian 

 Ooze is given when they amount to more than 25 per cent. 

 Radiolarian ooze is spread over a considerable part of the 

 central Pacific, and the east of the Indian Ocean where 

 the maximum depression occurs, but it is not found in 

 the Atlantic or the Southern Ocean. Diatom Ooze con- 

 tains about 50 per cent of diatom skeletons, mixed with 

 from 10 to 20 per cent of carbonate of lime. It is the 

 distinctive deposit of the Southern Ocean, where it occurs 



