266 



THREE CRUISES OF THE "BLAKE. 



In radiolarian (Fig. 183) and diatom (Fig. 184) oozes the 

 deposits consist of siliceous skeletons and frustules of surface 



Fig. 1 80. — Radiolarian ooze (Murray). "j5. Fig. 1S4. — Diatom ooze (Mirrray). ^^-. 



organisms, which have likewise fallen from the surface waters. 

 A radiolarian ooze has hitherto been met with only in the deep- 

 est waters of the Western and Central Pacific, and diatom ooze 

 appears to be confined to the Southern Ocean, a little north of 

 the Antarctic Circle. 



Thus it will be seen that the character of a marine deposit is 

 largely determined by its distance from land, and again by the 

 nature of the organisms living in the surface waters. The 

 dead shells of pteropods, of foraminifers, of radiolarians, and of 

 diatoms are heaped up on the bottom, some in one part of the 

 ocean, some in another ; and as no other materials reach these 

 distant regions to cover them, they form characteristic deposits. 

 Depth is, however, an important factor in reference to the com- 

 position of a deposit in any locality. There seems to be now 

 no doubt that the whole of the carbonate of lime shells, such as 

 those of mollusks and foraminifers, are entirely removed by 

 solution in very deep water during their fall from the surface to 

 the bottom, or immediately after reaching the bottom. It is 

 found that, with increasing depth, the pteropod and heteropod 

 shells are the first to disappear from deposits, then the more 

 delicate surface foraminifera, and finally the larger and heavier 

 ones. It is likewise observed, that the more numerous these 

 shells are in the surface waters, the greater is the depth at 

 which they will accumulate at the bottom. As a rule, a ptero- 



