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. 183. — Radiolarian ooze (Murray). т. 
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- 
