84 2T Proceedings of Boyal Society of Edinburgh. [sess. 
14 
cases 
under 500 
fathoms, m. p.e. 86 '04. 
7 
>» 
from 
500 to 
1000 
„ „ 66-86. 
24 
a 
1000 to 
1500 
„ „ 70-87. 
42 
}> 
» 
1500 to 
2000 
„ „ 69-55. 
68 
)} 
)} 
2000 to 
2500 
„ „ 46-73 
65 
>} 
)} 
2500 to 
3000 
„ „ 17-36 
8 
» 
3000 to 
3500 
„ „ 0-88. 
2 
>5 
}i 
3500 to 
4000 
„ „ o-oo. 
1 
over 
4000 
„ „ trace. 
The fourteen samples under 500 fathoms are chiefly coral muds 
and sands, and the seven samples from 500 to 1000 fathoms contain 
a considerable quantity of mineral particles from continents or 
volcanic islands. In all the depths greater than 1000 fathoms the 
carbonate of lime is mostly derived from the shells of pelagic 
organisms that have fallen from the surface waters, and it will be 
noticed that these wholly disappear from the greater depths. 
These figures are derived from a study of the “ Challenger ” 
deposits alone, but they are confirmed, as to the general result, 
by an examination of the deposits collected by the U.S.SS. 
“ Tuscarora” and “Blake,” by H.M.SS. “ Egeria” and “Investigator,” 
the ships of the Telegraph Construction and Silvertown Companies, 
and other ships. One other peculiarity as to the distribution of 
carbonate of lime organisms on the ocean’s floor may be noted. 
"Where these calcareous shells are most abundant on the surface, as 
in the tropics, the remains of the dead shells are as a rule found at 
greater depths on the bottom than in temperate or polar regions, 
where they are relatively much less abundant in the surface waters. 
In his paper on the Origin of Coral Reefs, published many years 
ago, Mr Murray pointed out that sea water, rushing in and out of 
the lagoon twice in the twenty-four hours, would take up and carry 
away large quantities of the carbonate of lime which, in the form of 
coral sand and mud, covers the bottom of these shallow basins. 
Just as the surface shells are dissolved by falling through the layers 
of ocean water, so in this case the dead coral fragments are dissolved 
by the sea water that continually passes over them; in this way 
chiefly he accounted for the formation of lagoons in atolls and barrier 
reefs. 
