ECOLOGICAL CHARACTERISTICS OF MARIANA ISLANDS. 9 
urged against its acceptance, but the biological facts are such as to demand just 
such a geological explanation. 
Clearly the character of the submarine territory in the neighborhood of the 
Mariana Islands is most important in the present connection. The available data, 
which are unfortunately scanty, are derived mainly from the survey of the U.S. S. 
Nero, conducted in 1899 with a view to the laying of a trans-Pacific cable to Guam.! 
In general, the submarine topography is such as to emphasize the present isolation 
of the Mariana Islands in all directions except the north. 
In the first place, the several islands of the group form the peaks of a con- 
tinuous range, for the depths between consecutive elements are far less than else- 
where in the neighborhood where soundings have been taken; between Guam and 
Rota, for example, one station gives 470 fathoms and another shows 690 fathoms, a 
little away from the direct line. In the second place, the geographical position and 
volcanic texture of the Bonin Islands would indicate their structural continuity 
with the Mariana Islands; by way of confirmation, the U. S. S. Nero found a sub- 
marine mountain range which actually connected the two groups in question. As 
the Bonin Islands themselves are structurally connected with Japan, evidently 
there is an extensive, continuous, and almost straight zone from Japan to Guam 
in which volcanic tectonics are most clearly marked. 
A third feature of great importance is the isolation of the Mariana Islands so 
far as the east, west, and south are concerned. Despite the seeming proximity of 
the Caroline Islands, the depth of 3,000 fathoms within 60 miles of southing indi- 
cates the present fundamental separation of the above-specified zone from the mass 
whose highest points constitute the extensive series of islands from the Pelews to 
the Marshalls. To the west of the Marianas the depth increases rapidly to about 
2,000 fathoms within 30 miles of dry land; although the bottom undulates somewhat 
for 600 miles, with the approach to the Philippine Islands it descends to 3,000 
fathoms and more, almost to the last-named group itself. Finally, to the east 
lies the profound abyss of the “‘Nero Deep,’—the ‘greatest known depth of the 
oceans—with soundings of more than 5,000 fathoms, or a little less than 6 miles. 
At a greater distance from Guam in this direction the ocean floor bears numerous 
reefs and submerged mountain ranges which diminish in frequency with the nearer 
approach to the Hawaiian Islands. 
In brief, then, the Mariana Islands of to-day prove to be well-separated from 
neighboring territories, saving only in the northward direction. The degree of 
isolation would seem to be such as to militate against the hypothesis of an origin of 
the present islands by the subsidence of a larger mass, while the theory of uplift 
seems to be directly supported by the geological nature of the islands, as well as 
by their location in an extensive zone of volcanic activity. But the vulcanism is 
that of to-day and of recent geologic time; what rock may constitute the core of the 
present islands and of the submarine cordillera beneath is of course unknown. 
However, if we may judge from the facts in the case of the Society Islands, the 
1 Report of the Secretary of the Navy, 1900, pp. 299-302, cited by W. E. Safford, The useful plants of Guam, Cont. U.S. Nat. Mus., vol. 
9, 1905. 
