118 CORALS AND CORAL ISLANDS. 
harbor, on the same island, there was no coral, with a depth 
of fifteen fathoms. 
Similar results were obtained about all the islands surveyed, 
as the charts satisfactorily show. ‘There is hence little room to 
doubt that twenty-five fathoms, or 150 feet, may be received 
as the limit in depth of flourishing banks of reef corals. 
It may however be much less, possibly not over half this, on 
the colder border of the coral-reef seas, as, for example, at the 
Hawaian Islands and the atolls northwest of that group. It 
is natural that regions so little favorable for corals on account 
of the temperature should differ in this respect from those in 
the warmer tropics. 
It may be here remarked, that soundings with reference to 
this subject are liable to be incorrectly reported, by persons 
who have not particularly studied living zodphytes. It is of 
the utmost importance, in order that an observation supposed 
to prove the occurrence of living coral should be of any value, 
that fragments should be brought up for examination, in order 
that it may be unequivocally determined whether the corals 
are living or not. Dead corals may make impressions on a 
lead as perfectly as living ones. 
As to the origin of this narrow limit in depth, tempera- 
ture may be one cause through the colder parts of the coral 
seas, it having been proved to be predominant with regard 
to distribution of lfe throughout the extent of the ocean. 
Yet it is not the only cause. The range of temperature 85° 
to 74° gives sufficient heat for the development of the greater 
part of coral-reef species; and yet the temperature at the 
100 foot plane in the middle Pacific is mostly above 74°. 
The chief cause of limitation in depth is the diminished light, 
as pointed out by Prof. T. Fuchs.* 
1 Verh. k. k. geologischen Reichsanstalt, 1882, and Ann. Mag. N. H. Jan. 1883. 
