MASAMARHU ISLAND. 
321 
level, and then descends at an angle of some thirty degrees. 
In the lower diagram we find no indication of this second 
ditch, but a long slope begins at the foot of the submarine 
cliff at a depth of about 850 feet, which is very nearly 
identical with that of the flat part of the glacis in the 
former section. 
It will be observed that the upper ditch (that common 
to both sections) has its bottom at a depth of full 500 feet, 
or about 85 fathoms — that is, at more than three times the 
average depth at which reef-building corals cease to live, 
while the least depth of the final submarine slope is 850 
feet, or more than 140 fathoms. These ditches seem irre- 
concilable with any idea of an outward-spreading growth 
of the reef, and must, I think, be indicative of a subsidence 
which isolated the outward and more flourishing edge of 
a shore reef, and progressed rather too rapidly to allow 
its corals to extend across the trench thus formed and effect 
a union with the main mass. Of course if a fissure-like 
hollow were once established between two masses of growing 
coral in a subsiding area, it would not be readily filled up, 
unless the edge of its outer wall were sufficiently near the 
surface to suffer much from the violence of the waves. 
The former section seems to me inexplicable under the 
conditions ordinarily admitted for coral growth, unless we 
suppose that the bottom of the lower ditch, now at a 
depth of over 1,200 feet (200 fathoms), was formerly situ- 
ated within about 25 fathoms of the surface ; so that a sub- 
sidence of more than 1,000 feet may fairly be claimed for 
the coral reef of Masamarhu. 
Professor Dana, 1 in an article which appeared while 
this sheet was in the press, adduces some new and very 
1 Points in the Geological History of the islands Maui and Oahu. 
By J. D. Dana, Amer. Jour. Sci. vol. xxxvii. p. 81 (February 1889). 
I am indebted to my friend Professor Judd for calling my attention 
to this article. 
