CORAL REEFS AND SUBMARINE BANKS “2077 
ground for drawing the volcanic cone with a broadly truncated 
surface ‘“‘cut away by the action of the sea” at the depth where 
igneous rocks were first encountered; almost any other form would 
accord as well with the recorded facts; indeed, in view of the strong 
variations of magnetic force on the island surface,’ it is eminently 
probable that the buried volcanic surface is uneven; if so, it is also 
probable that the depth of 245 feet is not the minimum depth of 
the volcanic foundation; for if the surface is uneven it is not likely 
that a single boring, located without any knowledge of the under- 
structure, would reach the culminating point. 
The-third example of a truncated volcanic island is Mangaia in 
the Cook group, which is briefly described by Marshall as having 
‘a, well-developed marine erosion surface forming the summit of 
the island 650 feet above sea level. An alluvial flat... . sepa- 
rates the high volcanic land from a ring of coral 125 feet above sea 
level’... . 200 of 300 yards from the volcanic land.”? ~ This 
example, being visible, is better attested than the other two. It 
would seem to represent a volcanic cone that was completely trun- 
cated by abrasion before any reef defended it, and then elevated; 
but the barrier-reef ring now surrounding it may well have been 
formed during a later depression before a later elevation. 
_It is not without careful consideration that I have been con- 
strained to reject the assumption that reef-building organisms were 
so completely killed during the glacial period as to leave the reefs 
an easy prey to the waves. The assumption is, as already noted, 
certainly a plausible one at first hearing, and it merits careful 
examination; but as the result of the best examination that I have 
been able to devise it proves to be erroneous. Thus a problem is 
laid before zoologists: If coral reefs are today limited by tempera- 
ture conditions and if the ocean were significantly cooled during the 
glacial period, why were not the reef-building organisms then 
killed? Perhaps the organisms were killed and the reefs were cut 
away on islands near the borders of the coral zone, as will be 
tJ. F. Cole, ““Magnetic Declination and Latitude in the Bermudas,” Terrestr. 
Magnetism, XIII (1908), 49-56. 
2P. Marshall, “Coral Reefs of the Cook and Society Islands,” Proc. Austral. 
Assoc. Adv. Sci., XIII, 1912, 140-45. 
