Barrier Reefs of Tahiti and Moorea

Abstract

THE surprising fact that fragments of volcanic rock occur in the barrier reef of Tahiti, as reported by Dr. Crossland in NATURE for April 23, must be welcomed by all students of that remarkable structure; but that fact does not, it seems to me, prove the “original continuity of the present barrier reef from the [island] shore to the ocean slope” outside the reef, as is stated in his second letter in the issue of July 2. The fact only permits the inference that the lagoon was formerly filled in some manner, perhaps by stream deltas instead of by a broadened fringing reef, so that island detritus could be transported to where the barrier now stands. This inference involves so extraordinary a series of changes from former lagoon filling to later lagoon excavation—for which there is no other satisfying evidence provided—that even the inference should not be accepted as valid until all other possible means of explaining the occurrence of the volcanic fragments have been excluded. Further details as to tho nature of the fragments and the manner of their occurrence are desirable.