Abstract
Borabora, one of the Society Islands, represents three eroded remnants of a once more extensive volcano. Quaquaversal dips of the lava‐flows and hundreds of radially striking, basaltic dikes locate the crater in the submerged lagoon between the large eastern island and the down‐faulted remnant of the western crater‐rim. Ellipsoidal structures of subaerial origin in the lavas extend from sea‐level to nearly 2500 feet and are believed due to rapid vesiculation and consequent billowing of the lavas. The lavas and dikes show a limited range of composition ranging from oceanites to olivine‐poor basalts. The only significant differentiation, other than concentration of olivine, is in a large dike in which the center shows a concentration of alkalies. The limited variation in composition is in contrast to the occurrence of trachytic and syenitic types on Tahiti and Moorea.
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