Gravity Relations in Samoa and Society Is. — Machesky 
371 
tainous and precipitous interior of Tahiti and 
the complete lack of roads precluded any over- 
land traverses, hence the present survey was 
confined to the coastal regions accessible by 
automobile. Viewed from the northwest, Tahiti 
presents a perfect shield-shaped structure, simi- 
lar to the island of Tau, American Samoa. 
The island of Tahiti is composed chiefly of 
alkaline basalts (Williams, 1933). The exposed 
caldera plug is composed of nepheline mon- 
zonites, theralites, and of subordinate nepheline 
syenites. The geological evidence indicates that 
a volcanic pipe is located near the geometrical 
center of Tahiti-Nui. The Taiarapu Peninsula 
volcanic pipe is geologically defined as being 
near the center of the peninsula just north of 
the highest topographic peak. The rock types 
that form the plug are generally more basic 
than those observed on Tahiti-Nui, consisting 
primarily of essexites and theralites. The orig- 
inal volcanic activity on the Society Islands 
chain is believed to have originated in the west 
and migrated eastward. Tahiti is extensively 
ringed by barrier and fringing reefs. 
Although the gravity picture on Tahiti-Nui 
and the Taiarapu Peninsula is incomplete, due 
to the absence of data from the interior, the 
magnitude of the anomalies and gradients ob- 
served are diagnostic. The Bouguer gravity 
anomaly contours on Tahiti-Nui (Fig. 4) out- 
line a circular form with the steepest coastal 
gravity gradients of 10 mgal per mile occurring 
in the north half of the island. The size and 
position of the gravity gradients suggest that 
the gravimetric center of the caldera will be 
found slightly north of the geologic position. 
The observed Bouguer anomaly values range 
from + 180 on the northeast coast to +230 
Fig. 4. Bouguer anomaly map of Tahiti I., French Polynesia. 
