PACIFIC EXPLORATION: /. P. IDDINGS 
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island toward the ocean, and show that the volcano originally was a 
flat-topped dome, the upper portion of which was mostly solid flows of 
lava, while the lower middle part of the volcano was largely tuffs and 
breccia. 
No core, or conduit, of coarsely crystallized rocks has been exposed 
by erosion, which is an indication that the volcano of Moorea was not 
as high above the sea as those of Tahiti and Taiarapu. The deep bays 
in Moorea show that this volcano has been flooded by the sea to a 
considerable depth, while the absence of bays on Tahiti indicates that 
this island and Taiarapu have not sunken sufficiently to submerge the 
deeply eroded valleys, although the lower portions of the stream channels 
are only a few feet above the surface of the sea. The principal lavas 
of Moorea are basalts like those of Tahiti, but there are subordinate 
though large bodies of trachytic and phonolitic lavas, and small bodies 
of peridotitic rocks. 
The Leeward Islands, or the Society Group, 100 miles northwest 
of Tahiti, are similar in structure and in the character of their lavas. 
Huahine, the most easterly, is a volcano so greatly eroded and flooded 
that a narrow, shallow, strait separates the northern from the southern 
portion, and connects the heads of east and west bays. The eroded 
center of the volcano is submerged in the head of the east bay. The 
mountains and ridges, with spurs sloping seaward, consist of lava flows 
nearly horizontal in the central peaks, but dipping down the spurs and 
ridges in all directions toward the coast. The basaltic lavas have the 
same general composition as those of Tahiti, but there are mountain 
masses of phonolite and trachyte in several parts of the island. 
Raiatea, the largest island of this group, consists of basaltic breccias 
at its center, with basaltic lavas in sheets which are horizontal in the 
upper parts of the central mountains, but slope toward the coast in 
the spurs. A great sheet of trachytic lava caps the long ridge forming 
the middle of the northern portion of the island, and reaches the shore 
at the ends of the northwest spurs. There are also several mountain 
masses of phonolite. The volcanic center of the island has been eroded 
to a deep valley draining east and flooded by the sea. There are several 
other drowned valleys on Raiatea. 
Tahaa, which is within the same lagoon and barrier reef as those 
surrounding Raiatea, is another dissected volcano having a deeply 
indented coast with bays extending to some distance inland, and having 
lava flows dipping down the spurs to the sea, and more or less horizontal 
in the central portion. 
Farther west, the smaller island. Bora Bora, is much more reduced 
