GEOGRAPHICAL DISTRIBUTION. 343 
therefore as we go northward, and the extent of the coral 
reefs increase as we go southward; no reefs occur about the 
northernmost islands, while they are quite extensive on the 
shores of Guam. This group, consequently, like the Hawai- 
an and Navigator, illustrates the influence of volcanic action 
on the distribution of reefs. 
A short distance southwest of the Ladrones, and nearly 
in the same line, lie extensive reefs. Mackenzie’s is an atoll 
of large size. Yap (or Hap), Hunter’s, Los Matelotas and 
the Pelews (Palao), are high islands, with large reefs. In the 
last mentioned, the reef grounds cover at least six times the 
area occupied by the high land. Still farther south, toward 
New Zealand, lie the large atolls Aiou, Asie and Los Guedes. 
South of the equator again:—The New Hebrides consti- 
tute along group of high islands, remarkable for the absence 
of coral reefs of any extent, though situated between two of 
the most extensive coral regions in the world,—the Feejees 
and New Caledonia. But the volcanic nature of the group, 
and the still active fires of two vents in its opposite extremi- 
ties, are a sufficient reason for this peculiarity. ‘Tanna is one of 
the largest volcanoes of the Pacific; and nearly all the islands 
of the New Hebrides, as far as known, indicate comparatively 
recent igneous action, in which respect they differ decidedly 
from the Feejees. 
The Vanikoro Group, north of the New Hebrides, accord- 
ing to Quoy, has large barrier reefs about the southernmost 
island, Vanikoro; but at the northern extremity of the range 
there is an active volcano, Tinakoro, and no coral. Tikopia, 
to the southeast of Vanikoro, is high and volcanic, according 
to Quoy, though not now with active fires; and it appears from 
the descriptions given, to have no reefs. Mendana, northeast 
of Tinakoro, according to Kruesenstern, as stated by Darwin, 
