188 DISTRIBUTION OF CORAL-REEFS. Ch. VI. 
Island probably contains a volcano, and it is only 
twenty miles distant from tbe barrier-reef of Mohilla. 
Ambil volcano, in the Philippine Archipelago, is dis- 
tant only a little more than sixty miles from the atoll- 
formed Appoo reef : and there are two other volcanos 
on the map within ninety miles of circles coloured 
blue. But there is not a single active volcano within 
several hundred miles of a group, even a small group, of 
atolls ; and it is clear that a group of atolls, surmount- 
ing a number of islands now all sunk beneath the level 
of the sea, implies a much greater amount of subsi- 
dence, than does a single atoll or a single encircling 
barrier-reef. It is a striking fact that two volcanos are 
known to have been in recent action in the Friendly 
Archipelago ; and the islands have here been formed 
by the recent elevation of a group of atolls. Again, 
extinct craters and well-preserved streams of lava occur 
on many of the encircled islands in the Pacific, and 
these by our theory have subsided at no very remote 
period ; but although thus plainly formed of volcanic 
matter, they do not offer a single active volcano. In 
these cases the volcanos seem to have come into action 
or to have been extinguished, in accordance with the 
latest movements of elevation or subsidence. 
Within the limits of our map, active volcanos occur 
on or near other coasts besides those which are fringed 
with reefs and coloured red ; and some of these coasts 
are known to have been upraised within the recent 
period. Thus I have shown in my Geological Observa- 
tions on S. America (1846) that the whole western shore 
