186 DISTRIBUTION OF CORAL-REEFS. Ch. VI. 
Islands, namely, Aitutaki and Manouai, are formed of 
upraised coral rocks, and have probably been elevated 
within a recent period ; nevertheless they are encircled 
by reefs extending so far from the land, that I have 
coloured them blue, though with much hesitation, as 
the space within the reef is shallow, and the encircled 
land is not abrupt. If these reefs really belong to the 
barrier class, we have here another instance of sub- 
sidence having followed elevation, both movements 
having been effected apparently within the recent 
period. There are also many cases of coral-forma- 
tions, such as Elizabeth Island, Metia, Mangaia, 
several of the Friendly and one of the Loyalty Islands, 
which it can hardly be doubted once existed as atolls, 
and were originally formed during subsidence, but 
have since been elevated, and are now surrounded by 
fringing-reefs. We have, however, no reason to feel 
surprise at occasional or even frequent alternations of 
level of the above two kinds. 
On the absence of active Volcanos in the areas of 
subsidence, and on tlieir frequent presence in the areas 
of elevation . 1 — The absence of active volcanos through- 
out the great areas of subsidence on our map, as 
shown by the pale and dark blue tints, — namely, in 
the central parts of the Indian Ocean, in the China 
Mr. Murray (p. 781) regards this reef as favouring his theory. Nar- 
rative of Challenger Voyage, p. 778.] 
1 It may be well here to state that all the reefs on the map were 
coloured either red or blue before the vermilion spots and streaks, 
showing the position of the active volcanos and volcanic chains, 
were added ; and indeed before I knew of the existence of several of 
them. 
