8 
GEOLOGY: W. M. DAVIS 
Proc. N. a. S. 
commonly of atoll-like loops and rings, 36 in number, from one to 5 miles 
in diameter, enclosing little lagoons from 10 to 17 fathoms in depth. The 
loops and rings of this half of the circuit are separated by as many passages, 
from V4 to 3 miles wide and from 15 to 35 fathoms deep. But the most 
remarkable features of this part of the barrier are the small or minute but 
high islands, here to be referred to as outposts, which rise in 22 of the reef 
loops. The largest of them is only 4 miles in diameter; their heights 
vary from 40 to 530 feet. Some of them appear to consist of schist, judg- 
ing by their trends ; but according to Maitland some of the others are vol- 
canic and a few are made of limestone. As elements of a barrier reef, 
these small but high outpost islands are so exceptional as to be almost 
unique. 
The great Tagula lagoon is divided by the Calvados chain of satellite 
islands into a smaller northern and a larger southern compartment; the 
northern compartment is of triangular outline, with its base along the di- 
viding chain and its vertex about 10 miles away at the most northern point 
of the reef; it occupies about of the entire reef-enclosed space, which 
must approach 2000 square miles in total area. The southern compart- 
ment measures 20 miles across, and extends east-west along the whole 
112 miles of the lagoon length; it occupies about ^/e of the enclosed area; 
the remaining Ve is taken by Tagula and the satellite islands. The 
greater part of the lagoon floor in both compartments is a gently undulating 
plain usually from 25 to 35 fathoms in depth. The depth of the south- 
ern compartment increases gradually for a moderate distance from the broad 
enclosing reef, and more rapidly from the islands of the Calvados chain. 
The greatest depths, 46 fathoms in the southern or windward compartment 
and 49 fathoms in the northern or leeward compartment, are in both cases 
found much nearer the dividing island chain than the outer barrier reef. 
The exterior slopes of the reef fall off rapidly into deep water ; a few sound- 
ings show depths of over 600 fathoms two miles from the reef on the west 
and northwest. 
A correct theory of the Louisiade reefs must take account of the great 
subsidence that the islands have suffered. It would therefore appear 
that the present sea-level reefs should be regarded as the successors of a 
long-lived series of upgrowing reefs which have been formed, essentially 
according to Darwin's theory, by more or less intermittent upgrowth from 
earlier shore lines of the subsiding mountainous islands. It is probable 
that where the island slopes were very steep, the reefs, presumably inclin- 
ing inwards as they grew up, remained attached to the shore as fringes; con- 
versely, where the island slopes were gentler or where low slopes have been 
broadly submerged, the reefs now form offshore barriers. During the up- 
growth of the reefs, some of their detritus must have been swept seaward, 
to form the submarine talus that descends into deep water ; the rest must 
