Vol. 8, 1922 
GEOLOGY: W. M. DAVIS 
9 
have been swept into the lagoons, where, reenforced by local organic detri- 
tus and probably in smaller measure by detritus from the islands, 
it appears to have aggraded the "moats" between the reefs and the 
islands. 
It thus seems that the formation of the great undermass of the Louisiade 
reefs, and especially of the Tagula reef, may well have been consistent 
with the conditions and processes of Darwin's theory. It should be added 
that the evidence for the strong subsidence of the Louisiade islands is, 
in view of their constitution, much more direct than that furnished for 
the similar subsidence of most reef -encircled volcanic islands in the central 
Pacific ; and that this well certified subsidence of the foundations on which 
the Louisiade reef -masses have been built up gives immensely greater sup- 
port for Darwin's theory than is afforded by the atolls of the open Pacific, 
where the occurrence of subsidence is indicated only by indirect evidence. 
It remains to inquire whether the Louisiade sea-level reefs, which surmount 
the great undermass, accord with or contradict other coral-reef theories, es- 
pecially the newly framed Glacial-control theory of sea-level reefs. This 
theory was proposed more particularly to account for the atolls and bar- 
rier reefs of the supposedly quiescent central Pacific than for the barrier 
reefs of much disturbed regions like the Louisiade archipelago: never- 
theless the Tagula reef in particular affords critical evidence against that 
theory, as will be made clear by the following considerations. 
The Glacial-control theory appears to be based on the conviction that 
it is the smooth lagoon floors rather than their enclosing reefs which are 
most in need of explanation, and that the bathymetric relation of the la- 
goon floors to the level of the ocean reached by the enclosing reefs is nor- 
mally so nearly constant in all the coral seas that their explanation by 
Darwin's theory in terms of reef upgrowth and lagoon aggradation on 
subsiding foundations of irregular form is impossible. A long period of 
nearly perfect stability of the mid-ocean floor is therefore assumed, altho 
instability is admitted for islands in the southwestern Pacific ; and instead 
of postulating that lagoon floors represent "moats" that have been heavily 
aggraded behind the upgrowing reefs during the subsidence of their founda- 
tions, a series of ingenious suppositions is invented, of which the chief are : 
that during Preglacial time many still-standing islands, more or less reef- 
surrounded, were either worn down to low relief by subaerial erosion or cut 
down to shallow platforms by marine abrasion; that during the Glacial epochs 
of the Glacial period the ocean surface was lowered by about 35 fathoms 
by the withdrawal of water to form continental ice sheets; that the sur- 
face waters of the ocean were then so chilled as to kill or greatly weaken 
reef -building organisms; that islands were then attacked by the waves, 
which cut low-level benches around them if they were high, or if they were 
low completely truncated them in platforms at a depth of 35 to 40 fathoms 
