10 
GEOLOGY: W. M. DA VIS 
Proc. N. a. S . 
below normal sea level; that as the waters warmed and rose reefs grew 
up on the margins of the benches and platforms, whereupon the lagoons 
behind them were moderately aggraded; and that the thickness of the 
aggrading deposits is greater, and consequently the lagoon depth is less 
in small than in large lagoons, because the detritus supplied from a linear 
front-foot of a reef has a smaller interior sector to aggrade in a small lagoon 
than in a large one. In brief, the long continued stability of reef founda- 
tions and the abrasion of sub-lagoon platforms upon them are leading 
factors of the Glacial-control theory. 
It should be noted here that neither the stability of reef foundations- 
nor the abrasion of sub-lagoon platforms is proved by any direct evi- 
dence; both of these leading factors are, like the subsidence of atoll 
foundations in Darwin's theory, assumed because they are thought to be 
necessary for the explanation of observed facts ; and both assumptions are 
believed to be true because of the apparent success of the explanation that they 
provide. Hence if it be shown, even in a single instance, that a lagoon 
floor of typical form and depth has been produced around an island which 
provides independent evidence contradictory to stability and abrasion 
and which demands strong subsidence, the fundamental assumptions of 
the Glacial-control theory will be seriously invalidated. 
The bearing of Tagula reef and lagoon on the Glacial-control theory 
may now be apprehended. Tagula is, as has already been shown, not in 
a region of long continued and nearly perfect stability, but in one of marked 
instability ; and as will next be shown it has not suffered abrasion by the 
lowered ocean; yet its lagoon floor is smooth and of a depth accordant 
with that of other large lagoons in various parts of the Pacific. 
Hence long continued stability and extensive low-level abrasion are not es- 
sential factors in the production of this fine example of a barrier-reef la- 
goon floor. But if these factors are not essential in Tagula, they should 
not be regarded as essential anywhere else; and their adoption as the 
leading postulates of the Glacial-control theory is therefore unnecessary; 
flatness of lagoon floors and their accordant depths may be explaned 
elsewhere as well as in Tagula as the result of long continued aggradation 
on subsiding foundations of uneven surface. 
The evidence that Tagula has not suffered abrasion by the low Glacial 
ocean, and hence that the reef-building organisms around Tagula were 
not seriously weakened by the lowered temperatures of the lowered ocean 
in the Glacial epochs, is found in partly in the absence of charted cliffs on 
the shores of the main island where the barrier reef becomes a fringe, 
partly in the absence of similar cliffs on the exposed sides of the satellite 
islands at either end of the Calvados chain where it approaches the bar- 
rier reef, and partly in the presence of the outpost islands in the barrier-reef 
loops around the northern lagoon compartment. 
