222 W. M. DAVIS 
This aspect of the problem deserves special consideration in 
connection with ‘‘almost-atolls,” or large reef-inclosed lagoons 
having one or more small and steep-sided, yet not clift, volcanic 
islands near their center, X, Fig. 4, as in the great reefs of 
Truk (Hogoleu), in the Caroline Islands, and of the Gambier 
Islands, southeast of the Paumotus, previously mentioned. A 
careful examination of the problem shows that it is difficult, if not 
impossible, to develop steep slopes on a small, non-clift central 
island without the aid of subsidence. The residual knob, X’, of a 
large, still-standing volcano would be surrounded in preglacial time 
by a broad lowland, U’Y’, or by a shallow wave-cut platform, 
L'’U"’, worn on volcanic rocks, outside of which would lie a broad 
exterior bank. If surrounded by a lowland, the lower slope, Y’, 
of the residual knob, X’, would today be of gentle declivity, unless 
its borders were clift by abrasion during the glacial period; but 
as small central islands do not possess spur-end cliffs, L’, with 
submerged bases, this possibility is excluded. If such islands were 
formerly surrounded by a preglacial wave-cut platform, L’’U”, 
some part of the platform should today remain as a shoal in the inner 
part of the lagoon inside of the cliff, K’’, cut by the lowered ocean— 
particularly within the polygon marked by the several residual 
islands of Truk—for it is not to be supposed that abrasion by the 
lowered glacial ocean should just suffice to cut away all the pre- 
glacial platform; but residual platforms of this kind are unknown. 
None of these difficulties arises under the theory of intermittent 
subsidence. ; 
Hence the actual features associated with barrier reefs cannot be 
matched by the features deduced from the theory of glacial control 
unless neither preglacial nor glacial cliffing of the spur ends on non- 
subsiding islands takes place to any great extent. This seems to me 
highly improbable; hence I am led to conclude that as a rule high 
volcanic islands, even if clift in their earlier history, had already 
subsided sufficiently in preglacial time to drown their cliffs and to 
allow the formation of barrier reefs before the ocean was chilled 
and lowered; and that, except along the margin of the coral zone, 
the flanks of the reefs, which emerged while the ocean was lowered 
and chilled, were occupied by living organisms of some kind so that 
the islands were continuously protected from wave attack. This 
