148 
GEOLOGY: W. M. DAVIS 
later pointed out for subsiding islands, afford critical evidence regard- 
ing the changes which the island suffered contemporaneously with the 
formation of the encircKng reef. These considerations may make it 
clear why my work has been almost wholly confined to the central 
islands of barrier reefs, though several examples of uplifted reefs were 
not neglected. 
Coral Reefs around Still-standing Islands. The various theories of 
coral reefs may be divided into two groups. Those of the first group, 
some six or seven in number, postulate a fixed relation between land 
and sea level during the development of the reefs; these will be called 
the still-stand theories. The two theories of the second group postu- 
late a change in the relative level of land and sea during the development 
of the reefs. As far as the barrier reefs that I have visited are concerned 
all the theories of the first group must be rejected, because within everyone 
of these barrier reefs the embayments or drowned valleys, C, C, C, figure 
1, by which the shoreline of the central volcanic island is indented, give 
through Dana's principle of shoreline development indisputable evidence 
of recent submergence. I beHeve that the same conclusion appKes to all 
barrier reefs, first for the reason that those which I visited were not 
selected because they were thought to be in any way unlike other mem- 
bers of their kind, but because they were easily accessible; and second 
because all the charts of other barrier reefs that I have examined show 
that their central islands also have embayed shorelines. For none of 
these islands can any one of the still-stand theories hold good. 
Coral Reefs and the Glacial Period. It remains to inquire which 
one of the two theories that postulate a change in the relative level of 
land and sea best accounts for the facts of barrier reefs and their asso- 
ciated central islands. One of these is Darwin's simple theory of a 
slowly subsiding ocean bottom, as a result of which the islands gradu- 
ally sink, diminish in size, and eventually disappear, while their fring- 
ing reefs grow upwards and are converted into barrier reefs and atolls. 
The other is the more complicated "glacial control theory" lately 
elaborated by Daly with especial reference to atolls. It begins by as- 
suming still-standing foundations, above and around which out-grow- 
ing reefs of less or greater size, with shallow lagoons or none, were de- 
veloped in preglacial times; a lowering of sea level is then inferred 
during the glacial period, when a significant amount of sea-water was 
withdrawn to form the continental ice sheets, and when in consequence 
of lowered ocean temperature the corals of most reefs were killed; 
next follows an abrasion of the unprotected preglacial reefs so as to 
