200 
GEOLOGY: W. M. DAVIS 
of the different islands are therefore best explained by local changes of land 
level, unlike in date, in direction, in amount and in rate. 
It is particularly the rapid rate, the recent date, and the considerable amount 
of subsidence often indicated that are of significance in the coral-reef problem; 
and this is true not only for the Philippines but also for the other archipelagoes 
between Asia and Australia. Embayed shore lines indicative of submer- 
gence are common though by no means universal in all this region, but well 
developed barrier reefs are rare. Before the reefs of Cebu and Negros were 
elevated, and before the platform of Palawan was depressed, barrier reefs must 
have been more extensive than now in the Philippines, and possibly in the 
other island groups also; but today no examples of these forms are to be found 
in the archipelagoes that can compare with the great barrier reefs of north- 
eastern Australia and of New Caledonia; few of the many small islands in the 
archipelagoes are enclosed by well developed encircling reefs, like those of 
the Fiji and Society groups; and atolls, which are so striking a feature of the 
open Pacific, are relatively uncommon in the archipelagoes. 
The best explanation of the small development of barrier reefs and atolls 
in the archipelagoes is to be found, not in the lack of subsidence, which is 
elsewhere so intimately associated with reef development, for geological and 
physiographic evidences of subsidence abound on many islands ; and surely not 
in the prevalence of unfavorable conditions as to the temperature and purity 
of sea water, for fringing reefs flourish; but largely in the occurrence of sub- 
sidence of so rapid a rate and in some cases of so great an amount, as to have 
submerged pre-existing reefs. Moreover the subsidences appear to be in 
many cases of so recent a date that the new fringing reefs are still narrow; it 
is presumably for this reason that the drowned barrier reefs and atolls have 
not yet had time to grow up again to sea level. Added to this is the frequent 
occurrence of recent uplifts in the archipelagoes, whereby weak marine sedi- 
ments, overwashed by an abundance of alluvium from rejuvenated rivers, have 
come to occupy the shore line of certain islands, thus discouraging even the 
growth of fringing reefs, as around much of the coast of Borneo and along 
the southern coast of Java; but this aspect of the problem cannot be discussed 
here. 
In view of these facts and inferences the Australasian archipelagoes must be 
considered much more unstable than the floor of the central Pacific. In that 
vast region, where reef upgrowth has generally kept pace with changes 
of level, and where atoll and barrier-reef lagoons have been filled with sedi- 
ments to a moderate depth, Darwin appears to have been right in conclud- 
ing that "the subsidence thus counterbalanced must have been slow in an ex- 
traordinary degree" (115). Not only movements of depression but modern 
movements of elevation also have been of moderate measure in the central 
Pacific, for none of the occasional elevated atolls found there have an altitude 
of more than a few hundred feet: it is only in Tonga and Fiji that greater 
measures of modern uplift are recorded, the greatest altitude of a reef in Fiji 
