26 Transactions. 



Atolls. 



Although sea-level atolls are, by themselves, inscrutable structures, it 

 sometimes happens that they occur at moderate distances from barrier- 

 reef islands : then the changes of level demonstrated for the barrier reef 

 may be plausibly extended to the atoll also. Thus it has been possible 

 to show good reason for ascribing certain small atolls in Fiji to upgrowth 

 during submergence, and to show also that the submergence was probably 

 due to relatively local subsidence (Davis, 1916d). The large atoll of 

 Ongtong Java, north of the Solomon Islands, can hardly have been formed 

 according to any of the still-stand theories, because the Solomon Islands 

 show many signs of diverse vertical movements. Similarh^, the uplifted 

 Loyalty atolls have probably suffered other movements than that of their 

 last uplift, for they are not far distant from New Caledonia, which has 

 had many disturbances. 



It may, of course, be urged that the atolls here mentioned, standing 

 near disturbed island groups, should not be taken to indicate the origin 

 of the more numerous atolls in the mid-Pacific ; but it may be answered 

 that, while the mid-Pacific region has very probably been less disturbed 

 by subsidences and upheavals than its western archipelagoes, nevertheless 

 the atolls which are associated with barrier reefs resemble mid-Pacific 

 atolls so closely in all essential particulars that the chief differences between 

 them are probably to be found less in the diverse conditions of their origin 

 than in the absence of neighbouring information-giving barrier-reef islands 

 in the one case and their presence in the other. 



It has been argued by some students of the coral-reef problem that 

 the uniformity of the depth of atoll lagoons is better explained in connec- 

 tion with a rise of ocean-level everywhere of the same amount than by 

 the subsidence of the atolls, which must vary somewhat from place to 

 place. In so far as the post-Glacial rise of ocean-level can satisfy the 

 demands of the problem this argument may be accepted ; but inasmuch 

 as the depths of atoll lagoons, as far as they are known, vary in a manner 

 more suggestive of varying than of uniform measures of submergence, 

 perfect stability of the atolls is improbable. Moreover, the reef -encircled 

 volcanic islands that occur in close association with certain atoll groups 

 demand a greater measure of submergence to account for their drowTied 

 Talleys thaii can be provided by Glacial changes of ocean-level. Finally, 

 the evidence of the Funafuti boring is, as noted above, strongly in favour 

 of subsidence during the formation of its reef rock. 



Elevated Keefs. 



Recently-elevated atolls not dissected sufficiently to disclose their 

 structure give little more testimony regarding their origin than can be 

 obtained from sea-level reefs. But if a recently-elevated fringing or 

 barrier reef lie unconformably upon its foundation, and if its limestones 

 enter into valleys between the ridges of its central island, as is mani- 

 festly the case with the elevated reefs of Oahu, Hawaii, submergence of an 

 eroded land-surface must have taken place before the reef was formed. 

 The measure of submergence can be inferred if the down-slope exten- 

 sion of the eroded land-surface beneath the reef can be determined. 



If elevated reefs have been out of water long enough to suffer dissection, 

 the details of their structure may be disclosed ; but so abundant is the 

 vegetation of tropical islands that observation of reef-structure is very 



