192 DALY. 



Rimless Banks All widths 22 82 62 



Fiji atolls only 1-5 km. inclusive 5 36 25 



6-20 " " 6 59 44 



Maldive atolls only 1-10 " " 9 37 27 



11-20 " " 5 64 48 



21-60 " " 11 73 56 



Tables I and II illustrate the following rules. 



(a) Both maximum and general lagoon depths increase with the 

 width of the platform, until that w^idth reaches a value of about 20 km. 

 These rules apply not only for world averages but also for averages 

 calculated respectively for the Fiji and Maldive groups. It appears, 

 therefore, that, where the conditions are not far from uniform, the 

 filHng of the lagoon is in direct proportion to the width of the platform; 

 yet more clearly than when many archipelagoes are considered. 

 Where the platform is extremely narrow, the lagoon is almost, or quite, 

 filled to sea-level. (Compare P'igs. 5-20, 38-43.) 



(b) Atoll and barrier lagoons of similar widths usually have 

 maximum and general depths of the same order of magnitude. 



(c) The lagoons of "drowned atolls" are deeper than those of 

 normal atolls having respectively the same widths of platform. This 

 fact is expected, since the periodic destruction of reef corals must 

 mean slower filling of a lagoon with coral detritus. (See page 214.) 



(d) The new theory demands that lagoon depths shall be generally 

 less than those on reefless banks; Table II shows the corresponding 

 fact. 



(e) The maximum depth of ordinary atoll lagoons is almost never 

 more than 91 m. (50 fathoms). In only one case is the lagoon depth 

 of a "drowned atoll" more than 100 m.; the Macclesfield bank, which 

 is not completely rimmed with a reef, bears water 110 m. deep. 



Since probably not more than 5 m. to 25 m. can be allowed for the 

 thickness of the Post-Glacial calcareous veneer in the wider lagoons, 

 the accordance of platform depths for the wider lagoons and reefless 

 banks seems clear. Their range of 60-90 m. represents magnitudes 



