238 W. M. Davis — Shaler Memorial Study of Coral Reefs. 



tinental rocks, the extent of which has unquestionably been 

 decreased by subsidence in the later geological ages. 



Furthermore, the " remarkable flatness " of the platforms on 

 which it is supposed that the existing reefs have been built, and 

 " their nearly uniform depth of 45 to 50 fathoms," are conclu- 

 sions which, I believe, are not supported by Daly's table of 

 lagoon depths. Measures for the 60 lagoons there listed do 

 not show that their floors have remarkable flatness, for, in the 

 first place, the " maximum depth '' of a lagoon is often 20 or 30 

 per cent, and sometimes 60, 80, or 100 per cent greater than 

 the " mean depth in deeper part," hence the depth is by no 

 means uniform in single lagoons ; in the second place, the 

 " mean depth in deeper part " varies from 20 to 40 fathoms, 

 that is, by double its smaller value, hence the mean depth is 

 not at all uniform from lagoon to lagoon ; and in the third 

 place, the " maximum depth " of different lagoons ranges rather 

 equably from 26 to 37 fathoms, and in six cases it is less than 

 25 fathoms ; in only about one-fifth of the 60 lagoons is the 

 maximum depth 45 fathoms or more ; hence a depth of " 45 to 

 50 fathoms " is distinctly exceptional. These figures do not 

 give, to my reading, sufficient ground for citing " the smooth- 

 ness of the plateaus " and " their steady adherence to the aver- 

 age depth of about 45 fathoms " (302) as evidence against either 

 the Darwin-Dana hypothesis of prolonged subsidence, or the 

 Murray hypothesis of solution. To be sure, if it be assumed 

 that smooth platforms of whatever origin exist at depths of 

 45 or 50 fathoms beneath the actual lagoon floors, it is con- 

 ceivable that the upgrowth of reefs near their margin and the 

 accumulation of lagoon deposits over their surface might give 

 the actual lagoons the large inequalities of depth (large in pro- 

 portion to their mean depth) discovered by soundings ; but the 

 soundings of Daly's table give no sufficient warrant for such an 

 assumption. As to the relation of the depth of the embay- 

 ments in barrier-reef islands to the glacial lowering of sea level, 

 I cannot accept the statement that " the Pleistocene deepen- 

 ing of the inter-tropical seas is precisely of the amount required 

 to explain the drowned valleys of the volcanic islands which 

 are now surrounded by barrier reefs" ; for the depth of 

 the embayments has been diminished by sedimentation : in 

 Tahiti and in the largest islands of the Fiji group, Yiti Levu, 

 and Vanua Levu, the embayments are in large measure con- 

 verted into delta plains ; the depth of their rock bottom is 

 unknown ; even in smaller islands some filling must have taken 

 place, because deltas are advancing at the bay heads and fring- 

 ing reefs frequently contribute much detritus near bay mouths. 

 It will be shown later that some of the embayments are prob- 

 ably filled with sediment and sea water to a depth of 600, 800 



