UNLIKE DEPTHS OF SUBMARINE BANKS 555 



in the southern members, the over-all distance being 400 iiautical miles. 

 Darwin was aware of these facts, yet said in the first edition of his booTi : 

 "I can assign no adequate cause for this difference of depth" (1842, 34) ; 

 but in the second edition he added, "excepting that the southern part of 

 the archipelago has subsided to a greater degree or at a quicker rate than 

 the northern part" (1874, 47). No one since Darwin has suggested" a 

 more adequate cause. It is interesting to note that the "drowned atolF' 

 known as the Great Cliagos bank lies farther south in the same line. 



VARIATIONS IN THE DEPTH OF SUBMARINE BANKS 



The numerous facts here pertinent must be briefly summarized. The 

 central depths of certain large banks ought, according to the Glacial- 

 control theory, to be of similar measure, and less by the thickness of their 

 aggrading sediments than the 30- or 40-fathom depth of their buried 

 platforms; but according to the subsidence theory, they need show no 

 close accordance. The facts are that while depths of less than 30 or 40 

 fathoms prevail, certain banks have greater depths : thus the Macclesfield 

 bank in the China Sea has central depths of 45, 55, and 60 fathoms; the 

 Tizard bank, farther south, in the same sea, 48 fathoms; the Vanguard 

 bank, farther southwest, 50 to 57 fathoms; the Say a de Malha bank, a 

 rimless bank in the southern Indian Ocean, has depths up to 64 fathoms ; 

 the Great Chagos bank, in the same ocean, is 48 fathoms in depth near 

 the center, though it has a rim of less than 20 or 10 fathoms. A great 

 bank in the Tonga group of the open Pacific slants to a depth of over 50 

 fathoms. 



None of these depths are fairly compatible with the requirements of the 

 Glacial-control theory; still less so, when it is recognized that several of 

 them, like Macclesfield and Great Chagos, have been changed by mar- 

 ginal reef growth and surface aggradation from their supposed initial 

 form of flat cones that abrasion would have given them to their existing 

 form of shallow saucers; and as this change demands a marginal reef- 

 thickness of some 50 fathoms and a decreasing aggradation from rim to 

 center, it must be supposed that a significant thickness of sediments has 

 been laid down over the central area also; hence the depths of 60 and 48 

 fathoms there measured must be significantly less than the depth of any 

 buried rock platform that may exist beneath. This aspect of tlie coral- 

 reef problem is more fully treated in another article (1918, a). 



BANKS AROUND REEF-FREE CLIFT ISLANDS 



It may be fairly urged that the amount of lowering of the Glacial ocean 

 ought, as long as it is in doubt, to be independently computed by each 



