June 24, 1921] 



SCIENCE 



563 



in the Glacial-control theory of coral reefs; 

 for if the Tutuila platform had been cut to a 

 width of a mile or two in volcanic rock under 

 such conditions, similar platforms should have 

 been cut around other volcanic islands, and 

 the tops of the platform-back clifFs should be 

 visible to-day above normal sea level; but as 

 a matter of fact such partly submerged cliffs, 

 or plunging cliffs as they may be called, have 

 not been often detected; besides Tutuila, the 

 other best known examples are Tahiti and the 

 Marquesas islands, as will be further told 

 below. 



To return to Darwin's text: a further ex- 

 amination of it discovers a remarkably close 

 parallel to the actual condition of Tutuila, as 

 the following statement will show. The 

 Tutuila barrier reef is now drowned ; its suc- 

 cessor is a fringing reef on the marginal slopes 

 of the abraded island; and these slopes are, 

 according to Mayor, steeper than the sides 

 of the valleys by which the island is dissected. 

 Kow in view of the association of fringing 

 reefs with rising or stationary coasts in Dar- 

 win's theory — as it is ordinarily quoted — it 

 might be thought that the occurrence of the 

 Tutuila fringing reef around a subsided island 

 contradicted his views. But that such is not 

 the case is made clear by this prophetic sen- 

 tence : 



If during the prolonged subsidence of a shore 

 ... an old barrier reef were destroyed and sub- 

 merged, and new reefs fcecame attached to the 

 land, these would necessarily at first belong to 

 the fringing class (124). 



That is precisely the case at Tutuila. Evi- 

 dently, it is immaterial whether the " old bar- 

 rier reef " here mentioned had been formed 

 by upgrowth froia the slopes of a non-abraded, 

 subsiding island, or by upgrowth from the 

 margin of a platform on an island that sub- 

 sided after the plaform had been abraded. 

 Darwin's suggested explanation is excellent; 

 it was only because he found no examples of 

 fringing reefs thus produced that he did not 

 pursue the suggestion further; but fringing 

 reefs of this kind abound in the Philippine 

 Islands.^ 



6 ' ' The fringing reefs of the Philippine Is- 



If it be true that the submerged barrier 

 reef of Tutuila was formed on a subsiding 

 platform of marine abrasion, one or two miles 

 in width, the cliffs at the back of the platform 

 should have been 1,000 feet or more in height. 

 Hence the upper part of their faces ought 

 still to be visible after a subsidence of some 

 400 feet; and it should therefore be on the 

 now submerged part of the cliff faces that the 

 present fringing reefs of Tutuila have been 

 formed. Mayor's accounts of Tutuila tell, 

 however, of narrow platforms backed by steep 

 cliffs a few hundred feet in height that have 

 been cut close to present sea level since the 

 submergence of the barrier-reef platform. It 

 would therefore seem that these new cliffs 

 must have been cut in the slanting faces of 

 the earlier and greater cliffs after their partial 

 submergence. This relation of the two sets 

 of cliffs has not been mentioned, as far as I 

 have learned, by any observer on Tutuila; 

 it is a " flier " of my own,'' based on the di- 

 mensions of the new cliffs and platforms as 

 reported by Mayor. The relation of the height 

 of these cliffs to the breadth of the platforms 

 at their base suggests that the inclination of 

 the preexisting spur-end surfaces in which 

 the new cliffs have been cut was much steeper 

 than the ordinary radial slope of the spurs on 

 a dissected volcanic island, but not steeper 

 than the precipitous descent which the earlier- 

 cut, spur-end sea cliffs might have had at the 

 back of their two- or three-mile platform ; and 

 as the cliffs at the back of so wide a platform 

 must have had some such height as 1,000 feet, 

 the upper part of their slanting faces should 

 be still visible as plunging cliffs after a 400- 

 foot subsidence. Furthermore, the idea that 

 the new cliffs of Tutuila are cut in the earlier 

 ones gains some support from photographs of 

 Tutuila by Mayor, and from photographs of 

 the Marquesas islands by Iddings; for these 

 islands appear to resemble Tutuila in many 

 respects, although their submerged platforms, 

 the presence of which is indicated by a few 

 soundings in front of their plunging cliffs, are 

 lands," Proc. Nat. Acad. Sci., IV., 1918, 197-204. 



7 ' ' The islands and coral reefs of Fiji, ' ' Geogr. 

 Journal, IV., 1920 ; see p. 218. 



