FACTS AND INFERENCES FROM CHARTS 503 



are only a conspicuous incident": and it may be added that the shore 

 outlines and offshore soundings given on hydrographic charts do not 

 suffice for the satisfactory study of such a complex. The flat floor of a 

 barrier-reef lagoon is well shown on charts, but it should not be thereupon 

 assumed to represent a platform produced by other than reef-making 

 agencies, for the floor is aggraded by an unknown thickness of detritus 

 on a foundation of unobservable form. 



Again, the change from a gentle slope to a steep pitch at a depth of 

 about 40 fathoms is a persistent feature in the exterior profile of many 

 reefs, but it is less satisfactorily explained as the edge of an antecedent 

 platform made independently of reef formation than as a miniature 

 continental shelf made by the action of marine agencies on reef detritus 

 with respect to present sealevel ; for, as Daly has pointed out, "the break 

 of slope'' on continental shelves formed by wave and current action is 

 "near the 40-fathom line'' (1915, 199). Likewise, the change at about 

 40 fathoms depth from a nearly flat floor to a steep pitch where a lagoon 

 is not completely inclosed by a reef, or where a reef stands back from an 

 exterior bench margin, finds competent explanation by marine aggrada- 

 tion of an earlier reef or reef -plain with respect to present sealevel, and 

 should not be taken to indicate the existence of a platform at that depth, 

 formed independently of reef -making agencies. I have treated this aspect 

 of the coral-reef problem more fully in an article on "Coral reefs and 

 submarine banks'' (1918, a). 



THE EVIDENCE OF VITI LEW, TAHITI, AND QUEENSLAND 



Viti Levu (Fiji), Tahiti, and Queensland (northeastern Australia), 

 associated in the above quotations from Vaughan's essays as if their reefs 

 had all been formed under similar conditions, are found to have really 

 had very different histories, when the coasts back of the reefs as well as 

 the charts of the reefs are studied. The present barrier reef of Viti Levu 

 has, I believe, been formed by upgrowth on the more or less aggraded 

 borders of a greatly eroded volcanic mass of complicated history, during 

 a slightly unequal subsidence, of greater measure on the northwest than 

 on the south; there is no sufficient reason for thinking that a flat plat- 

 form, formed independently of coral-reef agencies, serves as its founda- 

 tion, as will be further shown below. The French island of Tahiti is a 

 submaturely dissected volcanic island that has truly had a platform a 

 mile or two wide, backed by cliffs 1,000 or 2,000 feet in height, abraded 

 around its shores; the proof of this statement is, however, not found in 

 the soundings recorded on charts, but in the cliffs observed around the 

 island border, of which the charts give no proper indication and of which 



XXXVIII— Bull. Geol. Soc. Am., Vol. 29, 1917 



