98 J. D. Dana — Origin of Coral Reefs and Islands. 



of no subsidence, I was led to speak of it as one either of no 

 subsidence or of less subsidence than over the central area re- 

 ferred 'to. The difference between us is small. 



12. The true value of fringing reefs as evidence in the question 

 of change of level should be appreciated in this discussion, as is 

 apparent from the objections to Darwin's theory which have 

 been urged ; and I mention a few facts from the Pacific islands 

 in its elucidation. 



On Darwin's map, the Marquesas group is left uncolored, 

 which means doubtful as to subsidence or not. The Tahitian 

 group (Society Islands) is colored blue, that is, it is included 

 within the area of coral-reef subsidence. The Navigator or 

 Samoan Islands are colored red, or placed in the area of eleva- 

 tion ; the Feejees, blue ; the Sandwich Islands, red. The 

 facts are these. 



The Marquesas Islands are an example of absence of reefs to 

 a large extent with only small reefs where any. But the 

 shores are mostly too bold for reefs; and hence their smallness 

 bears no testimony as to elevation. Along the bold shores, 

 there are deep indentations and fiord-like bays separated in 

 some cases by narrow ridges sometimes in spider-leg style ; so 

 that the proof of subsidence is positive, as explained in §4. 



Tahiti presents none of the Marquesan evidence of subsidence. 

 Its erosion-made valleys, as already explained, die out at the 

 broad shore plain, and the island is comparatively even in out- 

 line. I found over it, like Darwin before me, no evidence of 

 elevation beyond one or two feet at the most. It has broad 

 reefs ; and the channel inside the barriers between Papieti and 

 Toanoa (2 miles) has a depth of 3 to 25 fathoms. From the 

 width of the reef, and the slope (6 to 8 degrees) of the land and of 

 the lava streams outcropping in the sides of the valleys, suppos- 

 ing this slope to be continued under water, I made the proba- 

 ble subsidence 250 or 300 feet. A slope of 6 degrees, and a 

 width of reef of half a mile, gives 240 feet for the depth of the 

 reef at the outer edge. 



The Samoan (or Navigator group) includes (beginning at 

 the east) Rose Island (an atoll), Maim a, Tutuila, Upolu and 

 Savaii. 



Manua has bold shores, a height of 2500 feet, and a narrow 

 reef where any.* Tutuila is of the Marquesan type in its bold 

 indented sides and this suggests a probable subsidence. Pango- 



* With regard to Manua, Mr. J. P. Couthouy, of the Wilkes Exploring Expedition 

 for two-thirds of its cruise, in his paper on Coral Formations (p. 50 of Proc. Bost. 

 Soc. Nat. Hist., Jan. 1 842) reported the occurrence of fragments of corals at a 

 height of 80 feet " on a steep hill-side rising half a mile inland from a low sandy- 

 plain." I was not on Manua. I found on Upolu fragments of coral limestone 

 and shells in the tufa of a tufa cone at a height of 200 feet, which had evidently- 

 been carried up by the erupting action of a slightly submerged vent. (Report 

 p. 328.) The facts on Manua need further study. 



