154 Dr. J. D. Dana on the Origin of 



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 subsi- 

 dence. Its erosion-made valleys, as already explained, die 

 out at the broad shore-plain, and the island is comparatively 

 even in outline. 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, supposing this slope to be continued under water, 

 I made the probable 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), Manua, 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. Pan- 

 gopango Bay, in which we anchored in 174 feet of water, has 

 a length of three miles. The coral-reefs are of the fringing 

 kind where any occur. Upolu, a few miles west, has bold 

 shores and small or no reefs for fifteen miles of its north 

 shore, east of its middle ; but elsewhere there are broad reefs 

 (mostly 5000 to 8000 feet in width) and a very gentle 

 slope (three to six degrees) in the land above, which is about 

 the slope of its underlying lava-streams. The great width of 

 the reef seemed to be evidence of subsidence. But the ab- 

 sence on the north side of the island of a channel in the reef 

 deep enough for any craft larger than canoes made it essenti- 

 ally a great fringing reef. A calculation from the width and 

 land-slope gave about 260 feet for subsidence; but I add (on 

 page 332 of my Report) my doubt as to any subsidence. The 

 facts known are against any elevation. 



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

 Expedition for two thirds of its cruise, in his paper on " Coral Forma- 

 tions " (p. 50 of Proc. Bost. Soc. Nat. Hist, Jan. 1842) 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. 



