J. D. Dana — Origin of Coral Reefs and Islands. 185 



over the same wide region ; but, with a tide of but two or three 

 feet, there is but little fall before the reef — which lies at low 

 tide level and a little above it — retards it by friction ; and 

 thus escape by the open entrances is increased in amount and 

 in rate of flow. The facts are the same in atolls where the 

 lagoons have entrances.* 



c. Examples of massive corals having the top fiat, or de- 

 pressed and lifeless, while the sides are living, are common in 

 coral-reef regions, wherever such corals are exposed to the 

 deposition of sediment, and where they have grown up to the 

 surface so that the top is bare above low tide. A disk of 

 Porites, having the top flat and the sides raised (owing to 

 growth) so as to give it an elevated border, is figured on plate 

 LY. of my Eeport on Zoophytes. Many such were found in the 

 impure waters of a shore reef at the Feejees. At Tongatabu 

 one flat-topped mass of Porites was twenty-five feet in diam- 

 eter ; and both there and in the Feejees, others of Astrseids and 

 Meandrinas measured twelve to fifteen feet in diameter. 



Over the dead surfaces, as Mr. Semper observes, the coral 

 may be eroded by the solvent action of the waters, and espe- 

 cially where depressions occur to receive any deposits, and 

 boring animals may riddle the coral with holes or tubes. But 

 generally the erosion is superficial; the la^e masses referred 



* The currents of the tropical Pacific Ocean are of very unequal rate in its dif- 

 ferent parts, and very feeble in the Paumotu Archipelago and the Tahitian and Sa- 

 moan regions. Capt. Wilkes reports that in the cruise of the Expedition through 

 the Paumotu Archipelago to Tahiti, a distance of a thousand miles, during a month 

 from August 13 to September 13, 1839, the drift of the vessels was only 17 miles; 

 and that during fourteen days in the first half of October, between Tahiti and 

 Upolu of the Samoan group, nearly 1 800 miles, the drift was only 43 miles. 



The Challenger, on her route from the Hawaian Islands to Tahiti, found, 

 between the parallel of 10° S. and Tahiti, "the general tendency of the current 

 westerly, but its velocity variable;" between the parallel of 10' S. and 6° N., 

 the direction was westerly with " the average velocity 35 miles per day, the range 

 17 to 70 miles per day," the maximum occurring along the parallel of 2° 1ST. 

 Farther west, about the Phoenix group, the equatorial current, as described by Mr. 

 Hague (loc. cit., p. 237), has "a general direction of west-southwest and a velocity 

 sometimes exceeding two miles per hour." At times it changes suddenly and 

 flows as rapidly to the eastward. The drifting of the sands about Baker's Island 

 (in latitude 0° 13' N., longitude 176° 22' E.) has much interest in connection with 

 this subject of current action, and the facts are here cited from Mr. Hague's 

 paper. The west side of the little island (Ixfm. in area) trends northeast, and 

 the southern east-by-north, and at the junction a spit of sand extends out. During 

 the summer the ocean swell, like the wind, comes from the southeast, and strikes 

 the south side ; and consequently the beach sands of that side are drifted around 

 the point and heaped up on the western or leeward side, forming a plateau along 

 the beach two or three hundred feet wide, and eight or ten feet deep over the 

 shore platform. "With October and November comes the winter swell from the 

 northeast, which sweeps along the western shore ; and in two or three months 

 the sands of the plateau are all drifted back to the south side, which is then the 

 protected side, extending the beach of that side two or three hundred feet. This 

 lasts until February or March when the operation is repeated. 



Am. Jour. Sci. — Thibd Series, Vol. XXX, No. 177. — Sept., 1885. 

 12 



