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



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

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

 coral-reef regions, wherever snch 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 liaised (owing to 

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

 Iv. of my Report on Zoophytes. Many such were found in 

 the impure waters of a shore-reef at the Feejees. At Tonga- 

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

 diameter ; 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 large masses referred 

 to showed little of it. Such dead surfaces in corals are 

 generally protected by a covering of nullipores and other 

 incrusting forms of life,, and the crusts usually spread over 

 the surfaces pari passu with the dying of the polyps. 



d. Every stream, saj^s Mr. Semper (when explaining, as 

 cited on a preceding page, the origin of the deep channel of 

 the large Pelew island, whose depth is " 35 to 45 fathoms "), 

 " has a natural tendency to deepen its bed." But there is a 

 limit to this action. The eroding or deepening power of a stream 



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° N. Further west, about the Phoenix group, the 

 equatorial current as descrihed by Mr. Hague (loc. cit. p. 237), has u a 

 general direction of west- south-west 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 (1 X f m. in area) trends 

 north-east, 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 south-east, 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-plat- 

 form. With October and November comes the winter swell from the 

 north-east, 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. 





