THICKNESS OF THE EEEF. 271 



whicli it was forming were more and more worn down by the 

 combined action of boring animals and plants, and of the currents 

 produced by the tides and by rain. Every stream has, as is well 

 known, a natural tendency to deepen its bed ; thus, while the 

 surface currents setting directly against the reef on the eastern 

 side prevented a perpendicular growth of the reef, and the hard 

 rock at the base of the coral rendered the formation of a channel 

 between the reef and the land by denudation impossible, such 

 a process could easily be effected on the western coast by the 

 destruction of the enclosed island, and thus the reef, forced into 

 perpendicular growth by the tangential current, was soon at 

 some distance from the coast. 



In a similar way all the other structural features of the 

 Pelew reefs can easily be explained by the hypothesis of a slow 

 upheaval during their formation. It appears superfluous to 

 discuss them here in detail, since it would involve the repetition 

 of the same arguments as have been applied to explain the two 

 princi[>al points. The presence of deep channels in spots where 

 an easily disintegrated tufa has served as the foundation for 

 the coral built up on it, the broad and almost horizontal sub- 

 marine plateau to the south of Ooroere, with the narrow channels 

 that intersect it, the large blocks of dead coral everywhere lying 

 on the raised outer ridge of the western reef and their total 

 absence on the eastern reefs, the gradual transition of the raised 

 cliffs of Polelew into the still living portion of the reef, the 

 drying up of the channel cut by man in the eastern reef of 

 Kriangle, and the absence of a channel on the eastern side of 

 Babelthuap — these and many other facts are easily comprehen- 

 sible if we suppose that the most important causes which deter- 

 mine the form of the reefs and the growth of the corals are 

 those that I have brought into prominence, and that they are 

 precisely such as would most easily prove effective during a 

 period of slow upheaval. 



In conclusion there are still one or two objections that must 

 be discussed, as they are certain to be brought forward against 

 my view. First, there is the enoi'mous thickness attributed, 

 according to Darwin's theory, to the reef rising abruptly from 

 the sea-bottom ; and secondly, the small depth in which only, as- 



