228 THE INFLUENCE OF INANIMATE SUItltOrNDINGS. 



divided current first strike the island further in, and still at an 

 angle. In this comparatively calm triangle of water, the 

 Madrepores, Astraa, Porites, and other corals do not grow 

 vertically upwards, but on the contrary in all directions, and 

 usually in isolated blocks; and though the branched sijecies 

 generally grow more in height than the more massive forms, 

 even in them it is impossible to overlook a tendency to increase 

 as much in breadth as in height. Hence, on the eastern point 

 of the island there is no perpendicular termination to the reef; 

 it sinks quite gradually to the bottom of the channel. But where 

 the two currents produced by the division of the main stream 

 impinge upon the island; we at once find the perpendicular 

 clifis again, and the upper surface of the reef is as narrow as in 

 the channel on the shore of Malaunavi. At the opposite end 

 of the island — the western end, that is to say — there is a second 

 calm triangle formed by the meeting of the two streams, and 

 there the reef again assumes the structure which has been 

 minutely described in speaking of the eastern end. Every- 

 where in the Philippines, and likewise in the Pelew Islands 

 in the Pacific, I have observed the same phenomena ; wher- 

 ever there was an eddy or a calm spot formed between the 

 current and an island, the corals grew in irregular masses, and, 

 more or less, in all directions ; and wherever the stream ran 

 parallel with the shore there was an abrupt and perpendicular 

 fall of the reef. 



It is not difficult to find an explanation of these phenomena. 

 It is the same as was given above in the case of individual 

 coral knolls ; where the sea impinges on coral reefs, with feeble, 

 irregular, and variable currents, flowing in various directions, 

 the reefs, like separate blocks, will be enabled to extend in 

 every direction, since they will not in the course of their growth 

 have to overcome a constant resistance in one direction. But 

 wherever strong and unopposed currents flow constantly in the 

 same direction parallel to the coast, impinging on the reef, the 

 corals forming it must be forced, individually and collectively, 

 to grow perpendicularly — assuming, of course, that their power 

 of growth is insufficient to overcome the resistance of the stream. 

 Between the perpendicular gTOwth thus induced and the perfectly 



