EFFECT OF CONSTANT CURRENTS. 227 



Commission of Hydrography at that time. The effect of the 

 ebb and flow of the tide is only visible in the increased or 

 diminished force of the current flowing through the channel. 

 Xow I willingly admit that possibly a return flow of the 

 current may sometimes take place ; but this certainly is not of 

 frequent occurrence, and there can be no doubt that a strong 

 stream passes through the channel for long periods in the same 

 direction. And it is this fact which, above all others, enables us 

 to understand the peculiar structure of the reefs which fringe the 

 islands, and consequently the channel, on both sides. Reefs 

 proper, with a raised margin, do not exist here ; the water in 

 the channel is always still, and, as even the waves raised by 

 storms on the main do not affect it, it offers to such small craft 

 as can navigate it a secure shelter in spite of its small extent. 

 But the current is often very strong, running sometimes at from 

 4 to 5 or even 6 knots an hour. 



Both banks of the channel are formed of coral, those of the 

 shore of Malaunavi on the north side being the most developed ; 

 they consist of the usual reef-forming species. Astrcea, Poritfs, 

 Ma'l reports, lire. Xow these, like all the species that form large 

 blocks, have a tendency to extend in all directions ; but here 

 they are interfered with by the strong current impinging on 

 them at an angle, and flowing, as has been said, during the 

 greater part of the year in the same direction. If it were weaker 

 than the growing power of the coral, its resistance would be 

 easily overcome ; but it is, on the contrary, strong enough to 

 force it to grow perfectly perpendicularly. Thus the reef on 

 the Malaunavi side is not more than a few paces wide, with an 

 abrupt perpendicular fall, though the depth is certainly incon- 

 siderable. 



The reef which fringes the little island lying in the western 

 outlet of the channel is quite different in its structure. The 

 same species are present as in the channel, but they here grow 

 quite differently in different spots. Where the current is met 

 by the island, of course it parts : thus, at the eastern end of the 

 island, Avhich is that turned inwards towards the channel, a 

 triangular space is found, where the water is full of feeble 

 currents flowing in various directions. Both branches cf the 



