226 THE INFLUENCE OF INANIMATE SUEBOUNDINGS. 



Any other explanation appears to me impossible. We 

 might be tempted to account for the elevation of the marginal 

 ring above the central plateau by assuming that it had grown 

 more rapidly than the centre ; but if this were the true explana- 

 tion the wall would be visible even in knolls of moderate size, 

 and this is never the case. Or it might perhaps be said that it is 

 well known that all corals grow most vigorously at the margin 

 of the block, and that the concavity of the upper surface may 

 be easily explained by assuming a subsidence; but the fii-st 

 statement would be simply untrue, and the second perfectly 

 absurd, for at any rate it is impossible to see why such a 

 subsidence should have taken place in the case of the lai'gest 

 masses and not in the medium-sized or .small ones. In point of 

 fact, I see no other explanation which agrees so perfectly with 

 the observed facts as that which I have given. 



As the last point under this head, we must now consider the 

 way in which the whole reef is affected where it is acted on by 

 currents of different force and flowing in varying directions. 

 I will illustrate this by a highly significant instance which I 

 myself carefully observed. 



To the south-west of Mindanao, and exactly opposite the 

 famous old Spanish colony of Zamboanga, is the little island of 

 Basilan. I visited it in 1859, when by far the larger part of it 

 was still occupied by hostile Mohammedans ; the Spaniards were 

 restricted to a village at the northern end of the island, which 

 lay opposite to the still smaller island called Malaunavi, 

 separated from Basilan by a narrow channel. This little strait, 

 which, though very narrow, is of some length, runs from north- 

 east to south-west ; to the east it opens with a wide mouth 

 into the straits of Zamboanga, while to the west it is barred by 

 a very small island lying between the two others. To this form 

 of the channel and to the particular divergence of the main 

 current in the straits between Basilan and Zamboanga we 

 must ascribe the fact that the current between these islands 

 flows always in one direction, and never changes with the turn 

 of the tides ; at least this was the case during the two months 

 I spent there, and it is so throughout the year a-ccording to the 

 information given me by Don Claudio Montero, the chief of the 



