522 CCELENTERATES. 



the steep slope, and surrounds the land which has risen above the sea, this terrace 

 being exposed at low tide. 



We have still to mention some of the causes of modification in the form and 

 growth of the coral-reefs. The presence of harbours in reefs and atolls can, as a 

 rule, be traced to the tides and to local ocean currents. There is generally an 

 outflow through the canals and openings in the reefs. This is apparently due to 

 the fact that water is constantly being thrown by the larger waves over the lower 

 portions of the reef into the canal or lagoon, and seeks either to escape as an under- 

 current in opposition to the flood-tide, or else strengthens the ebb-tide. These and 

 other similar disturbances of the water in the canals bring* with them much coral 



O 



detritus, and render the bottom altogether umsuitable for the growth of corals. 

 Where such currents are strong, they keep the canals clean and open. The action 

 of the oceanic currents is often increased by the fresh waters coming from the 

 central islands, and harbours are therefore very often found at the mouths of 

 valleys and of their small streams. The influence of the fresh water itself on the 



SECTION THROUGH A CORAL-BEEF. 



corals is not so great as is usually assumed, chiefly because it, being lighter than 

 salt water, flows away on the surface of the latter and hardly touches the animals 

 which grow below the surface. 



The form of the reef is again largely influenced by the form and constitution 

 of the sea-bottom. Where deep submarine fissures occur, dipping clown below the 

 level at which the corals flourish, no reef can be formed, as also in places where 

 firm ground alternates with sand and mud. All irregularities in the outline of a 

 reef or an atoll, and the formation of harbours in coral islands, can thus be simply 

 explained. 



The most important point which needs elucidation is why some reefs encircle 

 islands as a fringe extending from the shore, while others run parallel with the 

 land, no longer touching it ; others, again, forming circular lagoons with no island at 

 all in the middle. This was the question which puzzled the first discoverers of 

 reefs, and at one time it was supposed that instinct guided the animals in giving 

 their structures the form best suited to withstand the force of the waves. Accord- 

 ing to another hypothesis, put forth by Steffen in 1822, the reefs represent the 

 summits of volcanic mountains, the crater being filled by the lagoon, while the 

 channels through the reef indicate the points at which the edge of the crater was 

 destroyed by outbursts of lava. This superficially plausible view was disposed of 

 by Darwin thirty years ago. He argued that the volcanic cone thus assumed must 

 either once have stood upon dry land and then have been submerged, or else must 

 have been formed beneath the sea. In the former case, the crater would in almost 

 all cases have been destroyed during the gradual sinking ; while the formation of 

 craters by submarine eruptions and their subsequent elevation is hardly conceivable. 



