128 ON CORAL AND CORAL REEFS 



north side of the equator, where the trade-wind is a 

 northeasterly wind, the opening of the reef is on the 

 southwest side: while in the southern hemisphere, 

 where the trade-winds blow from the southeast, the 

 opening lies to the northwest. The curious practical 

 result follows from this structure, that the lagoons to 

 these reefs really form admirable harbours, if a ship 

 can only get inside them. But the main difference 

 between the encircling reefs and the atolls, on the one 

 hand, and the fringing reefs on the other, lies in the 

 fact of the much greater depth of water on the seaward 

 faces of the former. As a consequence of this fact, the 

 whole of this face is not, as it is in the case of the 

 fringing reef, covered with living coral polypes. For, 

 as we have seen, these polypes cannot live at a greater 

 depth than about twenty-five fathoms; and actual 

 observation has shown that while, down to this depth, 

 the sounding-lead will bring up branches of live coral 

 from the outer wall of such a reef, at a greater depth it 

 fetches to the surface nothing but dead coral and coral 

 sand. We must, therefore, picture to ourselves an 

 atoll, or an encircling reef, as fringed for one hundred 

 feet, or more, from its summit, with coral polypes 

 busily engaged in fabricating coral; while, below this 

 comparatively narrow belt, its surface is a bare and 

 smooth expanse of coral sand, supported upon and 

 within a core of coral limestone. Thus, if the bed of the 

 Pacific were suddenly laid bare, as was just now sup- 

 posed, the appearance of the reef-mountains would be 

 exactly the reverse of that presented by many high 

 mountains on land. For these are white with snow at 

 the top, while their bases are clothed with an abundant 

 and gaudily-coloured vegetation. But the coral cones 

 would look grey and barren below, while their summits 



