102 DYNAMICAL GEOLOGY. 



The position of the reef is shown by a snowy girdle of 

 breakers, within which, like a charmed circle, there is 

 calm sea in the wildest storm. Between the reef and the 

 island there is a ship-channel, often twenty or thirty 

 fathoms deep. Through breaks or tidal ways in the reef, 

 ships enter and find good harbor in the channel. If it 

 were not for the action of the waves, this would be all, 

 but the beating waves form little coral islands on the 

 reef, so that, instead of a continuous snowy girdle, it is 

 such a girdle gemmed on the inner edge with a string of 

 green islets. By sounding it is found that the inner slope 

 of the reef is gentle, but the outer slope is very steep, and 

 rapidly passes into abyssal depth. All these facts are shown 

 in the perspective view, Fig. 54, and the section, Fig. 55. 

 3. Circular Reefs, or Atolls. These are the most 

 remarkable of all. In this case there is no volcanic island 

 or preexisting land of any kind apparent, as a nucleus 



,:lhl. . 



FIG. 56. Perspective view of an atoll. 



for the growth of corals. The reef seems to have been 

 built up from abyssal depth, in an irregular circular 

 form, inclosing a lagoon of still water in the midst (Fig. 

 56). The position of the reef, r, r, is shown by a circle 

 of snowy foam inclosing and protecting a harbor of still 

 water. Through breaks in the reef-circle ships may 

 enter and find safe anchorage. The lagoon is ten, twenty, 

 thirty, or even fifty miles in diameter, and thirty or 



