ORGANIC AGENCIES. 



103 



forty fathoms deep. By sounding it is found that the 

 inner reef-slope is gentle, but the outer very steep, so 

 that at a distance of a mile more than a mile depth has 

 been found (Fig. 57). Thus far, the* action of corals 



FIG. 57. Section of an atoll. 



alone. Now add the action of waves, and the snowy ring 

 is gemmed on the inner edge with small green islets. All 

 these facts are shown in Figs. 56 and 57. 



4. Closed Lagoons and Lagoonless Islands. In 



the typical atoll the reef-circle is large, and only dotted 



FIG. 58. Map view of closed lagoons and lagoonless islands. 



with small islets, but in small atolls the land is more con- 

 tinuous (Fig. 58, n), or entirely continuous, but the 



