Geology of the Nicobars. 211 



whatever their geological structure may be, are more or less sur- 

 rounded by a very vigorous growth of corals. These animals, being 

 attached with their fabrics to a certain place, and not able to move 

 or to go in search of their food, are particularly fond of those places, 

 where this want is redressed by the motion of the water near 

 them ; and we accordingly find the growth of coral most vigorous 

 in those places where the water of the sea is most exposed either 

 to surf or to periodical currents. The following sketch (Fig. 2, Plate 

 IV,) is meant to show a very common section of these reefs, belonging 

 to what are termed by Darwin, fringing reefs, with the annexed 

 coast land : 



m. The older formation of the Island. 

 n. Limestone, 

 o. Coral. 



D C is high water line, and E B the low water line. From A to B 

 are the living corals, and here the surf is heaviest. The plain from 

 B to C, nearly dry at low water, is formed by a conglomerate of 

 fragments of corals, shells, and pieces of the surrounding rocks, 

 C J represents a small bank formed by the surf at high tides, (aided 

 by the action of the wind upon the dry coral sand) heaping up frag- 

 ments of corals, and from F to G the soil is found to be composed 

 of fragments of corals and gravel intermixed with vegetable matter ; 

 this part is no longer overflowed by the waves, but entirely incorpo- 

 rated with the Island; it is covered with a rich vegetation, and 

 affords the natives nearly all their supplies for shelter and food, 

 supporting their huts and nourishing their cocoa-nut trees. Suppose 

 the living corals at A B should take to building farther out, the 

 bank at C F and the jungle are likely to follow in the same direction, 

 and the land will increase according to the general geological laws. 

 But if this action is further conjoined with a general rising of the 

 land by subterranean forces, the augmentation of the land will go On 

 in a greater degree. It is very curious to see from the distribution 

 of tins coral-land how different the outlines of some islands have 

 been, even at a comparatively recent period, from their present form ; 

 to see how different parts of larger islands have by themselves before 

 formed smaller islands, and how smaller islands are in rapid progress 

 to be incorporated in larger ones. I often penetrated from the shore 



