CONSTRUCTION OF THE EARTH. 



155 



of the islands of the Polynesian Archipelago, as well as 

 many of those of the Indian Ocean, came from this source. 

 238. Atolls. This name is given to certain coral isl- 

 ands having a peculiar arrangement, one of which, Whit- 

 sunday Isle, is represented in Fig. 80. There is, as you 





fig. 80. 



see, a strip of land inclosing a lake called a lagoon. In 

 some atolls this strip of land is unbroken, while others 

 have one or more openings, so that the lagoon forms a 

 harbor. These atolls are of various shape, sometimes 

 almost circular, sometimes long and narrow, having va- 

 rious bends and indentations in the margin of land. 

 Their size varies from half a mile up to sixty miles 

 across. The manner in which they are constructed can 

 be explained by Figs. 81 and 82. We will suppose an 



Fig. SI. 



island around which the coral animals build reefs j a a, 

 Fig. 81, being the reefs, and b b the water between then 

 and the island. But in the figure a mere point in the 

 middle of the island is seen standing out of the water, 

 although the bottom of the reefs is far below the lowest 



