156 GEOLOGY. 



depth which coral animals reach. What is the explana- 

 tion? It is supposed to be this. When these animals 

 just began to make the reefs, the island stood far up out 

 of the water; but as fast as the reefs were built up the 

 island subsided, keeping the surface of the reefs all the 

 time of the subsidence under the water. The rate of 

 subsidence must have been very slow to correspond with 

 the slow growth upward of the corals, which has been 

 calculated to be only the one eighth of an inch in a year. 

 In a completed atoll, the island having wholly disap- 

 peared, we have a state of things indicated in Fig. 82, 

 b _ b A _ a a a representing a 

 ~ = ^ m -. '' x section of the mar- 



fi s- 82. gin of land, and b b 



the lagoon. An atoll, then, may be looked upon as "the 

 tomb and monument of an island altogether buried be- 

 neath the waves." 



There are groups of these singular islands, in some 

 cases extending over large spaces. In the Pacific Ocean 

 there is a band of such groups 4500 miles long, and va- 

 rying from 200 to 600 miles in breadth. 



Many atolls rise up from a depth of 2000 feet, and 

 therefore are mountains of coral rock standing in the 

 water over a third of a mile in height. At the rate of 

 an eighth of an inch a year, it took the coral animals 

 192,000 years to build such mountains. 



239. Calcareous Shells. I have said that in the for- 

 mation of the coral rocks there were contributions of 

 shells. In fact, they every where contribute material 

 for the limestones ; but in this case, as well as in others, 

 the great work of furnishing material is done by very 

 small, even minute animals. The shells of such animals 

 have formed extensive strata in various parts of the 

 earth's crust. In Fig. 83 you see represented the shells 

 of some of the foraminifera, a class of these minute ani- 

 mals, so named because their chambers communicate by 

 numerous foramens or pores. The microscopic animals 



