208 The Realm of Nature CHAP. 



found to be a piece of pumice, or the hard teeth or bones 

 of the larger creatures inhabiting the sea. Sharks' teeth 

 are very numerous, and also bits of the hardest bones of 

 whales. Red clay also contains in certain localities small 

 but perfectly formed crystals of the class of minerals known 

 as zeolites ( 286), which have evidently resulted from 

 chemical changes in the material of the clay. 



278. Permanence of Elevated and Depressed Regions. 

 From the scanty supply of materials out of which Red clay 

 is elaborated, it is evident that if the deposit has attained 

 any great thickness it must have been a very long time in 

 course of formation. There is no evidence as to the thick- 

 ness of the Red clay, but the teeth and bones found embedded 

 in its nodules are known in many cases to belong to species 

 of sharks which no longer live in the ocean, and must have 

 been extinct for an immense period of time. Moreover, if 

 the Abysmal and Continental Areas had ever changed places, 

 some rocks would almost certainly be found on the land 

 resembling a consolidated Red clay. None such have ever 

 been discovered unless in volcanic oceanic islands that have 

 been recently upheaved. Accordingly the existence of the 

 Red clay is a strong argument that the elevated and 

 depressed halves of the lithosphere have occupied their 

 present positions during past geological ages. 



279. Corals. Many oceanic islands and reefs are com- 

 posed of the stony framework of carbonate of lime which is 

 secreted by animals known generally as coral polyps. These 

 polyps belong to the same class as the sea-anemone, and 

 are of many different species, each characterised by some 

 peculiarity in the form of its calcareous support. Some 

 secrete a wide disc, the surface of which is starred with 

 their groups of waving tentacles ; others form little cups on 

 which they grow, these cups being either separate, as in the 

 deep-sea corals, or united by a solid stony stem forming 

 many branches. The branching corals of various species 

 are of most importance in reef-building. The distribution 

 of coral islands over the oceans depends on the suitability 

 of the water for the life of the polyps and the existence of 

 good foundations. The polyps flourish best in very salt, 



