CORALLINE ISLANDS. 619 



trade-winds namely, a circulation and distribution of the superfluous heat of 

 the equatorial regions, warming the northern countries ; and cooling, by the 

 return of under-currents, those in the tropics. The fogs of Newfoundland 

 are caused by the great current of warm water entering the cold region and 

 carrying with them surface-currents of moist air, which the cold condenses 

 into fog, just as the breath is visible in a cold atmosphere. England owes 

 its moist and mild climate to the same cause. The depth of the Gulf-Stream 

 itself is very little. It is a mere layer of warm water. (See Sir George 

 N ares' reports of the Challenger expedition.) 



In the foregoing pages you have now seen, and, we hope, gained, some 

 information concerning the sea sufficient, at any rate, to induce you to enter 

 more deeply into the subject than we can at present do. We have learnt 

 how the sea water is composed, and what goes on on the surface. We have 

 discussed waves, and referred to tides and currents, the wearing away and 



Fig. 705. Atoll, or Coral Island. 



the renewal of land by the sea; we have dived beneath the surface, and found 

 something to interest us at the bottom of the ocean. As we come up again 

 we are surprised to find islands or reefs where none existed when we went 

 down. What has caused this sudden appearance ? They may have been 

 slowly raised to the surface by coral insects, or suddenly by volcanic action. 

 Let us consider the coral, which plays a very important .part in our Physical 

 Geography, before we proceed to the volcanic island.* 



The low-lying islands are those formed by the skeletons of the coral 

 insects, and the Coralline Islands are some of the most wonderful productions 

 of nature. They are only found in warm climates, between the twenty-eighth 

 degrees of north and south latitude, and limestone pure and simple is the 

 chief component of the coral reef, as it is of the mountains erupted from the 

 depths of the sea. " The detritus of corals, echinodermata shells, reticularia, 

 and other living creatures," says a writer on this subject, " deposit not only 



* Those who wish to study the subject fully should read " Corals and Coral Reefs," by 

 the late Charles Darwin, and Dana's " Coral Islands." 



