VITAL CAIJSKS OF CHANGE, 287 





CHAPTER XI. 



Coral Islands. 



'*! saw the living pile ascend, 

 The mausoleum of its architects, 

 Still dying upwards as their labors closed; 

 Slime the materials, but the slime was turned 

 To adamant by their petrific touch." 



J. Montgomery. 



HAVING in the preceding chapter given a brief description of 

 some of the more common varieties of corals, we shall now con- 

 sider more fully the agency of these wonderful animals in the 

 formation of rocks. It has been generally considered that the 

 zoophytes cannot live in water of very great depths, and that 

 therefore their structures are based upon submarine mountains. 

 This view has been confirmed by the observations of Ehrenberg, 

 and more recently by the careful soundings of Captain Fitz Roy, 

 of the Royal English Navy. At a depth of ten fathoms the pre- 

 pared tallow invariably came up marked with the impressions of 

 the living corals, and as clean as if it had been dropped on a car- 

 pet of turf; at a greater depth the impressions were less numer- 

 ous, and the adhering particles of sand much more frequent, 

 until at a mean depth of twenty-five fathoms it was evident that 

 the bottom consisted of a smooth sandy layer. We may conclude 

 therefore that the reef building corals, do not usually flourish 

 much below this depth. The formation of the coral islets is 

 somewhat analagous to the growth of a tree which has been 

 headed. The zoophyte cannot endure even a short exposure to 

 the sun's rays in the air, their growth npw ards is therefore checked 

 as soon as the surface of the water is reached. They spread out 

 laterally however, not unlike the top of a tree. 



The appearance and formation of coral islands has been de- 

 scribed vnry minutely by a great mimb<>r of distinguished natural- 



