CGELENTERATES. 



?>1 



tralia, a thousand miles in length. It varies from ten to a hundred miles 

 in distance from the shore, and while between it and the land the sea is 

 rarely more than thirty fathoms deep, " at a few hundred yards outside 

 this "barrier reef no bottom can be obtained with a sounding-line of a 

 thousand fathoms." 



This leads to the mention of the fact that reef-building corals do not 

 exist alive at a greater depth than fifteen or twenty fathoms, and thai 

 the different species exhibit even within these limits considerable differ- 

 ences. How, then, are we to explain the coral islands so abundant in the 

 Pacific, which arise abruptly from an almost bottomless sea ? The most 

 satisfactory explanation of this and other phenomena is that given by the 

 late Mr. Darwin. It is that these islands and reefs occur onlv upon some 

 volcanic island, or in the place where one has existed within comparatively 

 recent times. As is well known, the Pacific Ocean is the seat of a marked 

 volcanic disturbance, and at times islands are suddenly thrown up from 



Fig. 39. — An island with fringing and barrier reefs. 



the bottom of the sea. Let us follow the history of one of these islands, 

 as it will explain many facts. 



As soon as the rock thus elevated from the bottom cools down to a 

 temperature where coral life is possible, coral embryos, drifted by the cur- 

 rents from other reefs, would obtain a foothold and begin their growth. 

 Little by little they would rise towards the surface, until at length they 

 form a fringe around the volcanic island, extending out from the shore 

 until the twenty-fathom line, the limit of coral life, was reached. Here 

 the reef would stop abruptly, and its outer edge would form a sheer preci- 

 pice a hundred and twenty feet in height. 



Coral needs for its nutrition an abundance of fresh water, bringing to 

 it food and oxygen ; hence that portion of the reef exposed to the ocean 

 waves would be best nourished, and consequently would grow most rapidly. 

 Suppose, now (a supposition warranted by many known examples), that 

 the island should slowlv sink at a rate not exceeding a few feet a century. 



