128 LOWER INVERTEBRATES. 



mentions a beach of coral sand on Socorro, one of the Rivillagigedo group. The 

 Sandwich islands are in part coralline. The Feejee, Paumotu, Society, and Friendly 

 islands are composed wholly or in part of extensive coral banks. The coral sea along 

 the northeastern coast of Australia is the most extensive coral reef in the world. In 

 the Indian Ocean, again, we find a large number of coral islands and reefs, among which 

 may be mentioned the Laccadives, Maldives, and i-eefs of the island of Madagascar. 

 Near the entrance into the Red Sea there occur coral banks of considerable size. 



The part which the coral plays in the formation of the coral island has been vari- 

 ously estimated, and many oj)inions have been advanced in regard to this point. The 

 difficulty comes oftentimes from a lack of an accurate knowledge of the relationship of 

 the solid matter of the coral to the animal which secretes it. The carbonate of lime 

 which makes up the great mass of the growing coral bank is the skeleton of the 

 animal, and is simply a secretion of the membranes of the body. The skeleton cannot 

 be said to be the work of the coral, except so far as it is an animal secretion. 



A growing coral plantation, with its multitudinous life, oftentimes arises from 

 great depths of the ocean, and the sea-bed upon which it rests is probably a submarine 

 bank or mountain, upon which have lodged and slowly aggregated the hard skeletons 

 of pelagic forms of life. When, through various sources of increase, this submarine 

 bank approaches to the depth of from one hundred to one hundred and fifty feet fi'om 

 the surface of the water, there begins on its top a most wonderful vital activity. It 

 is then within the bathymetric zone of the reef-building corals. Of the many groups 

 of marine life which then take possession of the bank, corals are not the only animals, 

 but they are the most important, as far as its subsequent history goes. As the bank 

 slowly rises by their growth, it at last approaches the surface of the water, and at low 

 tide the tips of the growing branches of coral are exposed to the air. This, however, 

 only takes place in sheltered localities, for long before it has reached this elevation it 

 has begun to be more or less changed and broken by the force of the waves. As the 

 submarine bank approaches the tide-level, the delicate branching forms have to meet 

 a terrific wave action. Fragments of the branching corals are broken off from the 

 bank by the force of the waves, and falling down into the midst of the growing coral 

 below fill up the interstices, and thus render the whole mass more compact. At the 

 same time larger fragments are broken and rolled about by the waves, and are eventu- 

 ally washed up into banks upon the coral plantation, so that the island now appears 

 slightly elevated above the tides. This may be called a first stage in the development 

 of a coral island. It is, however, little more than a low ridge of worn fragments of 

 coral washed by the high tides and swept by the larger waves — a low, narrow island 

 resting on a large submarine bank. 



The second stage in the growth of a coral island results from the formation on such 

 a ridge as we have just described of a quantity of fine coral sand. In the grinding 

 of the coral fragments which lie upon the fixed portion of the reef, a large quantity 

 of the finest sand is formed. This sand is sometimes held in a mechanical suspension in 

 the water, and in that way is transported from place to place. It is generally swept 

 along from the locality where it originates, and is ultimately, if not lost in ocean depths, 

 thrown up on the ridge of coral fragments which has been already mentioned. The 

 wind assists the waves, and, taking the sand which they cast from the waters, blows it 

 as it becomes dry higher and higher on the ridge. Thus we have formed the second 

 stage in the development of a coral island, which is simply derived from the former by 

 capping the coral fragments which form the foundation with a layer of sand. 



