Oct. 1, 1865.] 



SCIENCE-GOSSIP. 



221 



CORAL REEFS. 



Dejp in the wave is a coral grove, 

 Wliere the purple mullet and gold-fish rove ; 

 Where the sea-flower spreads its leaves of blue. 

 That never are wet with the falling dew. 

 But in bright and changeful beauty shme. 

 Far down in the green and glassy biine. 



rpHE formation of Coral Reefs is partly a vital 

 -*- and partly a chemical process. No more 

 wonderful instance of animal agency is to be found 

 than that which is exhibited in the work of the coral 

 zoophyte. In the warmer latitudes of every sea, 

 where the water is sufRciently clear and tranquil, 

 we find the homes of the polype, which, endowed 

 with the power of secreting lime from the waters 

 of the sea, rears with it its polypidom, and forms 

 those mountains commonly known as coral reefs and 

 islands. According to Cuvier's arrangement of 

 animals, the builders of these reefs belong to tlie 

 fourth, or lowest division of animal life, which 

 consists of zoophytes, or radiated animals. These 

 zoopliytes are subdivided into five classes, the fourth 

 of which contains the polypes, of which there are 

 three orders ; viz., fleshy polypes, gelatinous polypes, 

 and polypifcra polypes, the last of which are the 

 coral-builders. These tiny architects of mighty 

 works begin life in the shape of a gemmule, which, 

 dropping from a pile of coral into the sea, swims 

 about for some hours or sometimes days, until it 

 finally settles on another portion of old coral, or 

 other liard substance which is to be found at the 

 bottom of the sea. It then gradually lifts itself 

 into the shape of a tube, round the edge of which 

 there soon appears a delicate, thick-looking rim, the 

 beginning of the house in which it is to live. This 

 chalk, deposited by the live jelly out of its own sub- 

 stance, looks at first like a milky fluid, but it soon 

 becomes hard, and then tiie sides begin to rise up, 

 something like a fine pencil-case. Next comes the 

 grov/th of the mouth and the tentaculse of the 

 polype, which enable it to catch the food upon 

 which it is to live, and with which it carries on its 

 work of building. After the appearance of the 

 tentacula, the remaining jelly takes the form of a 

 tube polype, and for the rest of its life goes on ever 

 catching food and building therewith new stories to 

 its house. As the outside case grows higher and 

 higher, the polype follows up after the rising walls 

 of its house, always keeping its mouth just below 

 the top of the wall, but high enough to enable it to 

 thrust out its tentacula. The coralline polype does 

 not approve of solitude. Vast numbers, past 

 man's power to count, build their cells side by side. 

 Nor are their chalk pipes dead walls, but rather, so 

 long as the polype lives in them, living bones ; for 

 out of the body of the animal run little vessels into 

 tlic outer case, making their body and their waU one 

 living whole. 



Eurther, all the polypes in one pile are joined 

 together by their vessels into one great living crea- 

 ture; so that what one polype eats passes on to assist 

 in nourishing the rest. 



Very numerous are the family and genera of 

 these wonder-working zoophytes ; the more abun- 

 dant, accoi'ding to Darwin, being the Madrepores, 

 AstrcFcis, Forites, Meandrince, and Malllpores, at 

 moderate depths. Millepores, Seriatopores, and 

 other delicate forms, are found at depths varying 

 from 15 to 20 fathoms. Although we are used to 

 speak of coral reefs as rising from "unfathomable 

 depths," in ordinary cases reef-building polypifcrs 

 do not flourish at greater depths than 20 or 30 

 fathoms; a certain amount of light and warmth 

 being necessary to their existence. It is true, 

 however, that detached coral and coral drift have 

 been found at the depth of 270 fathoms ; but this 

 was not the work of the true rccf-building zoophyte. 

 As has been said, not only animal but also chemical 

 agency is at work in the building of coral reefs, 

 which, though chiefly effected by the lime-secreting 

 zoophytes, are in some measure owing to the pro- 

 miscuous aggregation of marine debris. 



Coral, as produced by the polypes, is almost pure 

 carbonate of lime, which, although soft and porous 

 at first, becomes in time so hard and compact as to 

 be used by the South Sea Islanders for architectural 

 purposes. However, in the formation process the 

 pure secretion of the animals envelopes sponges, 

 sea-weeds, star-fishes, sea-urcb'ns, drift-wood, drift- 

 coral, shells, and such-like ; the whole mass being 

 consolidated into a compact rocky mass by the 

 growth of the new coral and through chemical 

 action— that action consisting in the transfusion and 

 percolation of carbonated water among the particles 

 of lime of which the pure coral is mainly composed. 



All this agency combined with that produced by 

 the infiltration of carbonate of lime from decom- 

 posed coral, gives to the rock a brccciated appear- 

 ance, and renders it extremely analogous to some 

 older limestones of the secondary formations. 



On the other hand, coral reefs arc sometimes 

 found closely resembling in formation some of the 

 earthy varieties of chalk ; these arise from the con- 

 solidation, in lagoons and sheltered water-channels, 

 of particles of decomposed coral and other matter. 

 A third species of coral-stone has a sparry crystal- 

 line aspect; but this is only found when the reefs 

 have been upheaved by subterranean agency ; as for 

 instance, on the hills of Tahiti, where a stratum of 

 fossil coral exists. 



Thus, in the work of the polype we may find 

 almost every gradation of limestone ; varying from 

 the soft chalky mass formed by animals even now 

 alive, to the hard, compact texture of saccharoid 

 marble, M'hich has stood against wind and wave for 

 thousands and thousands of years. 



The rapidity of the growth of these rocks is a 



