POLYPES.-TIIE CORAL. 



All the coral at a moderate deptli below water is 

 living; all above is dead — being the debris of the 

 living masses, washed up by the incessant action of a 

 eeetliing and waving surge. 



On the lagoon side the water is necessarily calm — as 

 calm as a mountain-lake on a silent summer's day. 

 There the encircling coral chaplet shelves into the 

 snpiihire waves by successive terraces or ledges ; also 

 of living coral, but not of the same species as those 

 wliich lay the foundations and build the exterior wall. 

 It has been suggested that the luxuriant growth of the 

 latter is promoted by the perpetual change of water 

 wilh which the breakers bring them in contact. But 

 the same cause operates to deprive the whole of the 

 coral in the interior of the more nourishing part of 

 their food ; and species of a more delicate kind and a 

 slower growth take the place of the hardier corallines. 



The depth of the lagoon varies, in different atolls, 

 from twenty to fifty fathoms, the bottom being partly 

 detritus and partly living coral. Owing to the coral- 

 growth a few of the lagoons have been filled up ; but, 

 for the reasons already stated, the process is very slow, 

 and the growth of the coral is also checked by the 

 depredations of various marine animals. For here, as 

 in all nature, operates the great law that the prodigal 

 increase of one genus is always checked by the innate 

 hostility of another. 



The coral is of the most varied and graceful con- 

 figurations and the most exquisite tints, which gleam 

 beneath the azure waves like a rainbow against a blue 

 sky : dark brown, rich lustrous purple, vivid emerald 

 green, orange, pink, peach-bloom, dazzling white. 

 Among the gay branches of this submarine garden 

 dart fish of the most splendid hues, and the whole 

 scene is one of a fairy character, with a peculiar and 

 flaunting beauty distinctively its own. 



Lagoon islands are sometimes circular in form, but 

 more frequently oval, or else entirely irregular. They 

 are frequently found in groups ; sometimes they lie 

 scattered over the ocean, like dropped pearls; and 

 very generally they occur in elongated archipelagos. 

 In size these " fairy-rings of ocean," as they have 

 been happily called, vary from two to ninety miles in 

 diameter, and islets are frequently formed upon these 

 by the accumulation of the detritus, which the atmos- 

 pheric influences reduce, in due time, into a kind of 

 soil. Here the sea-birds drop a few seeds brought 

 from remote lands or other islands ; grasses and 

 mosses spring into existence ; the palm tree uplifts its 

 slender columnar stem ; and before long a miniature 

 world is formed. Those atolls, which are not filled 

 up, furnish in their lagoons the most admirable 

 harbours. 



Encircling reefs resemble atolls in every respect 

 but this; they inclose within their ring one or more 

 islands, generally mountainous, at a distance of two 

 or three miles from the shore, and separated by a 

 channel two hundred or three hundreii feet in depth. 

 Tahiti, the largest of the Society Islands, is an instance 

 of this kind. The lagoon encompassing it is like " an 

 enormous moat," thirty fathoms deep, and divided 

 from the ocean by a rampart of coial, at a distance 

 varying from half a mile to three miles. 



Barrier reefs only differ fjom the latter in their 

 position with regard to the land. They skirt the shore 

 in an elongated direction, like the great Australian 

 barrier reef, which extends one thousand miles along 

 the north-east coast of the continent of Australia. 

 The long ocean-swell of the Pacific, being suddenly 

 impeded by this colossal barrier, lifts itself, says Mr. 

 Jukes, in one great continuous ridge of deep blue 

 water, which, curling over, falls on the edge of the 

 reef in an unbroken cataract of dazzling white foam. 

 Each line of breaker runs often one or two miles in 

 length, with no break perceptible in its continuity. 

 There is in such a scene a display of power, grandeur, 

 and beauty, which almost rises to the sublime. The 

 unbroken roar of the surf, with its regular pulsation of 

 thunder, as each successive swell falls first on the outer 

 edge of the reef, is almost deafening, yet so deep-toned 

 that it interferes not with the slightest nearer and 

 sharper sound. Both sound and sight are such as 

 to impress him who hears and sees with a conscious- 

 ness of standing in the presence of an overwhelming 

 majesty and power. 



Coral reefs, or fringes, are mere edges of coral lining 

 the margin of a shore, and as they frequently surround 

 shoals, they are regarded by the mariuer with just 

 apprehension. 



Lagoon islands, as already stated, are built up by 

 various species of polypes, the most vigorous being 

 engaged in the erection of the exteiior wall. Even 

 these, however, cannot exist at a greater depth than 

 twenty or thirty fathoms, and they die immediately 

 the water ceases to cover them. Yet the coral preci- 

 pice shoots down to awful depths, and though the 

 whole of it is not the work of the corallines, the per- 

 pendicular thickness of the coral is known to be very 

 great, and to extend hundreds of feet below the deaths 

 at which they cease to live. IIow is this pheuomenon 

 to be explained ? 



The theory put forward by Von Buch is, that the 

 coral ring is only the edge of a submarine elevation 

 crater ; the latter having served as the foundation on 

 which the animalcules have raised their structure. In 

 support of this view its advocates refer to the circum- 

 stance of a lagoon island having been seen to rise in 

 1825, in latitude 30° H', accompanied with smoke; 

 and to the resemblance between the shape of many 

 lagoon islands and well-known volcanic craters. 



Another theory — Dr. Darwin's — is so concisely stated 

 by Mrs. Somerville that we shall give it in her words:* 

 — "Since there are certain proofs that large areas of 

 the diy laud are gradually rising and others sinking 

 down, so the bottom of the ocean is not exempt from 

 the general change that is slowly bringing about a new 

 state of things ; and as there is evidence, on multitudes 

 of the volcanic islands in the Pacific, of a rise in 

 certain parts of the basis of the ocean, so the lagoon 

 islands indicate a subsidence in others — changes arising 

 from the expansion and contraction of the strata under 

 the ocean bed. 



" There are strong reasons for believing," continues 

 Mrs. Somerville, " that a continent once occupied a con- 

 siderable area of the Tropical Pacific, and that soma 

 • Mrs. Souierville, Plij-aical Geography. 



