46 On the Formation and Groicth of Ccral Reefs and Islands. 



The corals principally instrumental in the formation of reefs, belong to 

 Lamarck's third order of polypes, the polypi vaginati, and the fifth divi- 

 sion, which he distinguislies as his polypiers lamellifferes, containing the 

 following recent genera : — carvophillia, fungia, pavonia, agaricia, raean- 

 drina, niouticularia, echinopora, explanaria, astrea, porites, pocillopora, 

 madrepora, seriatopora, and oculina ; in fact, the whole of the Linnean 

 genus madrepora. Of these, caryophillia, meandrina, astrea, porites, and 

 madrepora, form the main portion of the reef; while the ornamental parts 

 are made up by a difi'nsion of the other genera, together with gorgonia, 

 isis, corallium, melitea, corallina, spongia, alcyonium, actinia, &c. &c. 

 independent of the locomotive beauties, in the form of astrea, echini, and 

 testacea. 



For conciseness, it will be well to divide and treat of these islands, 

 under the different forms that they assume, which are as follows, yf«^, tiie 

 circular; secondbj, the flat; thirdly, the long narrow; ^\\A fourthly , those 

 which encircle high lands. 



The whole of the islands, exceeding fifty in number, which (with two or 

 three exceptions) form the gronpe called the Paumotus, are of the first 

 kind, and consist of narrow strips or belts of coral, of an annulate or circu- 

 lar form, each containing or enclosing a lagoon. These- strips of coral have 

 never been observed to exceed one mile, but are much more generally of 

 about four or five hundred yards in width ; seldom raised above the water 

 more than four or five feet, and presenting an abrupt line towards the 

 ocean. On these the waves break with much violence, and at a short 

 distance from them the depth is usually so great, as to be denoted by 

 navigators as unfathomable; this however is a vague mode of expressing 

 themselves, as it is now known that soundings may be had at more than 

 one thousand fathoms : and we have repeatedly obtained soundings about 

 a mile from the shore, at between two and three hundred, (mariners in 

 ordinary cases seldom using a line of more than one hundred and twenty 

 fatiioms deep, which is by them termed the deep sea line ; and in fact it 

 requires great care, time, and labour, to get soundings at much greater 

 depths;) while the interior of the lagoon is comparatively shallow, varying 

 from an inclined beach to a depth of one hundred fathoms or more. 



These islands vary from two and three miles in circumference to one 

 hundred or one hundred and fifty; and in many instances a fissure, or 



with saltwater seems necessary to the existence of the animalcules, for they do not 

 work, except in holes upon the reef, beyond low water mark ; but the coral sand and 

 other broken remnants, being rarely covered, their adhesive property, and remain- 

 ing in a loose state, form what is usually called a key, upon the top of a reef. 



The new bank is not long in being visited by sea birds ; salt plants take root 

 upon it, and a soil begins to be formed ; a cocoa nut, or the drupe of a pandanus is 

 thro^vn ashore ; land birds visit, and deposit seeds of shrubs and trees ; every high 

 tide, and still more every gale, adds something to the bank ; the form of an island is 

 gradually assumed; and last of all comes man to take possession." 



