CHANGES IN OCEANIC REGIONS. 383 



yellow, and sometimes of a blue colour ; while others resembled snails, and some were not 

 unlike lobsters and prawns in shape, but soft, and not above two inches long. The growth 

 of coral ceases when the worm which creates it is no longer exposed to the washing of 

 the tide. Thus a reef rises in the form of a gigantic cauliflower, till its top has gained 

 the level of the highest tides, above which the worm has no power to carry its operations, 

 and the reef, consequently, no longer extends itself upwards. The surrounding parts, 

 however, advance in succession till they reach the surface, where they also must stop. 

 Thus, as the level of the highest tide is the eventual limit to every part of the reef, a 

 horizontal field comes to be formed coincident with that plane, and perpendicular on all 

 sides. The reef, however, continually increases, and, being prevented from going higher, 

 must extend itself laterally in all directions ; and this growth being probably as rapid at 

 the upper edge as it is lower down, the steepness of the face of the reef is preserved ; and 

 it is this circumstance which renders this species of rocks so dangerous in navigation. In 

 the first place, they are seldom seen above the water ; and in the next, their sides are so 

 abrupt that a ship's bows may strike against the rock before any change of soundings 

 indicates the approach of danger." As an instance of the caution requisite in navigating 

 amongst coral, Captain Hall mentions, that his ship, the Lyra, was at one time within 

 three or four yards of a reef, the ragged tops of which were distinctly visible two or three 

 feet below the surface, while, at the same moment, the leadsman on the opposite side of 

 the vessel sounded in nine fathoms. 



Captain Flinders, while surveying the coasts of Australia, examined the coral 

 formations in process there, particularly those of Half- Way Island, on the north coast of 

 that region. " This little island, or rather the surrounding reef, which is three or four 

 miles long, affords shelter from the south-east winds ; and being at a moderate day's run 

 from Murray's Isles, it forms a convenient anchorage for the night to a ship passing 

 through Torres' Straits. I named it Half-Way Island. It is scarcely more than a mile 

 in circumference, but appears to be increasing both in elevation and extent. At no very 

 distant period of time, it was one of those banks produced by the washing up of sand and 

 broken coral, of which most reefs afford instances, and those of Torres' Straits a great 

 many. These banks are in different stages of progress : some, like this, are become 

 islands, but not yet habitable ; some are above high-water mark, but destitute of vege 

 tation ; while others are overflowed with every returning tide. It seems to me, that 

 when the animalcules, which form the corals at the bottom of the ocean, cease to live, 

 their structures adhere to each other, by virtue either of the glutinous remains within, 

 or of some property in salt water ; and the interstices being gradually filled up with sand 

 and broken pieces of coral washed by the sea, which also adhere, a mass of rock is at 

 length formed. Future races of these animalcules erect their habitations upon the rising 

 bank, and die in their turn to increase, but principally to elevate, this monument of their 

 wonderful labours. The care taken to work perpendicularly in the early stages, would 

 mark a surprising instinct in these diminutive creatures. Their wall of coral, for the 

 most part, in situations where the winds are constant, being arrived at the surface, affords 

 a shelter to leeward of which their infant colonies may be safely sent forth ; and to this, 

 their instinctive foresight, it seems to be owing, that the windward side of a reef exposed 

 to the open sea, is generally, if not always, the highest part, and rises almost perpen 

 dicular, sometimes from the depth of two hundred, and perhaps many more, fathoms. To 

 be constantly covered with water, seems necessary to the existence of the animalcules ; 

 for they do not work except in holes upon the reef, below low-water mark ; but the coral, 

 sand, and other broken remnants thrown up by the sea, adhere to the rock, and form a 

 solid mass with it, as high as the common tides reach. That elevation surpassed, the 

 future remnants, being rarely covered, lose their adhesive property, and, remaining in a 



