CHANGES IN OCEANIC REGIONS. 381 



formation very distinctly. On approaching its eastern side, Captain "Wilkes sounded 150 



Aurora Island. 



fathoms from its perpendicular cliff, but found no bottom with 150 fathoms of line. The 

 cliff, which, when measured, proved to be 250 feet high, appeared to be worn into caverns : 

 the coral shelf was found to be 550 feet wide, extending on the northern side of the 

 island, and gradually diminishing in width until it lost itself at the western end. 



" As far as our observation went," Captain Wilkes remarks, " the upper portion of this 

 island is composed of limestone or compact coral rocks ; the cliff, on its eastern side, 

 where we first landed, appeared stratified, horizontally, in beds of ten to twelve feet thick, 

 of a sort of conglomerate, composed of shells, coral, and pieces of compact rock, cemented 

 together by a calcareous deposit. The under part of this bed had been much worn by the 

 sea ; the rich soil was composed of decayed vegetable matter and decomposed limestone, 

 and the slabs that were lying loose on the surface had a clinky metallic sound when 

 struck. The island has unequivocal marks of having been uplifted at different periods, 

 the cliff, at two different heights, appearing to have suffered abrasion by the sea. Stalag 

 mites were observed under the cliff, and also some stalactitic columns, fourteen feet high 

 by six in diameter." 



Another island of this group was surveyed by the expedition the Arutua Island, 



Dean's Island. 



which was found to be connected with Nairsa, or Dean's Island. The coral blocks here 

 showed themselves more conspicuously and in greater numbers. 



The zoophytes are frail gelatinous creatures of varying size, apparently limited in their 

 range, operating only from high-water mark downwards to the depth of eighteen or 

 twenty fathoms. The various species of these animals appear to be furnished with minute 

 glands, secreting gluten, which, upon exudation, converts the carbonate of lime in the ocean, 

 and other earthy matters, into a fixed and concrete substance, twisted and fashioned in every 

 variety of shape. The formation of coral is one of those chemical processes in the great 

 laboratory of nature, which the skill of man has not enabled him either to imitate or to compre 

 hend ; but the fact is clear, that huge masses of solid rock are formed by these diminutive 

 living agents sea-workers toiling and spinning to the music of the waves, whose construc 

 tions are capable of resisting the tremendous power of the ocean, when most agitated by 

 winds and tempests, and ultimately become a secure habitation for man himself. The 

 coral substance appears to bear the same relation to the insect, as the shell of a snail or an 

 oyster does to either of those animals, without which they cannot long exist ; and it is 

 upon the death of the animalcules, that their separate homes and skeletons are knit 

 together by some mysterious cement, and serve as a basin for the memorials of fresh races, 

 which, as they die off, increase the growth of the firm and solid fabric. 



