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. This last-named island 

 seemed a reef washed by the sea. The coral blocks here showed themselves more con- 

 spicuously and in greater numbers than had been before observed, as will be seen from 

 the accompanying sketch. 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 comprehend ; 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 constructions 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 

 structures become firmly knit together by some mysterious cement, and serve as a basis 

 for the erections of fresh races, which, as they die off, increase the growth of the firm 

 and solid fabric. 



