Voyages of Discovery and Survey. 325 



1200 feet, as the case may be. On the weather or more exposed 

 side of the reef, great blocks of coral, six to nine feet across, are de- 

 tached by the force of the breakers from the main mass, and the sur- 

 face of the reef is described as having the appearance of a great flat 

 of sandstone. Loose slabs lie about, with here and there an accumu- 

 lation of dead coral branches, or banks of white sand, and the whole 

 is checkered with holes and hollows, in which living corals are grow- 

 ing. Coarse sand occurs inside the barrier, and near the reefs, while 

 finer matter is found more towards the mainland. 



We have here, upon a great scale, occupying an area which may 

 be roughly estimated at 30,000 square miles, that variable mixture 

 of organic, mechanical, and chemical accumulations which has been 

 so oi'ten remarked among coral reefs and islands, and of which Mr 

 Darwin has given so valuable a summary, illustrated with such im- 

 portant original reasoning, in his work on Coral Islands. Although 

 it would be out of place here to enter into the interesting details 

 afforded by Mr Beete Jukes, it is important to notice that he found 

 corals able to sustain life when left by the tide several inches out of 

 the sea. He observed living Astrsese, the tops of which were 18 

 inches above water, and he believes that an exposure to the sun and 

 air for two or three hours will not kill many coral polyps, the cells 

 retaining moisture so long as they are in a position of growth. 



When we consider the accunmlations of the great barrier reef as a 

 mass of matter obtained by animal life for its uses, having formed the 

 hard parts needful to it (including the shells of molluscs, the spines 

 and coatings of echinoderms, and the like), before it became sand, and 

 furnished the materials for chalky and calcareous mud, or crystallised 

 out in fitting situations, and under the proper conditions, we are 

 forcibly struck with the means by which the same matter has pro- 

 bably passed from the solid form (often perhaps from the fossil re- 

 mains of pre-existing life), into solution, whence it vias abstracted by 

 the coral polyps, molluscs, and other creatures for their wants, again 

 to be accumulated in a solid form, partly in that given to it by ani- 

 mal life, partly as sand and fine mud, and partly in a crystalline 

 state. Mr Darwin has pointed out the mixture of organic and me- 

 chanical matter forming coral islands, the gi'owth of the reef-making 

 corals outwards, their abrasion in part by the breakers, and the ac- 

 cunmlation of an outside talus at a high angle, over which the living 

 corals gradually extend, and cover the fragments and worn portions 

 by masses of their calcareous secretions. The observations of Mr 

 Beete Jukes would confii-m these general views, but, at the same 

 time, he remarks on the possibility of corals, with whose habits wo 

 may still remain unacquainted, laying the foundation of reefs and 

 islands in deeper water than is assigned to the existence of the known 

 reef-constructing corals, which flourish from near the surface of the 

 sea to the depth of twenty or thirty fathoms. 



