126 BULLETIN OF THE 



Wherever in that region the fringing reef has attained any consider- 

 able width, we find that upon the portion nearer shore, where the corals 

 once flourished, they have died because the extension of the reef towards 

 the shore has excluded them from contact with the fresher sea-water 

 outside. This part of the reef has been corroded and eaten off, or was 

 dissolved, aa Murray suggests, by the action of the carbonic acid held 

 in sea-water, which absorbs a large amount of carbonate of lime. 



But it should not be forgotten that this solvent action of carbonic acid 

 in sea-water cannot be considered as the chief agent in the formation of 

 barrier reefs. Take, for example, the case of the Florida reefs, or that 

 of the great barrier reef of Australia. The former are so far from the 

 neighboring chain of keys, and the latter so distant from the adjacent 

 mainland, that such an explanation of the presence of the channel sepa- 

 rating the one from the keys, the other from the shore, would involve the 

 solution and disappearance of the reef itself. 



We are inclined to look upon the depth and extension of the ridge or 

 plateau upon which a barrier reef first establishes itself as the chief 

 cause of its growth and final form. Such a plateau having reached the 

 level at which corals flourish, the reef begins to grow, and its distance 

 from the mainland or from the adjacent islands is thereafter determined 

 by the contour lines of the submerged extension of the land seaward or 

 landward. Nevertheless, the effect of the relatively clear or muddy 

 water on the sea and land faces of the incipient barrier reef cannot have 

 failed to exercise an important influence on its seaward or landward 

 development. 



Murray ^ clearly shows that the solution of dead carbonate of lime 

 shells and skeletons by sea-water is as constant as its secretion by living 

 organisms. He considers it probable that on the whole there is more 

 secretion than solution, and that there is at the present moment a vast 

 accumulation of carbonate of lime going on in coral areas no one familiar 

 with the subject will deny. This secretion diminishes with the depths, 

 while the rate of solution perhaps increases under pressure. He com- 

 pares it to the action of aqueous vapor in the atmosphere over land sur- 

 faces. When precipitation is in excess of evaporation fresh-water lakes 

 are formed ; when evaporation, on the contrary, exceeds it, salt lakes 

 are formed in inland drainage areas. 



The discussion on the theory of solution which has taken place in 

 " Nature " between Reade and other geologists, does not appear to cover 

 the ground. The objections are mainly made by investigators who know 



1 Nature, March 1, 1888, p. 414. 



