LIMIT OF CORAL GROWTH 91 



elusion, yet, as events have proved, in the main 

 correct. 



Chamisso's opinion was not destined to remain long 

 unchallenged, for two famous French naturalists Quoy 

 and Gaimard asserted, as the result of their observa- 

 tions, that the coral rock of an atoll is only skin-deep, 

 i.e., it forms, according to them, a mere superficial crust, 

 not more than about 25 feet in thickness ; the rest 

 Chamisso's " table-mountain " being, on this view, of 

 volcanic, or at all events of inorganic, origin. 



Few of the arguments by which it was attempted to 

 sustain this erroneous conclusion strike one as being very 

 satisfactory, but they include one highly important 

 observation, viz., that reef-building corals do not live 

 at greater depths than 25 feet below the level of low 

 tides. Subsequent inquiry, while fully confirming the 

 existence of a limit, has at the same time extended it 

 down to a depth of as many as 25, or perhaps even 40, 

 fathoms. Yet, even with this modification, the un- 

 expected discovery of Quoy and Gaimard seems to stand 

 in flagrant contradiction to the views of Chamisso. If 

 corals cannot grow below a depth of 25 fathoms, how 

 could they possibly have built up islands of over 100 

 fathoms in thickness? 



The answer to this question, as is well known, was 

 given by Charles Darwin. If we admit the truth of both 

 the apparently conflicting statements, it is obvious that 

 the corals at the base of a reef 100 fathoms in thickness 

 must have been situated within the limit of 25 fathoms 

 at the time they were alive. But in order to bring them 

 within this limit it is only necessary to suppose that the 

 foundation on which they grew originally stood 75 fathoms 

 nearer the sea-level than it does now ; or, in other words, 

 that since the lower layers of the reef were alive and 

 flourishing, the ground which supported them has sunk 



