392 ON THE GROWTH OF 



these narrow passes, where one is always sure of finding 

 plenty of water. And this circumstance also comes in 

 support of what we have advanced ; for, if these perpen- 

 dicular walls were entirely composed of madrepores, they 

 would present no deep openings in their continuity, be- 

 cause it is the property of zoophytes to build in masses 

 that have no interruption ; and because, again, could they 

 raise themselves from very great depths, they would end 

 with encumbering and shutting up these passages ; a cir- 

 cumstance which does not take place, and probably never 

 will, from the causes which we have related. 



If these facts prove, that madrepores cannot exist at 

 very great depths, the submarine rocks, which they only 

 increase in height, are not, therefore, exclusively formed 

 by them. 



We now come to the second part of the argument ; 

 and we assert, that there are no islands of any magnitude 

 and constantly inhabited by man, that are formed by co- 

 rals ; and that the layers which they construct under the 

 water, are not more than a few fathoms in thickness. 



We shall commence with the second part of this ques- 

 tion. The impossibility of penetrating to the bottom of 

 the sea to examine at what precise depth the solid zoo- 

 phytes establish themselves, constrains us to confine our- 

 selves to what has taken place in former times; and the 

 monuments which the ancient revolutions of the globe 

 have disclosed to our view, will serve to prove what is 

 going on in our own days. We shall mention what has 

 been seen in several places, and we shall first speak of the 

 island which Peron took for the theatre of the great works 

 of these polypi, namely the island of Timor. 



The banks of coral which the sea has left exposed m 



