The Mighty Deep 



Equally impossible was it to photograph these 

 fairy grottoes, "seen through twenty feet of 

 surging water," though some snapshots were 

 taken in moments when the vast billows with- 

 drew themselves. 



One difficult question in connection with coral 

 reefs has been How can they rise, as they do, 

 out of considerable depths, when coral-polyps 

 cannot live below some twenty fathoms from the 

 surface ? 



Two chief explanations have been offered, 

 each of which has been warmly taken up and 

 defended. 



One meets the difficulty by means of the 

 theory of a rising and sinking earth-crust. A 

 mountain-top might have been first heaved up 

 to within twenty fathoms of the surface. Then 

 later, while the polyps were building upon it, 

 the mountain might have slowly sunk. In that 

 case the polyps below would die, as the water 

 deepened ; but the wall of dead coral would 

 remain, reaching in time very far down. 



The other explanation also admits that coral- 

 reefs have been generally built upon sub-ocean 

 heights. But it supposes that height to have 

 been partly raised by volcanic forces, and after- 

 wards built up to the needed height by a growing 



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