INTRODUCTION. 25 



Now, while this is the way in which the theory accounts 

 for the formation of barrier reefs surrounding islands, and 

 of atolls where no island remains, it is equally applicable 

 to barrier reefs extending longitudinally for many miles 

 along the coast of a continent at a considerable distance from 

 the shore, for at first there were fringing reefs close to the 

 land, but when by upheaval the shore became a raised beach, 

 the reefs, when built up by the polypes, were far out at sea. 



The theory accounts satisfactorily also for those gaps or 

 gateways, one or more of wliich are found in every barrier 

 reef or atoll. The existence of such gaps might at first 

 seem fatal to the theory, for it might be said, had the po- 

 lypes reared the reef from the foundation, they would have 

 carried on their work uniformly, and would have left no 

 gaps; and yet had there been no gaps no vessel could ever 

 have fled for refuge into these lagoons of broad, peaceful, 

 sheltered waters. Wherever there is a high-peaked island, 

 the clouds, attracted and caught by the peaks, are condensed 

 into water ; the water forms rills, and the converging rills 

 in many cases become rivulets flowing into the sea. In an 

 island of considerable size we may well suppose two or three 

 of these mountain streams at different places will enter the 

 deep. Wherever a stream enters there is no fringing reef 



