114 CRUISE OF THE BARRERA 



the reef is a very flourishing one and occupies the 

 outermost edge only of the island shelf is altogether 

 certain. The gradient of depth without the reef 

 is exceedingly steep, the dark blue of ocean depths 

 being entered at once upon crossing the barrier. 

 For lack of proper apparatus for obtaining bottom 

 samples from the deeper water without the reef 

 we were unable to determine whether or not a line 

 of dead reef exists on the slope without. From 

 other indications, however, we were of the opinion 

 that the platform (the island shelf) is of fairly recent 

 submergence to about its present depth and that 

 the present living reef is too young to have ex- 

 panded very far in a seaward direction. The 

 coral polyps find here almost ideal conditions for 

 growth, — shallow water, high temperature, and a 

 steady current, richly laden with food sweeping the 

 edge of the shelf. No rivers with burden of silt 

 interfere and the water is of exceptional purity. 



Along a rising coast successive lines of fringing 

 reef with occasional inner patches of dying coral 

 might be expected, the outer ones being in the 

 most flourishing condition. The Colorados, how- 

 ever, present but a single line of almost continu- 

 ous living reef without the inner patches of living 



