252 TOILERS IN THE SEA. 



the coast line. Having found the declivity of the 

 land, assuming the slope to be regular and con- 

 tinuous, it would not be difficult to estimate the 

 depth, at any given distance from the land, but there 

 must be uncertainty as to the submarine slope, 

 although extensive observation has shown that, in 

 general, these slopes nearly coincide with those of 

 the exposed land. In this, or a similar manner, 

 Dr. Darwin estimated that the thickness of some of 

 the reefs in the Pacific islands, at their outer limits, 

 would be at least two thousand feet. After esti- 

 mating the outer reef of the Gambier group at 

 eleven hundred and fifty feet, of Tahiti at two 

 hundred and fifty feet, and of Upolu at four hundred 

 and forty feet, Professor Dana remarks that "the 

 results are sufficiently accurate to satisfy us of the 

 great thickness of many barrier reefs ;" and again, 

 " with regard to Tahiti and Upolu, information bear- 

 ing upon this point was obtained, and the above 

 conclusions may be received with much confidence. 

 Many of the Fiji reefs, on the same principle, cannot 

 be less than two thousand feet in thickness." The 

 objection which will at once be urged against such a 

 thickness of coral, on the ground that living coral 

 can only be found at a limited depth below the 

 surface, will be adverted to hereafter, and perhaps 

 reconciled with these estimates. For the present we 

 leave the data as they stand. 



