THE CORAL-REEF PROBLEM. 69 



the evidences of subsidence. In other words, there would be an 

 interassociation in the same archipelago of both fringing reefs 

 and atolls, for it can scarcely be conceived that all the project- 

 ing land-masses of the archipelago could, at the time when 

 movements of one kind or another set in, have been equally 

 elevated above, or depressed beneath, the surface of the water. 

 Hence, unequal developments must have taken place. 



Such are the principal circumstances connected with the 

 history of coral islands. If the theory of subsidence cannot, 

 perhaps, be considered to be absolutely demonstrated, it ac- 

 cords best with the facts, and, indeed, may be said to be in 

 substantial harmony with them. Furthermore, it helps to ex- 

 plain the significant fact, first pointed out by Dana, that a 

 very large, if not the greater, number of coral structures are 

 ranged along the line of greatest depression in the sea. 



The question here naturally suggests itself: Is there any 

 evidence supporting the theory of assumed subsidence of the 

 oceanic basins beyond what is furnished by the coral islands? 

 It must be admitted that our positive knowledge on this 

 point is very limited — indeed, almost nothing. But various 

 considerations lead to the belief that the present site of the 

 oceanic basins is a very ancient one, and possibly one that has 

 not materially changed, except in so far as intensification is 

 concerned, since it was first marked out as the most prominent 

 feature of the earth's crust. While manifestly we can have no 

 proof of this condition, it seems but reasonable to assume that 

 if this vast depression was formed through an early flexure of 

 the crust, and as the result of weakness in certain parts of that 

 crust, it has retained its position of depression from the first. 

 With a contracting or moving crust, moreover, particularly 

 under the special conditions of loading (sedimentation) and 

 continental unloading (denudation), it is likel}' that a depres- 

 sion of this kind would tend to sink or to subside, and force a 

 relief from strain in the uplift of the continents. This is the 

 view now held by probably the greater number of physicists 



