817 



iiiipuritj of the water, and volcanic eruptions. If, iiowever, reefs do 

 develop, the shape of the living reefs depends largely on the stage 

 of development of the cruslal movements at that moment. In our 

 discussion we assume a stable sealevel, because our conclusions will 

 also hold for a moving sealevel. If (he coastline remains stable for 

 a considerable time or undergoes only slightly horizontal displacements, 

 horizontal and thick reefs may possibly l)e formed ; slightly vertical 

 movements will perhaps increase especially the thickness of the reef, 

 if the movement is a positive one, while negative movements will 

 soon cause the reef to rise above the sea, even though its thickness 

 and extent be only slight yet. 



All these phenomena may appear simultaneously at |)oints of the 

 geanticline remote from each other, so that already while the material 

 is forming which is to help in the composition of the reefcaps, 

 considerable difference in the shape may occur. 



The development of reefcaps. 



After the reef has i-isen above the sea, the morphological changes, 

 which were the combined result of the character of the crustal 

 movements and the growth of the corals, aie at an end. During the 

 continued movements the reefs move along curves, whose shapes 

 vary and are determined by the character of the crustal movements. 

 These movements may again be alternately vertical or horizontal 

 and downward, each type manifesting itself during a longer or 

 shorter period. The shape of the curves is determined by the evo- 

 lution of the geanticline on which the reefs were formed. The 

 reefcap observed by us is the final product of these continual and 

 varying movements. The reefs formed at a certain epoch on the 

 surface of the sea, which, initially, were all lying in the same 

 horizontal level, are, in a later stage of development, located in a 

 plane of irregular shape. The oldest parts of the reefcap have 

 undergone this change longer than the other portions. 



Besides by the character of the crustal movements the form of 

 these reefcaps is to a great extent also determined by erosion. 



Influence of erosion. 



In rising areas subject to strong erosion, it is no matter of sur- 

 prise to tind that of the portion of a reefcap that has for a long 

 time been elevated above the sea-level, only some remainders are 

 left, whereas the younger portions still present an unbroken cap. 

 This will sometimes happen, but it is not the rule. There are namely 



