402 



several other forms of life which possess calcareous skeletons outrun the 

 reef-corals in bathymetrical range, and it is likely that they (calcareous 

 algae, deep water corals, &c.) first populate the summit of the bank. 



The process now becomes less a matter for hyopthesis and more 

 one for actual observation, for the growth tendencies of reefs and of 

 colonies may be more easily studied. It is claimed that the tendency is 

 for such reefs to become "basin-shaped reefs," and to develop as flat 

 banks, with edges raised from their general surface and abundantly 

 covered with coral colonies. The chief factor in this process is again 

 the action of sedimentation. The surface waters still drop their burden 

 of suspended matter over the reef, and it is deposited upon the uneven 

 surface of the coral colonies, for, though it could no longer come to rest 

 upon the open sedimentation bank, it more easily finds a lodgment upon 

 the broken coral surface of the reef. At the edges of the reef the sedi- 

 ment becomes more easily washed off by wave action , and the corals of 

 the circumference of the reef flourish most. 



To obtain a concrete picture of the process it is only necessary to 

 turn to the colonies to be found any day in quiet pools in which sediment 

 is accumulating. A colony of Pontes grows as a spherical mass. In time 

 it develops to such a size that its rounded upper surface becomes suffi- 

 ciently flat to afford a lodgement for sediment. Then the activity of its 

 central zooids wanes, and, by the upgrowth of the peripheral ones, the 

 flattening increases. At length the central area dies — the zooids choked 

 by sediment, — and a raised ring of active living zooids surrounds a 

 central depressed area — an atoll in miniature. 



That this process is not due to the colony reaching tide-level 

 (Darwin, Semper) is proved by the abundant finding of such colonies 

 developed many feet below the level to which the tide ever falls. 



The process that may be seen any day in the myriad colonies around 

 an atoll, is presumed also to occur on the reef as a whole, for it is 

 merely a question of substituting colonies for individual zooids to picture 

 the development of the submerged basin-shaped reef. 



The basin-shaped reef continues to grow upwards until tide limit 

 arrests the growth of its margins. At this stage the waves begin to act 

 upon it and hammer fragment against fragment .with the production of 

 a quantity of coral débris at the point of maximum intensity of the 

 waves. This débris becomes cemented into solid breccia by the deposi- 

 tion of calcium carbonate around the particles that compose it. This 

 is the beginning of the breccia platform , and its origin may be looked 

 for upon the windward side; and on that side it will always remain best 

 developed. 



The breccia platform follows the raised rim of the reef in its 



