J. S. Gardiner — Origin of Coral Reefs. 207 



center, and channels due in the first place to lesser growth 

 would .tend to be formed, along which the currents would more 

 particularly rush off, doubtless carrying with them a certain 

 amount of material in suspension. The result at about 75 

 fathoms would be that we would have a great broad rim cut 

 by channels of 100 or 120 fathoms, in other words a series of 

 banks all round a plateau, reaching a depth of 75 fathoms from 

 the surface of the ocean. 



Up to 75 fathoms the chief organisms that formed our bank 

 were those which aj^proximate to the deep sea type in being 

 essentially holozoic forms. Now, however, we are approach- 

 ing the minimum depth towhich exjDerience shows us that these 

 forms can survive, perhaps owing to physical causes, perhaps 

 owing to competition. At any rate a fresh series of organisms 

 comes into account, and gradually becomes more and more 

 dominant, reaching its maximum at about 45 fathoms but con- 

 tinuing to exist on to 25 or 30 fathoms, at which depth in the 

 tropics reef corals obtain full sway over all other sedentary 

 forms of life. This medium depth group of organisms is a 

 most varied one. It includes in the first place many corals 

 such as Goniopora^ Alveopora^ Dendrophyllia^ Ileliopora, 

 Millepora and many solitary Madreporaria. Polytreraa is 

 not unimportant from its consolidating (binding) growth. 

 Sponges are few but molluscs of all sorts are very abundant. 

 Litliotliaranion is known to completely cover banks at ^^ 

 fathoms, and not improbably has had a considerable share in 

 building them up to this height; various forms of Halimeda 

 and similar algae no doubt also exist. 



There is no upward limit to the growth of these medium 

 depth forms. Most enter into competition with the actual 

 organisms of the reef, but apparently few are able to compete 

 with the reef corals. The larvae of the latter settle in some 

 numbers at about 40 fathoms, and commence a struggle with 

 the possessors of the ground for supremacy. The reef corals 

 feed almost entirely by means of their commensal algae, and, 

 as the depth decreases, the light becomes more powerful so that 

 at about 25 fathoms they finally obtain the upper hand and 

 only a few of the deeper organisms (notably Millepora and 

 Heliopora) manage to maintain their position. Our bank, in 

 effect, soon becomes crowned with a surface reef. 



It is scarcely necessary to point out that the smaller a bank 

 the greater the relative circumference it presents, and the more 

 the water over it will be mixed together ; and that the shallower 

 the bank, irrespective of its size, the less will such admixture be. 

 The smaller the bank the more will the current be divaricated 

 on either side of it, and the less will its rate be increased. The 

 larger the bank the more will the rate of the current around it 



