54 THE BERMUDA ISLANDS. 



extensive terrestrial movements, and presupposes the existence 

 of numerous land-masses rising from the deepest water. Around 

 these, under favorable conditions, reef-building and other corals 

 would flourish in abundance, the submerged cone affording a 

 suitable base for the development of the coral animal. The 

 external margin of the coral barrier or buttress, which may be 

 assumed to grow from a possible depth of 120 feet, owing to the 

 invigorating action of the beating surf, and an increased food- 

 supply, would probabty rise more rapidly than the inner parts, 

 whose development would also in a measure be checked by the 

 out-pouring of detrital sediment. A shelving inwardly -slop- 

 ing collar or bank, having a land-nucleus in its center, would 

 thus be produced. In the ring thus forming, whose outer mar- 

 gin, through the breaking and heaping action of the sea, would 

 be lifted somewhat above the general water-level, we have the 

 skeleton of the future atoll. We may now distinguish three 

 elements in its construction : the outer ring or collar of coral, 

 the central nucleus of land, and the encircling body of water 

 which separates the two. 



If at this stage of its formation we conceive the enclosed 

 island to undergo a slow and gradual subsidence the following 

 phenomena may be assumed to present themselves. The outer 

 border of the reef would slowly but steadily build itself up to the 

 level of the water, the growth of the coral colony keeping pace 

 with the gradual sinking of its substratum, provided this be not 

 too rapid. The parts sinking below the line of 120 feet would die 

 out, and their future purpose would be merely to afford a base 

 for the super-structure. The island portion, on the contrary, 

 would sink deeper and deeper, until eventually it might com- 

 pletely disappear. We would then have an outer barrier and 

 an inner lagoon, with probably one or more communicating 

 passages between the latter and the sea cut through the coral 

 growth. This is the typical atoll. 



When a reef is separated by a considerable body of water 

 from the adjoining land it is termed a " barrier " reef, of which 

 two distinct types, the "encircling" and the "linear" barrier 



