140 Daly — The Coral-Reef Zone. 



islands had been occasioned: partly because of the 

 removal of snbcrnstal lava to the earth's surface, entail- 

 ing local collapse of the crust; partly because of the 

 compacting of ash, agglomerate, and porous lavas, now 

 deeply buried and therefore strongly compressed by the 

 younger masses which were erupted during the growth 

 of each high cone. 5 



Subsidence due to any of the causes mentioned was 

 local and took place at different times. 



Conversely, local uplifts at different times had pro- 

 duced coastal plains, elevated beaches and sea-cliffs, and 

 probably elevated reefs of the fringing type. 



Out to sea there must have been many banks of highly 

 varied history. Some of them represented ancient 

 islands — volcanic or otherwise constituted — old enough 

 to have been completely truncated by waves and currents 

 before protecting reef-societies of corals and algae had 

 yet been evolved. Other, younger islands had been 

 similarly truncated, more or less completely, because of 

 the failure of the coral societies to get foothold in their 

 shallows, for reasons which still keep many tropical 

 shores reeness. 



One of these reasons needs particular attention. The 

 cone of an explosive volcano, during the upbuilding of 

 its ash-beds into and just above the level of energetic 

 wave-action, is subject to prolonged truncation by the 

 waves. Between explosions the loose materials are 

 levelled, with a tendency to produce a broadening bank 

 covered by 20 to 40 fathoms (36 m. to 73 m.) of water. 

 Judging from the time required to build up cones like 

 Vesuvius, such contemporaneous truncation might form 

 a tolerably wide detrital bank. After later eruptions 

 have succeeded in establishing a stable island, the shelf 

 is further broadened by the addition of land detritus and 

 of organic debris. With extinction of the volcano and 

 with sufficient broadening of the shelf, the mass may 

 finally become the site of a stable reef. Totoya of the 

 Fiji otoup, with its young cone and wide crater sur- 

 rounded by a rather wide shelf, seems to be a modern 

 example ; many similar cases might be cited. In any case 

 there can be no question as to the contemporaneous trun- 



3 The hypothesis of purely isostatic subsidence seems to be weak on the 

 quantitative side; Barrell doubts the sensitiveness of the earth's crust to 

 loads no greater than the average submarine volcano. (J. Barrell, Jour. 

 Geol., vol. 22, pp. 28-48, 1914.) 



