President's Address. It 



unsubmerged portions of any extensive continent or mass of 

 land such as D.irwin's theory requires. Whether built up 

 above the sea-level into islands, or brought up to varying 

 heights below that level, the volcanic eminences of the ocean 

 may conceivably be brought into the condition of platforms 

 for reef-builders by two causes. In the first place, the 

 erosive force of waves and tidal scour must tend to reduce 

 all prominent oceanic summits to the lower limit of breaker 

 action, and thereby to produce truncated cones or flattened 

 domes and ridges, on which coral-reefs, if not already estab- 

 lished, might spring up. In the second place, submarine 

 eminences may have been brought up to within the zone of 

 the reef-builders by the deposit of organic detrihts upon 

 them. One of the most remarkable results of recent deep- 

 sea explorations has been the accumulated evidence of the 

 extraordinary profusion of pelagic life in the tropical surface 

 waters. From experiments made during the cruise of the 

 " Challenger," Mr Murray estimates that, if the organisms 

 are as numerous down to a depth of a hundred fathoms as 

 they were found to be in the track of the tow-net, there 

 must be more than sixteen tons of carbonate of lime, in the 

 form of calcareous shells, in the uppermost hundred fathoms 

 of every square mile of ocean. The shells and skeletons of 

 these organisms fall in a constant rain to the bottom, where 

 their organic matter supplies food to the fauna which there 

 subsists upon the mud. By the accumulation, partly of 

 these superficial exuvim, partly of the remains of creatures 

 living at the bottom, an organic deposit is growing over the 

 sea-floor in the tropical regions wherein coral-reefs flourish. 

 Owing probably to the greater solvent action of the carbonic 

 acid of sea-water at great depths, or to the greater mass of 

 water through which they must sink, the shells of the upper 

 waters seem never to reach the abysmal bottom, or, at least, 

 soon disappear from it, for they are seldom met with in deep 

 dredgings. But in the shallower portions of the ocean they 

 abound; consequently it may be legitimately inferred that 

 the rate of growth of the calcareous organic deposit on the 

 sea-bottom must be more rapid in the shallower waters. 

 The tops of submarine peaks and banks, being constantly 



