SUBMARINE MOUNTAINS 75 



On the contrary, it is reasonable to find in the prolonged 

 stability of a Bermuda, Truk, Suva Diva, or Funafuti a measure 

 of the strength of the sub-oceanic crust. 



Of course, proof that the sub-oceanic crust, though heavily 

 burdened with piles of lava, has not sunk during the last mil- 

 lion years or so by no means implies stillstand in earlier times. 

 In fact, the sea floor had to keep sinking while each volcanic 

 mountain was slowly growing up, flow on flow, ash-bed on 

 ash-bed. The mere transfer of the huge total mass of lava to 

 the surface meant removal of the underpinning of the rocky 

 floor of the ocean round about, and therefore subsidence in the 

 same region. Moreover, any older volcanic mountain that had 

 been built on the outlying part of the belt of subsidence had 

 also to go down. It is a case of action at a distance — true sub- 

 sidence compelled by the elastic strength of the earth's crust. 

 A possible example may be represented by the moderately 

 drowned Bora Bora Island of the Society group; here the 

 drowning is conceivably the result of the eruption of the 

 voluminous lavas constituting the younger volcanic mountain 

 of Tahiti, not far away. 



For still other reasons, quite unconnected with volcanism, 

 an atoll or reef-rimmed island, which had been stable long 

 enough for the development of a broad lagoon, may have been 

 forced to subside. Examples may be sought where an oceanic 

 region has been disturbed by the forces of mountain-building. 

 Such may be the case with the "drowned" atolls of the China 

 Sea and the atolls of the Philippine region (Figure 43). 



On the other hand, the "drowning" of these same atolls 

 may be only apparent, not real. They lie in the broad belt 

 tormented by typhoons, storms capable of beheading the reefs 

 to depths of five to ten fathoms below sealevel. According, 

 then, to an alternative hypothesis there is no need to assume 

 any departure from that stability of reef foundations which is 



