292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



Many authors have correlated the observation that island arcs (and 

 hence mountain building) develop in the oocean basins along the 

 margins of continents with the concept that the continental massifs 

 are strong and the oceanic crust weak, thereby accounting for the 



localization. However, if mountain-building forces are related to 



FiGUBE 11. — Fathometer recorder trace of guyot in latitude 14°20' N., longitude 

 165°55' W. Ship's speed, 13.7 knots; course 059° true. 



convection currents within the earth (Griggs, 1939), the most satis- 

 factory of the present theories, then the localization can more reason- 

 ably be explained on the basis of heat relations within the crust. 

 Being warmer under continents and cooler under oceans the down- 

 ward-flow part of the convection cell would be more likely to be 

 localized under the ocean and would be supplemented in some cases by 

 the outward flow of warm material from beneath the continental area. 

 Having concluded that the Pacific Basin was in general strong and 

 stable, it is now appropriate to turn to exceptions in detail to these 

 generalities. All volcanic islands of the ocean basin proper (exclud- 

 ing from this discussion the highly unstable island arc areas) are 

 subject to frequent vertical movements as long as vulcanism is active. 



