SECT. 2] 



OCEANIC ISLANDS, SEAMOUNTS, UUVOTS AND ATOLLS 



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The Pratt- Welker group of guyots in the Gulf of Alaska, however, has relatively 

 concordant peaks at a depth of 700-800 m ; and the guyots among the Austral 

 Islands are concordant at about 1500 m. 



Individual guyots have the sizes and shapes that would be produced by 

 planing off existing volcanic islands. The first random crossings of the "flat" 

 tops indicated that the center was flat and that the margins sloped gently. This 

 has not been confirmed by surveys. Instead the entire top slopes gently to a 

 point in the center or a in places few pinnacles rise from near the center. The 



Fig. 4. Pratt Guyot in the North Pacific. 



topography, in short, is exactly the same as that of an insular shelf except that 

 the islands have been eroded away. Islands have terraces in their submarine 

 slopes in some j3laces, and random echogram crossings have suggested to 

 various writers that similar terraces have been cut in the sides of guyots. 

 Detailed surveys show that terraces that can be traced around a seamount or 

 guyot are very rare at best. The broad sound cone of an echo-sounder normally 

 causes a pinnacle on a steep slope to appear as a terrace. Consequently, many 

 so-called "terraces" may be parasitic cones. In any event, any conclusions about 

 changes in sea-level suggested by unsurveyed terraces appear unwarranted. 



