The largest seamount encountered on the cruise (Fig. 2a) is located 

 in the south-west Pacific about 1,000 miles west of New Zealand in a 

 sparsely-sounded region. This feature rises from a sea-floor depth of 

 2,700 fathoms to a minimum depth of 600 fathoms. The summit area 

 is extensive but irregular, so that there is no evidence of wave truncation. 

 The north slope has a declevity of 14°, compared with 21° for the south 

 slope. Both slopes are slightly concave. 



All of these seamounts have an appearance which suggests that 

 they are volcanoes which are probably extinct. Such an origin is 

 suggested by the simplicity and symmetry of form, the slightly concave 

 slopes, and the average angle of slope which is always in excess of 10', 

 but never exceeds 21°. In addition, the seamounts off Baja, California, 

 are located at no great distance from the volcanic Guadalupe Island 

 and in a region of known volcanicity. Yet there is a slight possibility 

 that the seamount shown in Fig. 1b is a south-facing fault escarpment 

 rather than a volcanic mass. The slight asymmetry of the feature and 

 the straightness of the south slope suggests this alternate interpretation. 

 However, although it is difficult in most cases to conceive of any other 

 origin, the volcanic explanation for these seamounts must remain 

 tentative because it is postulated purely on a geomorphic basis. Such 

 criteria for determining the nature of sea-floor features are not well 

 established, particularly when only sounding profiles are available. 



None of the seamount profiles show any good evidence of terracing 

 or summit truncation such as would result from wave 'cutting if the 

 seamounts had stood above or near sea-level during a part of this 

 history. This is in contrast with the discovery by Hess(l) that most 

 seamounts in the west central Pacific that rise to depths shoalerthan 

 1,000 fathoms show summit truncation. If the ocean floor has been 

 relatively stable so that summit truncation is to be explained by a 

 former lower stand of sea level, then these seamounts must be younger 

 than those in the region described by Hess. 



During the Pacific passage the writer was impressed by the paucity 

 of large seamounts. Vast stretches of the ocean showed no sounding 

 less than 2,000 fathoms. Most of the seamounts discovered are 

 located off Mexico at no great distance from the continent. Existing 

 charts indicate numerous seamounts in this region which show that it 

 has had a volcanic or diastrophically active history. Although large 

 seamounts are rare on these fathograms of the deep Pacific from south 

 of the Mexican area to the Antarctic Ocean, except in the Marquesas 

 Islands region, the eye-ear soundings show that the oceanic floor is 

 rarely smooth, but is, rather, generally rugged and irregular. 



III. Easter Island Swell 



A profile across the northermost portion of the Easter Island Swell 

 is reproduced in Fig. 2b. Existing charts such as H.O. 2562 show 

 that this swell has a linear extent of several thousands of miles between 

 Easter Island and the Antarctic. This swell is apparently a system of 

 submarine mountain ranges comparable to some of the large Cordilleras on 

 the continents, and it must be the combined result of folding, faulting, 

 and volcanism. Soundings obtained by the eye-ear method prior to 

 passing over the swell revealed a flat bottom, with no indication of a 

 foredeep. The profile across the swell shows a general absence of concave 



338 



