were recorded automatically on a tape, and soundings of greater depths 

 were obtained bj^ the eye-ear method every hour. Because almost all of 

 the ocean bottom traversed is deeper than 2,000 fathoms, a continuous 

 automatically recorded profile was obtained of only a few topograpically 

 positive features such as the tops of seamounts or escarpments. In 

 this report a few features displayed on the " Henderson " fathogram are 

 reproduced and discussed because they are of special geological interest. 



II. Seamounts 



A number of new seamounts were discovered during the San Diego 

 to Antarctic passage.* Four were found in the abyssal ocean off Baja 

 California ; an especially large seamount was discovered in the south- 

 west Pacific off New Zealand. 



Figure 1a shows two symmetrical seamounts that were located about 

 300 miles south-west of Allaire Bank, off Baja Cahfornia. The larger 

 seamount rises to a sharp peak at about 860 fathoms and the smaller 

 rises to a peak at 1,400 fathoms. The sides of these seamounts are 

 fairly steep, having a maximum average slope angle of 19"( M . Extension 

 of the bottom echo when crossing the peaks indicates steep slopes 

 parallel to the ship's track and suggests that the vessel " side- 

 swiped " the seamounts rather than passing directly over their highest 

 peaks. 



Another seamount (Fig. 1b) was discovered off Baja, California, 

 about 130 miles north-west of Allaire Bank, rising 1,400 fathoms from 

 the abyssal sea-floor to a peak at 900 fathoms. The north side is 

 irregular and concave and has a slope of 12° ; the south side is straight 

 and featureless and has a slope of 16°. The summit is pinnacled and 

 shows no evidence of terracing or truncation. 



The seamount shown in Fig. Ic rises to a sharp peak at 540 fathoms. 

 This symmetrical feature was discovered 220 miles west of Alijos Rocks, 

 off Baja, California. Its slightly irregular and upwardly concave sides 

 have an average slope angle of about 12°. The terrace which is present 

 on the north slope at about 800 fathoms may be the result of wave 

 erosion, but such a wave-cut origin is very questionable because echo 

 extension suggests that this terrace is of rather limited horizontal extent 

 normal to the recorded profile. 



Another seamount (Fig. Id) was crossed near Hiva Oa, in the 

 Marquesas Islands, the presence of which is suggested by soundings 

 on published charts. The flanks of this feature have a slightly concave 

 form. The steeper northern side has an average declivity of 13°, with 

 a maximum slope of 21° near the summit. The somewhat gentler 

 south flank is marked by a series of pinnacles. A deep and irregular 

 plateau extends to the north-east of the seamount. Some of the atolls 

 of the Marquesas Group presumably rise from this plateau. 



(1) In this report the apparent slope angles are corrected assuming a 30° 

 effective half-angle of the sound-beam. In other words, the sound-beam is 

 assumed to be semi-directional. However, the effective half-angle of the beam 

 cannot be accurately determined since it \aries with numerous factors such as 

 gain setting, roll of the ship, depth, &c. This correction is significant only for 

 slopes greater than 15°, and the true slope is always greater than the apparent 

 slope. The apparent slope is obtained by correcting the fathometer tape-slope 

 for vertical exaggeration. 



336 



