SKCr. 2] TRENCHES - 419 



Table I — continued 



Aleutian 



7679 (uncorrected, taken with nominal sounding velocity of 1500 m/sec) 



Nansei Shoto (Ryuku) 

 7507 



Java 



7450 (after van Riel, 1934) 



New Hebrides (South) 



7070+20 (this Chapter)" 



Middle America 

 6662 ±10 



" These soundings were taken during Proa Expedition, April-Jvme, 1962, aboard H.\ . 

 Spencer F. Baird. A Precision Depth Recorder was employefl, and the ship's track crosseil 

 over (within the limits of celestial navigation) points from which maximiun depths had 

 been reported. 



^ This is the maximum sounding obtained in the vicinity of the Vitiaz Depth (Udintsev. 

 1959) by French and Japanese vessels in connection with dives of the bathyscaph Archi- 

 mkh, July, 1962. 



future work will modify the ranking very little, except perhaps in the case of 

 the Marianas-Tonga pair, where the spread in recent depth measurements is 

 less than 250 m. As the preceding discussion and Figs. 1 and 2 indicate, more 

 absolute values must await precise ship positioning and the development of 

 powerful narrow-beam sounders with precise timing. 



Recently prepared topographic charts, based largely or wholly on data from 

 continuously recording sounders, include plots of the following trenches : 

 Tonga (Raitt et al, 1955), Middle America (Fisher, 1961), Peru-Chile (Fisher 

 1958), Philippine (Kiilerich, 1959; this Chapter), Kermadec (Brodie and 

 Hatherton, 1958), Kuril-Kamchatka (Udintsev, 1955), New Britain-Bougain- 

 ville-New Hebrides (this Chapter), Puerto Rico (Ewing and Heezen, 1955) and 

 the western portion of the Aleutian Trench (Gibson and Nichols, 1953; Gates 

 and Gibson, 1956). 



The Philippine Trench (Fig. 3) is representative of the deep narrow trenches 

 bordering island chains or small land areas of the western Pacific and associated 

 with all the gravity and seismic characteristics discussed above. In plan nearly 

 straight, its axis lies almost equally distant from the 4000-m contour at the 

 base of the Philippine island chain and from the 6500-m contour marking 

 the lip of the deep Philippine basin to the east. There is no ridge or swell 

 paralleling this trench on the seaward side. The most marked change in trend 

 occurs east of southern Mindanao, where the trench veers southeast but the 

 negative-gravity-anomaly belt associated with it farther north continues 

 southward to the west of Halmahera (Vening Meinesz et al., 1934). A short 

 southwest-trending ridge lies east of the trench in the vicinity of the change in 

 trend. Northward the Philippine Trench ends against a broad area shoaler 



