organic in origin. There were no gravels, and animal shell remains them- 

 selves build up the bottom substratum where the currents are greatest. 

 The tongue of hard bottom deposits extends from the harbour out to 

 North Head. There are also lens-shaped hard areas outside the harbour, 

 amidst the mud, and these again are attributable to increasing current 

 velocities. 



Dr. van Baren referred to detailed mapping of the Sunda Sea and the 

 southern part of the China Sea. Sampling of the bottom had revealed, 

 he claimed, hardl}'^ any connection between sea movements and bottom 

 material. We might expect the east to west current strongl}^ developed 

 in the Java Sea to affect the distribution of sediments, but no such result 

 was to be found. It was agreed that restudy of the area in light of 

 Dr. Shepard's conclusions would be valuable. 



Dr. Shepard asked whether it was not true that there was some 

 amount of coarse material in the Sumatra- Java Strait. It was stated 

 that muddy sediments predominate. Dr. Emery observed also that the 

 sediments are probably not all detrital, which would complicate the 

 matter. 



Dr. Kuenen stated that the results tended to confirm that the classical 

 grading with fine sediments outermost does not hold good on the conti- 

 nental shelf. He asked whether waves with a churning action at higher 

 velocity had been considered as well as slower-moving currents. It was 

 answered that this had been thought of, and evidence was cited from 

 California, where detailed work had been carried out on wave effects. 

 The sediments off-shore were very coarse, as the depth was too great for 

 wave action. A programme was under way at Scripps Institute on 

 currents and waves at different depths. 



THE WHITE ISLAND TRENCH: A SUBMARINE GRABEN IN 

 THE BAY OF PLENTY. NEW ZEALAND 



By C. A. Fleming, New Zealand Geological Survey 



An expedition of the New Zealand Department of Scientific and Indus- 

 trial Research to White Island in January, 1947, stimulated interest in 

 the characters of the sea bottom adjacent to this active volcano. Isobaths 

 drawn on Admiralty Chart No. 2527 revealed a well-defined trench 

 (Fig. 1) extending north-east from the Bay of Plenty coast. The name 

 " White Island Trench " is proposed for this steep waUed depression 

 which has a width of about 7 miles, measured from the summits of its 

 walls, and a depth, below the general level of the continental shelf, of 

 about 3,000 ft. The trench can be traced clearly from the 75 fathom 

 isobath to a depth of 850 fathoms at a point 15 miles north-east of White 

 Island. This volcano rises to a height of 1,000 ft. on a ridge which ex- 

 tends seawards from the Rurima Rocks, to delimit the west side of the 

 trench. There is a fall of 5,000 ft. in 5 miles from the top of White 

 Island to the bottom of the trench. Under such conditions in an area 

 of active seismicity, submarine avalanches are likely, and may account 

 for in-egularities in the walls of the trench. 



Shorewards from the 75 fathom isobath, recent sediments doubtless 

 obscure the trench, but its persistence to the south-south-west is attested 

 by tlie known structure of the Whakatane district, where Macpherson 

 (194^) has mapped a narrow graben occupied by the deep alluvium of 



210 



