ECHO-SOUNDING AND H YDROG R APHIC AL STUDIES 35 



uncertain to that extent, it is possible that our sounding was made in the 

 immediate vicinity of her actual station. On the other hand, she may have 

 been further north, in which case we failed to strike it on our cruise. There 

 is thus no basis for cither confirming or invalidating the Cape Johnson'?, 

 sounding, so that we must assume the existence, cither inside or just out- 

 side our trawling area, of a restricted pocket of 10,497 m^'trcs, whi(h thus 

 remains the greatest known depth of the Philippine Trench. However, 

 since the discovery of depths of 10,863 metres in the Mariana Trench, 

 by the British Challenger Expedition in the autumm of 1951, the Philip- 

 pine Trench has lost its supremacy as the greatest ocean deep. 



The eastern slope of the northern and central portion of the Philippine 

 Trench rises steeply from the bottom for the first 1,000 — 1,500 metres, 

 but then, immediately to the east, falls again by 500 — 700 metres to a 

 new valley parallel with the main valley. The greatest depth measured 

 by us here, opposite the great depths in the main valley, was about 9,500 

 metres. Further south we had no opportunity of exploring it; but our 

 southernmost section, at about latitude 9° N, shows the valley as having 

 no cliff to the east but gi\ing place immediately to a lc\cl floor at a depth 

 of 8,500 — 9,000 metres. 



A comparison of all the sectors obtained by the Galathea Expedition 

 gives the general picture that, though the stretch of the Philippine Trench 

 between latitudes 12^^50' N and 9° N is one coherent and extensive de- 

 pression of the ocean bed, it falls into several sections. Southward to lati- 

 tude 10° 45' N it seems to proceed regularly, its depth steadily increasing 

 from 8,000 metres to 10,000 metres. At this point irregularities in the 

 eastern slope cause a sudden contraction of the valley, but then follow 

 the great depths of the central valley. Further south come fresh irregula- 

 rities and a stretch less than 10,000 metres deep. Between lalitudes 10° N 

 and 9° N we have two areas where the trench widens out a little and 

 where it just passes the 10,000-metre mark, the more northerly of the two 

 areas being at about the position of the Emden. 



This picture of the topography of the Philippine Trench, based on the 

 Galathea s echogram, conforms extremely well to the mode of its forma- 

 tion. Like other trough faults along the fringe of the ocean, it originated 

 at the same time as the inland mountain ranges, and in association with 

 volcanic eruptions and earthquakes. There are still 20 active volcanoes 

 in the Philippines and, on an average, between one and two recorded 

 earthquakes annually with their centres along the western edge of the 

 Philippine Trench. The terraced structure of the trench sides is due to 

 displacements in the earth's crust along lines of fracture. That such dis- 



