to the nature of the ocean deeps.[1] They are, with a few exceptions of probably different formation, always opposite the outer (convex) side of the island festoons, where the latter impinge against the old oceanic floor, whilst on the inner side, where the newly exposed floor crops out, window-wise, a deep is never found. It appears as if only the old ocean-floor is in a position to form deeps, because its cooling and hardening extend down to a greater depth. Perhaps they may be considered as marginal rifts, one side of which is formed of the sial of the festoons, and the other of the sima of the deep-sea floor. The profile of this area, shown in Fig. 26, in reality very flat, need not lead one astray, for naturally it is strongly levelled down by gravity.
_-_figure_26.jpg)
Fig. 26.—Section through the Yap Deep (after G. Schott and P. Perlewitz), exaggerated five times.
(The upper interrupted line shows the true proportions.)
The origin of the deep channel, bent at right angles, south and south-east of the island of New Britain (Neu-Pommern) obviously depends on the violent dragging of the island towards the north-west, as a consequence of its adherence to New Guinea; the island block, reaching down to a depth of 100 km., ploughs up the sima, which, flowing behind, has not yet quite filled the furrow. This is perhaps just the case in which we can give the most accurate account of the formation of an ocean deep.
Still another explanation seems to be possible for the Atacama Deep west of Chile. If we bear in mind
- ↑ The term “ocean troughs” (deep sea graben) is less fortunate, since it implies the assertion that they are similar to the trough faults of the continental blocks.
