Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/166

140 the average depth of the Pacific as 4028 m., of the Indian as 3897 m., and the Atlantic as 3332 m. A faithful picture of these depth relationships is also given by the distribution of the oceanic sediments (Fig. 24), to which Krümmel personally drew my attention some time ago. The red deep-sea clay and the radiolarian ooze, both real “abyssal” (deep-sea) sediments, are confined essentially to the Pacific and the eastern Indian Oceans, whilst the Atlantic and the western Indian Oceans are covered with “epilophic” deposits, the greater lime content of which is causally connected with the lesser depth of the sea. That these differences in depth are not accidental, but are systematic, and that they are connected with the distinction between Atlantic and Pacific types of coast, is best shown by the Indian Ocean, of which the western half has an Atlantic and the eastern half a Pacific character. For here the eastern portion is also considerably deeper than the western. These facts have a special interest in connection with the displacement theory, for a glance on the map plainly shows that it is the most ancient oceanic floors which have the greatest depths, whilst those which have been uncovered for the first time in relatively recent time show the smallest depths. Thus on Fig. 24 we see in a surprising manner the trace of the displacements.

The reason for these differences of depth might lie naturally in the different specific gravities of the recent and ancient sea-floors. It is conceivable that the composition of the sima has altered during the course of the earth’s history through the crystallizing out of certain components, or from other causes. It recalls, for example, the mineralogical difference of old and recent eruptive rocks, and also the distinction of the present-day Atlantic and Pacific lavas. But, according to this, it might well be expected that the