62 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



Bahamas, in the gravity profile West of Cuba, 

 at the West coast of America between Panama and 

 San Francisco, and in many places in the Nether- 

 lands East Indies. The basins in the Bahamas 

 are an exception to this rule. 



Instances of the first rule are found in the Nether- 

 lands East Indies, where a narrow strip of strong 

 negative anomalies is found, bordered on both 

 sides by fields of positive anomalies and where 

 other strips of slighter negative anomalies can 

 likewise be detected. The same disposition is found 

 in the West Indies, where the Gulf of Mexico, the 



Caribbean and the sea West of Cuba show positive 

 fields while negative strips are found North of 

 Porto Rico and Haiti, West of N. W. Cuba, near Gr. 

 Cayman Island, near Jamaica and in the inland 

 seas of the Bahamas. A third instance has been 

 stated in the investigated part of the Atlantic, 

 where an extensive field of positive anomalies was 

 found, interrupted by strips of lesser anomahes near 

 the Azores and in a few other places. 



We shall consider these rules again at the end of 

 this report. 



THE GRAVITY RESULTS IN THE NETHERLANDS EAST INDIES AND 



ADJOINING REGIONS 



Because of the world-crisis the results have not 

 yet been supplemented by gravity research on the 

 islands but the field is already complete enough 

 for drawing conclusions and these conclusions are 

 important for many problems of the Earth's crust. 



The results show strong deviations from isostasy 

 of which the main feature was mentioned above: 

 a strip of great negative anomalies (maximum — 

 204 mgal; mean about — 100 mgal) of a width of 

 only 50-100 miles, running through the whole 

 archipelago and bordered on both sides by fields of 

 positive anomalies (maximum -|- 166 mgal, mean 

 about -f 45 mgal). Considered as a whole, the 

 region is nearer to isostatic equilibrium; the mean 

 of all the anomalies is -|- 20 mgal. 



The correlation of the strip with the distribution 

 of earthquake-centers confirms the obvious supposi- 

 tion that it is connected with the tectonic action 

 in the crust. Taking this for granted, it gives a 

 valuable indication of the course of the geosyiicline, 

 that is considered responsible for this action. 

 It proves that the Alpine-Himalayan geosyncline, 

 which is knowii to continue through Malacca and 

 Sumatra, does not go on through New Guinea, but 

 that it bends Northwards in the Eastern part of the 

 Archipelago and that it continues in the Pacific 

 geosyncline running along the east coast of Asia. 

 It proves likewise that the Australian continent does 

 not play the prominent part in the tectonics of this 

 region that the supporters of the Wegener hypothe- 

 sis of migrating continents think; instead of the 

 anomalies being more intense, where the strip is 

 bordering on this continent, as it ought to be 

 expected in the light of this theory, they are just 

 as pronounced where the strip is bordering on the 

 Indian or the Pacific Oceans. If we follow the 



geologists in considering this geosyncline as a region 

 of strong lateral compression of the Earth's crust, 

 we may draw the important conclusion that the 

 crust under these parts of the oceans offers the same 

 resistance to the compressional stresses as the Aus- 

 trahan continent. 



The strip shows only in a slight degree some 

 dependence on the topography and this dependence 

 corresponds to the second rule, the strip is mostly 

 coinciding with a submarine ridge, while the 

 positive fields coincide with the deeper parts. The 

 strip shows often a correlation with the deeps but 

 it is besides them, as it is for instance the case for the 

 Java Deep and the Weber Deep. The fact that the 

 anomalies for those parts of the strip that are near 

 deeps are not stronger than for other parts, clearly 

 indicates that deeps are not independent features, 

 but that they are accompanying features of much 

 greater phenomena. Gravity surveys of deeps will, 

 therefore, have to encompass wider areas than the 

 deeps themselves. 



The fact that the strip is not coinciding with the 

 deeps but mostly with submarine ridges, prevents 

 explaining it by incomplete compensation of surface 

 features. It is neither acceptable to explain them 

 by assuming recent surface movements that are not 

 yet compensated ; the course of the strip makes this 

 assumption unlikely as it would assume the down- 

 ward pressing of the ridges. So the only explana- 

 tion that is left is to assume abnormal light masses 

 in the upper layers of the Earth. 



It will, however, hardly be feasible to locate this 

 whole mass-defect in the sialic layer, because we 

 should then have to assume densities that are 

 too small for being acceptable. For a great part 

 at least we shall have to ascribe it to a protuberance 



