66 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



according to this view the islands are surface loads 

 on the crust, that are not in local isostatic compen- 

 sation and so the gravity on the islands is greater 

 than over the neighboring basins. 



For further details of the gravity results in the 

 West Indies I may refer to the publications of the 

 two expeditions of the U. S. Navy, that have been 

 mentioned in the introduction. 



The four gravity profiles that have been made, 

 perpendicular to the coast, between Panama and 

 San Francisco all show rather strong positive 

 anomalies over the foot of the shelf and about nor- 

 mal gravity over its top. In two instances this 

 profile can be continued in the continent, in Mexico 

 and near San Francisco. In Mexico this continua- 

 tion shows fairly strong negative anomalies in a 

 strip parallel to the coast and so we get the im- 

 pression that we have here another instance of the 

 same feature : a strip of negative anomalies bordered 

 on both sides by positive anomalies. The profiles 

 perpendicular to the coast look as if they are the 

 outer parts of profiles over this strip. The correla- 

 tion with the earthquake activity appears to con- 

 firm this view. More research will be necessary 

 before we can be sure about it and before we know 

 if this strip is the continuation of the critical zone 

 in the West Indies, which appears to continue 

 through the Bartlett Trough. 



The second profile, over San Francisco, confirms 

 our supposition up to a certain degree, but the strip 

 is less intense than those in the East and West 

 Indies. The greatest negative value, found outside 

 the coast, is only —57 mgal. Another deviation 

 of this gravity profile from the normal profile in the 

 Indies is, that the anomaly remains slightly negative 

 in the U. S. up to a great distance from the coast, 

 instead of showing the positive values found else- 

 where besides the strips. 



After this more detailed discussion of two regions, 

 we may shortly consider again the two general rules 

 mentioned in the beginning. We found both 

 rules vahd in the East and West Indies and we have 

 discussed a hypothesis about what is going on in 

 these parts. If this hypothesis is true we have to do 

 with a buckling towards the inside of the Earth's 

 crust along a curved line and this buckUng brings 



about a thickening of the crust along this line. 

 This is accompanied with strong negative anomalies 

 because of the accumulation of fight crustal ma- 

 terial and in most parts with a submarine ridge 

 because of the outward bulging of the surface layer. 

 We find thus both rules realized by this phenome- 

 non: the coming into existence of strips of negative 

 anomalies and a tendency of the anomaly to in- 

 crease, in a positive sense, when going from smaller 

 to greater depths. 



We have further found that the deep basins in the 

 East and West Indies show positive anomalies and 

 this is also in harmony with the two rules. A well- 

 founded explanation of these positive anomalies is 

 still lacking, but there seems to be little doubt 

 that there is some connection with the tectonic 

 phenomena in these regions. 



We are now confronted with the important ques- 

 tion whether the other instances, where these rules 

 have been found valid, can also be explained in the 

 same way, or, in other words, whether all these 

 instances can be considered to be related to the same 

 tectonic phenomena, active or extinct, that are going 

 on the East and West Indies. Or are there other 

 phenomena, bringing about similar results for the 

 gravity field? 



This important question cannot yet be answered; 

 future research will have to provide the data for 

 attacking this problem. As far as the gravity 

 research is concerned, the following program seems 

 indicated. First the further investigation of tec- 

 tonically active areas for the elucidation of the great 

 geophysical problems of these regions. Secondly 

 the investigation of the distribution over the Earth's 

 surface of the fields of positive anomalies, that 

 occur in some regions, e.g., in the Atlantic and, 

 though less intense, in the Pacific, in order to see if 

 they are of the same kind as the fields of positive 

 anomalies in the deep basins of the East and West 

 Indies. Thirdly, the investigation of the gravity 

 transition for all steep submarine slopes, e.g. near 

 continental shelves and near island coasts. This 

 further research may provide us with important 

 possibilities for the investigation of the Ea/th's 

 crust under the oceans. 



