82 THE FLOOR OF THE OCEAN 



in general a strongly positive isostatic anomaly; and that a 

 strongly negative free-air anomaly means a strongly negative 

 isostatic anomaly. 



East Indian Chain. — When the isostatic anomalies were 

 plotted on the map, there appeared a most remarkable "strip" 

 of negative anomaly — that is, deficient mass — all across the 

 region and keeping a fairly constant width of about ioo miles. 

 The actual length of the strip exceeds 3000 miles, for it goes 

 far beyond the limits of the East Indies. The scale of the 

 phenomenon is appreciated when we reflect that the archipel- 

 ago has an over-all area greater than half that of the United 

 States. The map of Figure 46 represents the serpentine strip 

 by shading, the intensity of which is proportional to the size of 

 the negative anomaly. Along the axis of the strip the anomalies 

 commonly exceed 100 milligals. To bring this axial belt, with 

 negative anomaly greater than 100 milligals, into balance with 

 the rest of the earth's crust, about 3000 feet of granite or 

 equivalent matter would have to be added to the belt. 



The astonishing discovery of the negative strip led auto- 

 matically to the question of origin, which became still more 

 intriguing when another was found in the West Indies. A 

 sketch map of it is given in Figure 47. In scale and other essen- 

 tials it is the twin of the East Indian strip. Note the broken 

 line, representing the axis of the strip, winding its way through 

 the complex of islands, shoals, sea troughs and sea-basins.* 



Aided by the geologists of the Netherlands, Professor Ven- 



* It is reasonable to suspect that a third negative strip, also thousands of miles 

 long, would be demonstrated if measurements of gravity were made in the South 

 Adantic region stretching from Patagonia through South Georgia to the South Sand- 

 wich Islands and across to Graham Land of Antarctica. Here there is strong curvature 

 of an island-crowned structure with a pronounced "fore-deep," analogous to the Porto 

 Rico deep of the West Indies and the Java deep of the East Indies. The South 

 Atlantic region further resembles the other two by being subject to heavy earthquake 

 shocks. One of these originated at the center indicated in Figure 9, a point near the 

 South Sandwich fore-deep. Such disturbances are to be expected if, here too, the 

 earth's crust is not in equilibrium. 



