80 THE FLOOR OF THE OCEAN 



the sialic islands to say about the nature of the earth-shells? 

 From Chapter I we recall that the islands of the second kind 

 are partly or wholly made of granite or other rock character- 

 istic of the continents. These relatively small bits of dry land 

 appear as (i) lonely individuals, or (2) in the form of wide 

 archipelagoes, or (3) arranged in long, distinct lines or belts. 

 Each of these three classes has its own problems, now under 

 discussion by geologists. Because of limited space we shall con- 

 centrate attention on two sets of the islands and shoals belong- 

 ing to the third category — islands in linear arrangement. One 

 set can be traced more than 2500 miles through the East Indian 

 archipelago; the other for about the same distance through the 

 West Indian archipelago. The significance of these two belts 

 for the student of planetary mechanics was not guessed until, 

 within the last dozen years, Professor Vening Meinesz and 

 other investigators inspired by him reported on measurements 

 of gravity in the two regions. 



Information is most abundant about the East Indian case, 

 to which our chief attention will be given. The dry lands of 

 this extensive region make a complex map, and an unusually 

 dense network of soundings has proved the relief of the sea 

 floor to vary in comparable degree. Probably no equal area of 

 the globe is so "accidented." It is natural that geodesists, seek- 

 ing the best figure of the earth, should wish to know how the 

 shape of sealevel is affected by the positive gravitational attrac- 

 tion of the island masses and also by the negative attraction of 

 deep, water-filled hollows. Examples of these hollows are the 

 Banda, Celebes, and other "mediterranean" sea-basins. Pro- 

 fessor Vening Meinesz undertook the gigantic task of measur- 

 ing the intensity of gravity in a submarine, successively located 

 at hundreds of selected points in the East Indian waters. 



From each measured value of gravity at the depth of the 

 submarine there was computed the value at sealevel, imme- 



