FOUNDATIONS OF THE GREAT DEEP 5 



tools; he must probe the rocky layers on which the ocean rests. 

 His interpretation of the dry lands has taxed his will and 

 skill; all the greater should be courage and technical efficiency 

 as he continues to explore his earth under water. 



Only thirty years ago geologists were baffled in the effort 

 to demonstrate the mechanical conditions below the salt-water 

 cover. Since then, however, special methods of diagnosis have 

 been invented and applied. Some of the results are described 

 in the following pages, which attempt to tell the complex story 

 in words perhaps not unbearably technical. First, the composi- 

 tion and thickness of the earth's "crust" under the veiling 

 ocean will be considered. The second chapter will deal with 

 mountain structures now known to feature the sea floor. The 

 third and final chapter will specialize on the sloping, sub- 

 merged flanks of the plateau-like continents, and on the sys- 

 tems of great valleys shown to make those "continental slopes" 

 rugged in marvellous degree. The investigation of the ocean's 

 bed is still in its initial stage; yet already the gains for science 

 are so spectacular and so full of meaning that even a report of 

 progress can have value. That report will illustrate the dra- 

 matic quality of ocean geology, and also the genius of the men 

 who have been using the new methods in a scientific recon- 

 naissance of two thirds of the globe. 



A summary of the detective methods and results may well 

 begin with the problem of the rocky structure on which the 

 open, truly deep, ocean rests. Logically that underpinning 

 extends down 4,000 miles, all the way to the earth's center, 

 but we shall see that in general our geological inquiry need 

 reach down no more than 100 miles from the surface. After 

 learning some figures about the size of the watery envelope, 

 we shall review the recent studies on the sub-oceanic part of 

 the solid skin of the earth. A composition in decided contrast 

 with that of the remaining, continental, part will be indicated. 



