PREFACE. 
The literature of the natural sciences during the past few years exhibits', 
in a remarkable way, the profound influence our fuller knowledge of the 
deep sea has had on all general conceptions concerning the modifica- 
tions the surface of the earth is now undergoing and has undergone 
in past geological times This could not well be otherwise. When- 
ever science is enriched by a large addition of new facts, a change 
in theoretical views invariably follows. No complete theory of the earth 
was possible so long as we were ignorant of the conditions prevailing over 
the three-fifths of the globe covered by the waters of the ocean. It may 
fairly be said that since the discoveries of Columbus, Gama, and Magellan 
in the thirty years from 1492 to 1522, there has been no addition to the 
knowledge of the surface of our planet that can in any way compare with 
that acquired by the Challenger and other deep-sea expeditions during the 
past quarter of a century. 
The difficulties connected with the exploration of the deeper waters of 
the great Ocean Basins arise from the fact that the vast majority of the 
observations are from the nature of the case indirect. At the surface of 
the ocean direct observation is possible, but our knowledge of the conditions 
in deep water, and of all that takes place beneath the surface, is wholly 
dependent on the correct working of instruments, the actions of which are. tor 
the time, hid from sight. A few years ago the apparatus necessary tor the 
