FIRE, ROCK, AND SEA 33 



Umbgrove and others prefer to think of a more permanent 

 ocean floor; and some scientists think that the ocean floor has 

 folded in somewhat the same way that the continents have. 

 Thus, just as the land was thrown into huge mountain ranges 

 during the great revolutions or periods of uplift, so folds of 

 the deep ocean floor may have been raised to the surface dur- 

 ing the past and have enabled plants and animals to become 

 dispersed along island chains or intercontinental connecting 

 ridges like the Isthmus of Panama. It is unlikely that very 

 great changes have taken place in the total amount of sea 

 water in the oceans over the past hundred million years. If 

 this is so, then large areas of deep-sea floor could not have been 

 raised above the surface without a simultaneous and equally 

 deep submergence of large areas of the continents. But none 

 of the sediments typical of the deep-sea floor have ever been 

 found for sure in the rocks of the present continents, and so 

 there is no evidence that the present continents have ever 

 formed part of the deep-ocean floor. If this is so, then it is 

 unlikely that large and widespread uplift of the deep-sea floors 

 has happened except in the very distant past. 



Hans Pettersson, leader of the Swedish Deep-Sea Albatross 

 Expedition during 1947 and 1948, was able to bring back new 

 evidence obtained with some of the recently invented instru- 

 ments for probing the secrets of the deep-ocean floor. The 

 results were surprising, though they are capable of different 

 interpretations. Underwater depth charges were used in order 

 to study the thickness of sediment layers in deep water. Echo- 

 like reflections of the explosive shot were carefully recorded, 

 so that the time required for them to return to the ship from 

 the sea floor was known. Part of the explosive wave passes 

 through the sea floor into the sediment layer and is then 

 reflected from the underlying rock. The time taken for this 

 second echo to return to the vessel on the surface is used to 

 estimate the thickness of the sediment layer. By this ingenious 



