FIRE, ROCK, AND SEA 23 



must have displaced a considerable volume of water and so 

 brought about a rise in sea level. The enormous weight of the 

 sediment also played its part in lowering the land by depress- 

 ing the margins of the continents into the underlying basalt, 

 which even today is still plastic. A further process was also 

 taking place, during at least some of these periods. The im- 

 mense amount of water locked up in glaciers was released as 

 they melted during the later times of subsidence, and this 

 alone was sufficient to raise the water level by as much as 250 

 feet. The result of these different processes was a widespread 

 raising of the water level relative to the land, so that the ocean 

 waters invaded the continents and formed shallow seas well 

 inside the former continental edges. 



The records of these changes are to be found today in the 

 rocks themselves. Since we live in a geological age when the 

 continents are all well above water, we are able to inspect a 

 great part of the rocks formed by earlier periods of sedimen- 

 tation. Not only are we able to discover something of the long 

 history of alternating mountain building and inundation by a 

 study of the sedimentary rocks, but the ancient marine sedi- 

 ments also trace out for us the old margins of seas. These 

 strata are not, of course, continuous from continent to conti- 

 nent. The marine sediments of land origin were laid down 

 only in the shallow seas and on the continental margins. It is 

 their very absence from the deep sea floor that leads us to 

 believe that the greater part of the bed of the Ocean River, 

 the deep Atlantic basin, has not in recent time formed part of 

 the continents. 



Lack of continuity of sediment layers across the ocean floor 

 cannot prevent us from linking together in a proper time 

 sequence the events that have taken place on both sides of the 

 ocean. The use of radioactive methods of measuring the age of 

 rocks has already been mentioned. But life in the ocean is the 

 main key to the synchronization of these long-past events, for. 



