THE MINERAL MATTER OF THE SEA 483 



of rock would remain above sea-level, while about 17,000,000 would 

 have been removed. The deposition of 17,000,000 cubic miles of 

 mineral matter in the sea would raise its level between 500 and 600 

 feet, and such a rise would submerge an area of about 12,000,000 

 square miles of the present land, or more than one-fifth of its area. 

 Considering the much larger area which would be submerged by an 

 equal rise of the sea-level when the lands had been lowered to the 

 extent indicated, there would be a submergence of continents com- 

 parable to those which have repeatedly marked the beginnings and 

 ends of geologic periods. 



The facts (1) that the notable changes in relative level of land and 

 sea have been periodic, and (2) that the sea-level seems to have been 

 lowered at times, and not to have been continually rising, as it would 

 be if affected by sedimentation alone, are probably to be accounted 

 for by crustal warpings. 



In so far as sedimentation has been a cause of subsidence of the 

 ocean bed, the subsidence has probably lagged behind the sedimen- 

 tation. In so far as the sinking of the sea bottom is independent of 

 sedimentation, it is unlikely that it always, or even generally, kept 

 pace with sedimentation. It would appear, therefore, that rise of the 

 sea-level due to sedimentation may have been an important factor, or 

 even, perhaps, a chief factor, in the submergence of the continents, 

 at various periods in the earth's history, while the. unequal rates at 

 which sediment has been transferred from land to sea may have had 

 an influence on the rate at which submergence was brought about. 

 It is highly probable that crustal warpings, by increasing the capacity 

 of ocean basins, have, on the whole, tended to reduce the amount of 

 change of sea-level which sedimentation alone would have effected. 

 Volcanic material extruded into the sea, on the other hand, has 

 worked in the opposite direction. 



Rate of change in relations 0} sea and land. — It is not to be sup- 

 posed that such changes in the level of the sea as those mentioned on 

 p. 482 have actually taken place as the result of sedimentation, for 

 the ocean bed has probably been sinking through the ages, though 

 perhaps not continuously, and probably not at a constant rate, even 

 during periods of sinking. Subsidence of the ocean bottom would 

 tend to counteract the effect of sedimentation, so far as its effect on 



