AND ITS INHABITANTS 39 



THE ORIGIN OF OCEAN BASINS 



The relations of crustal density to ocean basins. The fluid 

 earth had a surface as level as the ocean, and the process of 

 solidification which has been outlined does not account for 

 those marked variations in density and in surface form which 

 are expressed by the outer crust of the solid earth being divided 

 into continents standing high above the ocean floors. A sketch 

 of the formative period is therefore not complete unless the 

 processes are briefly discussed which are thought to have 

 shaped the earth's surface, giving rise to the existence of lands 

 even before the period of the oldest known rocks. 



Reasons will be given below for holding that the ocean 

 basins have been formed by subsidence of broad areas of the 

 crust, owing to the weight of magmas of high specific gravity 

 rising widely and in enormous volume from a deep core of 

 greater density into these portions of an originally lighter 

 crust. This regional subsidence was especially characteristic 

 of primordial times, but the process did not wholly cease then; 

 since certain lines of evidence suggest that some ocean basins 

 have been extended in later geologic ages, breaking into once 

 wider continental platforms. The resultant increase in the 

 , volume of the ocean basins has led to a drawing off of the 

 ocean waters from the continental areas, and a marked diminu- 

 tion of the shallow seas of earlier ages. 



The cause of the continued generation of new bodies of 

 molten rock in the sub-crustal shell, adequate to account for 

 the observed results of later geologic time, is thought to lie 

 in the slow accumulation of heat from radioactivity in these 

 depths below the crust. This is discussed in the subsequent 

 topic on the rise of basic magmas. 



Some very thick bodies of intrusive rock are observed to 

 be more dense and basic below, lighter and more siliceous 

 above. The lower part is a gabbro, whereas the upper may 



