AND ITS INHABITANTS 43 



In order to reestablish isostatic equilibrium such a region must 

 subside. 



Most of the igneous rock of later geologic ages which has 

 been intruded into the outer continental crust clearly has not 

 increased the density sufficiently to produce a foundering and 

 would appear therefore either to have come from somewhat 

 higher levels or to have risen in lesser quantity. In some 

 regions, however, as in that of the Lake Superior basin, large 

 masses of basic magma do seem to have overweighted the crust 

 in an early geologic period and produced a tendency to settle 

 as a basin. The same effect may have taken place to even a 

 larger degree in some regions of notable subsidence, as in the 

 Mediterranean basins. In the earliest times, following the 

 solidification of the earth, the forms and relations of the ocean 

 basins suggest that dense molten matter from the depths of 

 the earth broke into or through the outer crust, on a gigantic 

 scale, eruption following eruption until the widespread floods 

 had weighted down broad areas and caused their subsidence 

 into ocean basins. 



As seen in the lava plains of the moon, such an action, once 

 started at a certain point, is conceived to have gone forward 

 with widening radius, leading to the origin of the many rudely 

 circular outlines characteristic of the ocean basins. The 

 process left great angular segments of the original lighter crust 

 as continental platforms standing in relief between the coales- 

 cent basins. The waters gathered into the basins and the 

 continents emerged. 



The Reign of Surface Processes and Beginning of the 



Archean 



It IS possible that shallow ocean basins began to form nearly 

 as fast as the waters gathered, tending to maintain some land 

 areas above the level of the primordial sea. Or the lands 

 may have emerged later, as the ocean basins spread and deep- 



