288 Nature and Bearings of Isostasy. 



in geologic thought, and in this summary attention "will 

 be given to these views alone. 



Suess developed, without regard to the theory of isos- 

 tasy, his theory of continental fragmentation as a pro- 

 cess continuing through geological time. In fact he 

 formulated his views before the very word isostasy had 

 been coined and long before the quantitative data prov- 

 ing it were available. These data indicate that a conti- 

 nental platform cannot break down broadly into an ocean 

 basin unless there has been a previous or accompanying 

 increase in density in the lithosphere. Such an increase 

 in density might be made locally by the rise of great 

 masses of basic or ultra-basic magmas, but there is no 

 independent geologic evidence that this has occurred on 

 the scale demanded by the theory of Suess, nor that it 

 could be effective on such a scale. The possibility of 

 broad density changes cannot, however, be excluded. 

 Therefore if the geological evidence of continental 

 fragmentation should become regarded as compulsory, 

 the evidence that isostatic compensation is mostly re- 

 stricted to the outer fiftieth of the earth's radius and 

 that large sections of the crust lie at a level giving 

 approximate isostasy would go to show that density 

 changes otherwise unsuspected have progressively gone 

 forward in the lithosphere through geological time. Thus 

 the two conditions of fragmentation and isostatic com- 

 pensation may conceivably be reconciled by the existence 

 of a third and otherwise unsuspected condition of changes 

 of density in the lithosphere. 



Let us turn next to Chamberlin's views on the origin 

 of continents and ocean basins. He has advanced the 

 hypothesis that the continents have been built up from 

 lighter material segregated by surface processes during 

 the growth of the earth by planetesimal infall. In ac- 

 cordance with this mode of origin he holds to a view 

 that the earth is composed of a limited number of large 

 and strong conical sectors, the bases of the cones being 

 the ocean basins, the apices lying deep in the central 

 core of the earth. Diastrophism, under the terms of 

 this hypothesis, occurs by means of movements of ad- 

 justment of these cones with respect to each other and 

 a mashing of the weaker continental sectors which con- 

 stitute yield tracts between them. The continental sectors 

 are assumed to be in approximate hydrostatic equilib- 



