278 B. WILLIS — DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



elements, the Colombian, Guianan, Brazilian, and Patagonian, with sur- 

 rounding negative elements. The Andes may represent a crushed posi- 

 tive element or elements. Europe, Africa, and Australia exhibit group- 

 ings of areas vs^hich have been either deeply buried or deeply eroded, and 

 may be interpreted accordingly in terms of heterogeneous structure. 



Thus, on geologic evidence it appears conclusively that the structure 

 of continents is complex as to the distribution of density in the under- 

 body.^^ The magnitude of masses of approximately like density through- 

 out each mass ranges from one hundred to many hundred kilometers, 

 measured horizontally. The least horizontal dimension is c'etermined, 

 probably, not altogether by variations of density, but chiefly by the bend- 

 ing or shearing moment of the slab or beam which can support the 

 gravitational strain to which its own weight and any excess load or de- 

 ficiency of load subject it. This least horizontal dimension is a function 

 of the depth or thickness of the slab. Barrell's researches, already cited, 

 appear to indicate that the least horizontal dimension of a mass which 

 may respond isostatically to gravitational stress is between 100 and 300 

 kilometers. Bodies smaller than this would not demonstrate their exist- 

 ence by independent movements which would be recorded by erosion or 

 deposition, though their presence may be detected by observations of 

 gravity. 



In the underbodies of ocean basins the distribution of masses which 

 differ as to density can not be traced on geologic evidence. We are 

 thrown back on the general theory of isostasy and on determinations of 

 the intensity of gravity. Since oceanic waters protect their basins from 

 superficial disturbing activities, except that of deposition in the littoral 

 regions, we might expect that isostatic balance would be more nearly 

 complete throughout oceanic underbodies. It appears, however, as will 

 be argued in a subsequent section, that the great deeps are loci whose 

 underbodies are especially liable to fusion, and that the molten material 

 flows from under them, leaving them occasionally defective as to gravity. 

 Hecker's observations of the intensity of gravity on the oceans appeared 

 to demonstrate that the suboceanic masses are close to isostatic equilib- 

 rium, except for negative anomalies over certain deeps and positive 

 anomalies on volcanic islands; but his methods of observation have been 

 challenged and it is wise to withhold judgment as to their value. They 

 were calculated, furthermore, according to Hayford's assumption of the 



35 The term underhody has come, in the writer's thought, to indicate the mass under- 

 lying any area, not including the superficial veneer of rocks in the zone of clastic defor- 

 mation. It is used to designate the column extending from, say, 10 kilometers helow 

 the surface down to the bottom of the lithosphere. 



