THE PLAN OF THE EARTH AND ITS CAUSES. 



385 



a quiet period of slow restoration of the spheroidal form, causing the 

 series of marine "trausgressious" which are the dominant feature of 

 the geological history of the Mesozoic era. 



VERTICAL RANGE OF DEFORMATION. 



Reluctance to admit the possibility of such changes is reduced when 

 we recollect how insignificant are the differences in level, when com- 

 pared with the size of the earth. The use of exaggerated diagrams 

 leads to unconscious magnification of the extent of the polar flattening, 

 and of the difference between the continental summits and the oceanic 



Sphere without Polar flattening 



romboli 



Vesw 



jius M£j!lanc 











! Depth of Atlantic 





! Lisbon earthquake 







Depth of Too th. earth's radius 







a 







Fig. 7. 







DIAGRAM OF RELATIVE EXTENT OF INEQUALITIES ON THE EARTH'S SURFACE, a, 

 A TRUE SCALE CURVE OF PART OF EARTH'S SURFACE; b, SECTOR OF CIRCLE, 

 SHOWING RELATIVE SIZE OF ZONE INCLUDED WITHIN a TO THAT OF THE EARTH. 



depths. The study of large-scale maps has been author- 

 itatively recommended. The examination of true-scale 

 curves and outlines may help us to realize the actual con- 

 ditions. The accompanying figure 1 shows a section of 

 the earth's crust from Stromboli to Vesuvius. The thick 

 black band represents the section across the Mediter- 

 ranean; the line a b marks the depth of the Atlantic; 

 the upper curve shows where the surface would be if 

 there were no flattening. The lowest line marks the 

 depth of one-hundredth of the earth's radius. The thick- 

 ness of this zone in comparison with the size of the earth is shown 

 on fig. 7, bj which is a sector of a circle, with the zone of a shown, 

 reduced to its true relative size. The polar flattening is barely recog- 

 nizable, and the difference between sea bottom and mountain summit 

 is marked only by variations in the thickness of a line. 



The diagram illustrates the insignificance of the deformations re- 

 quired ; and that crustal disturbance occurs much deeper than the layer 

 with which the tetrahedral theory is concerned is shown by the fact 

 that the estimated center of origin of the Lisbon earthquake lies far 

 below. 



This diagram also serves to show that the amount of contraction in 

 the earth necessary to allow tetrahedral deformation is very small. 



SM 98- 



-25 



Based on Lingg's Erdprofil. 



