402 B. WILLIS — A THEORY OF CONTINENTAL STRUCTURE 



hypothesis of the conditions to which they owe their positive or negative 

 character. We may, however, frame a hypothesis in accordance with 

 Button's original statement of isostacy,* which runs : 



"If the earth were composed of homogeneous matter its normal figure of 

 equilibrium without strain would be a true spheroid of revolution ; but if 

 heterogeneous, if some parts were denser or lighter than others, its normal 

 figure would no longer be spheroidal. Where the lighter matter was accumu- 

 lated there would be a tendency to bulge, and where the denser matter existed 

 there would be a tendency to flatten or depress the surface. For this condition 

 of equilibrium of figure, to which gravitation tends to reduce a planetary body, 

 irrespective of whether it be homogeneous or not, I propose the name 

 isostacy." 



In his explanations, Button laid emphasis on effects of erosion to a 

 degree which has led to a general oversight of the fact that his primary 

 conception dealt with an earth composed of lighter and denser bodies, but 

 that original idea was clearly his. The misleading discussions of the 

 efficacy of erosion and sedimentation to produce isostatic adjustment 

 should be set aside, that cause being clearly subsidiary and self-destructive 

 where effective at all. Through investigations of the deflection of the 

 plumbline observed in the primary triangulation of the United States, 

 Hayford reaches the conclusion that for the United States and adjacent 

 areas it may be assumed, with a close approximation to the truth, that 

 the earth is in the condition called isostacy. f He uses the term in the 

 sense quoted above from Button. 



Hecker's determinations in the Atlantic and Pacific basins J suffice 

 to show that the intensity of gravity is what it would be on a litho- 

 spheroid of the radius of sealevel; in other words, there is an excess of 

 material beneath the ocean bottoms very nearly, if not exactly, equivalent 

 to the dejDth of the ocean at the point of observation. There are some 

 exceptional results, which do not agree with the rule, but as a whole the 

 observations give very strong support to the theory of the existence of 

 an isostatic balance between the suboceanic and the subcontinental 

 masses. 



In applying the hypothesis of density differences to the distinct parts 

 of the continent, we tread on speculative ground. A firm foundation 



* C. B. Button : On some greater problems of physical geology. Bull. Phil. Soc. 

 Washington, vol. xl, 1889. 



t O. H. Tittmann and ,T. F. Hayford : Geodetic operations in the United States, 1903- 

 '06. A report to the fifteenth general conference of the International Geodetic Associa- 

 tion. Washington, 1906. 



t Bericht liber die Schwericrafts messungen aufdem Meere, O. Hecl^er. Nature, .lune, 

 1904, p. 104 ; Sitzungsbericht, Ali. der Wissensch. zu Berliin, Pebr., 1902 ; Veroffent- 

 lichung des k. preuss. geodiit Instituts, Neue Polge no. 11, Berlin, 1903 ; Bericht zura 

 XV Intern, geodiit Konferenz zu Budapest, 1906. 



