ASSUMPTIONS INVOLVED IN THE DOCTRINE OF ISO- 

 STATIC COMPENSATION, WITH A NOTE ON 

 HECKER'S DETERMINATION OF GRAVITY AT SEA 1 



WILLIAM HERBERT HOBBS 

 University of Michigan 



CONTENTS 



Introduction 



A Rigid versus a Plastic Earth Shell 



Pratt's Hypothesis 

 The Hayford Conception of Isostatic Compensation 



Scope of Dr. Hayford's Investigations 



Methods and Assumptions within the Field of the Exact Sciences 



Hayford's Negative Argument for a Failing Earth, Based upon Data Now 

 Shown to Be Inapplicable 



His Positive Argument and the Facts upon Which It Is Based 

 Hayford's Fundamental Assumption Concerning the Distribution of 



Mass within the Earth's Surface-Shell 



Lack of Knowledge Concerning Distribution of Mass beneath the Earth's 

 Surface 



Preponderant Effect of Near Masses Due to the Law of Inverse Squares 



Hayford's Explanation of Anomalies Found in Systematic Regularity as 

 Contrasted with Local Irregularity in Distribution of Mass 

 Evidence of Local Irregularities in Distribution of Gravity and 



Magnetic Constants 



Evidence from Russia and from Southern Italy 



Evidence from India 



Mutual relationships of Abnormal Gravity, Abnormal Earth Magnetism, 

 Dislocations, and Seismicity 



1 Criticisms of the conception of isostatic compensation from other viewpoints 

 than those here presented are those of Professor T. C. Chamberlin (" Diastrophism 

 and the Formative Processes," Jour. GeoL, XXI [October-November, 1013], 577-87; 

 November-December, 1913, pp. 673-82, and succeeding numbers); and those of Pro- 

 fessor Joseph Barrell ("The Strength of the Earth's Crust; Part I," ibid., XXII 

 [January-February, 1914], 28-48, and succeeding numbers), which appeared after its 

 formulation. A sharply critical mathematical discussion of Hayford's trial hypothesis, 

 used in fixing the depth of complete compensation, had already been published (Har- 

 mon Lewis, "The Theory of Isostacy," ibid., XIX [191 1], 603-26). Hayford's 

 rejoinder appeared in the Journal of Geology in 191 2 (XX, 562-78). 



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