INTRODUCTION 251 



periods of general planation, while more nearly attained during epochs 

 of continental uplift and mountain growth, such as the present is, for 

 instance. In the long retrospect of the history of the lithosphere, states 

 of strain due to notable departures from isostatic equilibrium have been 

 more prolonged than the alternating states of approximate equilibrium 

 relatively free from strain. 



Provisionally, for convenience of discussion, the depth of the zone of 

 compensation is taken as in the neighborhood of 120 to 150 kilometers 

 (75 to 100 miles), as indicated by the studies of Barrell, following 

 Hayford and Schweydar.- In the discussion referred to, Barrell gave 

 definite form to the concept of a zone of weakness, which he called the 

 "'asthenosphere,'' and which should underlie the zone of compensation. 

 Gilbert had previously discussed the "horizon of mobility."^ Chamberlin 

 and Lunn had shown how such a zone might result from the outflow of 

 internal heat and its concentration at a level where the constant pressure 

 would be overcome by the rising temperature and local fusion would 

 result.^ 



I accept the concept of the asthenosphere as an inevitable consequence 

 of the conditions postulated by Chamberlin. I prefer, however, to use 

 round figures, such as 100 miles, to express the probable depth at which 

 the zone of compensation passes into the upper part of the asthenosphere, 

 because a precise figure, such as 122 kilometers, for example, gives an 

 idea of definiteness to a transition zone, which must be by its very nature 

 indefinite and variable. 



For the purpose of this discussion, then, the lithosphere is taken to be 

 a shell of solid, rigid, elastic crystalline rock of indefinite depth. It com- 

 prises : 



(a) An outer shell, approximately 120 to 150 kilometers thick, the zone 

 of isostatic compensation or the isostatic shell. 



(b) A zone of local and temporary, but periodic, fusion, extending 

 from the bottom of the isostatic shell to a depth of at least 1,000 kilo- 

 meters, more or less, and equivalent to the asthenosphere of Barrell. 



Within the lithosphere, as above defined, there is a mass which it is 

 convenient to call the centrosphere, in order to indicate that nothing is 

 postulated as to the state and constitution of the interior of the earth 



2 J. Barrel! : The strength of the earth's crust. Chicago Jour, of Geol., vol. xxii, 

 1914, p. 680 ; also Ibid., vol. xxiii, 1915, p. 428. 



' G. K. Gilbert: Interpretation of anomalies of gravity. U. S. Geol. Survey, Profes- 

 sional Paper 85 C, 1913. pp. 34.35. 



*T. C. Chamberlin: Geology, vol. 1, p. 629. Hypothesis of 1909. 

 H. C. Lunn : Geophysical theory under the planetesimal hypothesis. Carnegie Insti- 

 tution of Washington, Publication 106. 



