280 B. WILLIS DISCOID AL STRUCTURE OF THE LITHOSPHERE 



DEPARTURE FROM AVERAGE DENSITIES 



Considering the major differences in density of the lithosphere, we 

 contrast the nnderbody of a continent as a whole with that of an ocean 

 as a whole. Minor differences also are recognizable, and, according to 

 the maps, the separate masses within a continent are lighter bodies lying 

 in a somewhat heavier general mass, whereas the distinctive elements 

 beneath the oceans are heavy bodies lying in a somewhat lighter general 

 mass. The details of form are interpretations of partial knowledge and 

 are subject to change, but the relations seem to indicate that there is a 

 continental underbody of average density for the continent and an 

 oceanic underbody of greater average density for the ocean, while within 

 each there are masses which depart from the average toward greater 

 lightness on the continent and toward greater density beneath the ocean. 



It is obvious that where isostasy is imperfect the resultant stresses set 

 up in the lithosphere will be of moderate degree in the marginal zones 

 of juxtaposed masses of average continental density and average sub- 

 oceanic density, whereas greater stresses will develop where extremes of 

 density characterize neighboring masses. 



The preceding statements deal with the horizontal distribution of 

 densities. The vertical distribution, which is equally important, has 

 been discussed in a preceding section. Geologically, any assumption of 

 uniform densities or of uniformly varying densities from the surface 

 down is unreal. The igneous rocks within reach of observation vary 

 widely in density. The average density of the earth as a whole greatly 

 exceeds the average density of surface rocks. The processes of igneous 

 intrusion and extrusion tie the superficial masses of rock to their deep- 

 seated sources. The heterogeneity exhibited in the visible masses must 

 characterize all the shell which has been subject to igneous activity, and 

 that shell certainly comprises not only the zone of compensation, but also 

 the asthenosphere, or zone of fusion. We cannot avoid the conclusion 

 that vertical heterogeneity exists to great depths. 



DIRECTION OF ELASTIC STRESS 



We turn to the consideration of elastic stresses which are produced in 

 the lithosphere by the weight of the rocks themselves. A familiar treat- 

 ment of the subject, based on the assumption that isostatic equilibrium 

 is initially perfect and extended to the condition of imperfect equilib- 

 rium, is given by Barrell.^^ He, however, introduces an artificial condi- 



37 J. Barrel! : strength of the earth's crust. Chicago Jour, of Geol., vol. xxii, 1914, 

 pp. 655-670. 



