68o JOSEPH BARRELL 



deficiency. To the westward is a broad region of average negative 

 anomaly reaching a maximum at centers near the Pacific coast 

 and no marked excess is shown near the mouths of the great 

 rivers. Such a lack of regional relations would appear to show that 

 the anomalies are due much more to local loads and irregularities 

 upon and within the lithosphere, and to bowings due to great 

 compressive movements unrelated to isostasy, rather than to the 

 existence of an isostatic gradient leading from the ocean borders 

 to the interior fields of great erosion. Therefore either the idea 

 of strong viscous drag by undertow or else the very doctrine of 

 isostasy — one or the other — must be abandoned. But it has been 

 seen that if undertow exists in a comparatively plastic stratum, then 

 that physical condition will cause it to be the bottom of the zone 

 of compensation. Thus the application of every pertinent engineer- 

 ing principle reduces the initial hypothesis of surface folding by 

 isostatic undertow, and, especially by undertow within the zone of 

 compensation, to an absurdity. 



Undertow restricted to a sphere of weakness — the asthenosphere. — 

 All of this accumulative argument has not been advanced merely 

 to show that a certain view is wrong. Rather has it been the inten- 

 tion to prepare the ground for what would appear to be a sounder 

 theory of the mode of maintenance of isostatic equilibrium. 



As for the basis of that theory, Schweydar, from the mathe- 

 matical analysis of the measurement of the tides in the crust by 

 means of the horizontal pendulum, has found that they are in 

 accord with the assumption of the existence of a slightly plastic 

 zone about 600 km. thick beneath a more rigid crust 120 km. thick.^ 

 It would appear that the geodetic evidence of isostasy points 

 also toward the existence of such a thick and somewhat plastic 

 zone beneath the more rigid lithosphere. It gives no knowledge 

 of the exact thickness or depth, but for convenience the figures 

 given by Schweydar will be assumed. It is a matter of importance 

 to note however that, although the quantitative limits are uncer- 

 tain, the suggestions given both by the tides and by isostatic 



I " Untersuchungen iiber die Gezeiten der festen Erde und die hypo the tische 

 Magmaschicht," Verdjfentlichung des k. k. Preusz. geodat. Institutes, Neue Folge No. 54, 

 Leipzig (191 2, B. G. Teubner). 



