662 JOSEPH BARRELL 



sea-water and the stress gradient becomes less high. The maximum 

 thrust occurs at the bottom of the ocean and is from the land toward 

 the sea. Below this level the density of the sea-column is greater 

 than that of the land-column. This, with increasing depth, 

 gradually balances the excess pressure, and at the base of the litho- 

 sphere both the lateral and vertical pressures of both columns by 

 hypothesis are equal. 



In this diagram the pressures of the columns are imagined to 

 act hydrostatically, but, in reality, for stresses within the elastic 

 limit, this would not be so. Further, in so far as the partition is 

 much wider than the difference in elevation of the columns, it has 

 a gentle surface slope and will tend to give the upper part of the 

 land-column competence to hold itself in by its own strength and 

 that of the partition. The approximate ratio which the actual 

 lateral pressure-differences on the two sides of the partition hold to 

 the assumed hydrostatic pressures may be perceived from the 

 results of a recent work by Love entitled Some Problems of Geo- 

 dynamics.^ In chaps, ii and iii he considers the problems of the 

 isostatic support of continents and mountains. As a basis for the 

 analytic treatment he assumes, first, the existence of complete com- 

 pensation within a depth of one-fiftieth of the earth's radius, 

 127 km.; second, that at this depth all stress-differences disappear, 

 the pressures below being of the nature of hydrostatic pressures, 

 the only kind which could occur if a fluid layer existed at and below 

 the depth of 127 km.; third, it is known that the heterogeneities of 

 mass in the lithosphere only slightly modify the form of the geoid, 

 and it is accordingly assumed that there is no such effect. Love 

 thus treats of the limiting case of a crust exhibiting perfect isos- 

 tasy, its surface relief not modifying the form of the geoid given by 

 the ocean surface, and resting with its base upon a fluid zone. As 

 such, his solution is of great value, but he states: "It must, how- 

 ever, be understood that the special form (of the hypothesis of 

 isostasy) is introduced for the sake of analytical simplicity rather 

 than physical appropriateness."^ 



The artificiality of the assumption of the existence of no stress- 

 differences below the zone of compensation is shown by the law of 



^Cambridge University Press, 1911. - Op cit., p. 7. 



