274 B. WILLIS DISCOTDAL STRUCTURE OF THE LITHOSPHERE 



Since the outermost crust, from the surface down to a depth of 40 

 miles, is relatively strong as compared to the load of its own mass, no 

 adjustment of isostatic equilibrium is likely to occur, except below 40 

 miles from the surface. In the neighborhood of 40 miles the weight of 

 the superincumbent column equals the strength of the rock, and thence 

 downward increasingly exceeds it. Below 40 miles from the surface the 

 temperatures rise above 2,000 degrees centigrade and minerals presuma- 

 bly approach a state of unstable equilibrium. The conditions below 40 

 miles, therefore, favor adjustment toward uniform stresses, such as define 

 the depth of compensation. 



Hence it seems reasonable to conclude, on geological and physical 

 grounds, that there is a zone of adjustment below 40 miles and extending 

 to the base of the asthenosphere ; that imperfections of equilibrium in 

 the deeper asthenosphere are less apparent on the surface than similar 

 imperfections in the upper part of the asthenosphere and isostatic shell, 

 and that the latter are chiefly compensated in the zone which lies be- 

 tween 40 and 100 miles below the surface. 



Furthermore, the present geologic period corresponds with unusual 

 orogenic activity, which must have reduced departures from isostatic 

 equilibrium to a minimum. It is, therefore, not surprising that refined 

 geodetic investigations should demonstrate an existing state which is a 

 close approach to complete isostasy, or that they should place the depth 

 of compensation at 76 or 60 miles. 



Development of the discoidal Hypothesis 

 distribution of heterogeneous masses 



This section treats of the size of masses which differ as to density, and 

 of their distribution within continental areas or oceanic underbodies. 



If a dike of basalt of density 3 cuts a granite of density 2.6, there is 

 a certain distribution on a small scale of masses whose density differs 

 more than 12 per cent. The differences of density which are indicated 

 by isostatic compensation are, as a rule, under 5 per cent; and yet no 

 one would ascribe an isostatic eifect to the basalt dike in granite, because 

 of its insignificant size. 



The illustration may serve to indicate that heterogeneous masses which 

 may be competent to produce a tendency toward isostatic compensation 

 in the lithosphere must be of notable dimensions. Barrell, by an ex- 

 haustive analysis of the evidence bearing on the strength of the earth's 

 crust, arrived at the conclusion that — 



