380 
GEOLOGY: B. WILLIS 
in direction, and continuity of action during prolonged periods. These 
conditions are regarded as giving effectiveness to the stresses propagated 
in the elastic, rigid rocks far beneath the surface by imperfections of 
isostatic equilibrium. 
The postulated directive stresses tending to orient foliation in curved 
surfaces have coexisted with conditions favorable to recrystallization 
since an early stage in the growth of the earth, under the planitesimal 
hypothesis. Under that hypothesis any resulting structure and its 
effects were characteristic of the growing earth and still pervade its 
mass from near the surface to the depth to which crystalline textures 
endure. Also under any other hypothesis of the earth's past history, 
which recognizes the effects of heterogeneity as to density and the 
periodicity of dynamo-metamorphism, structures involving curved 
foHation should result from isostatic stresses. 
Upon the preceding reasoning is based the postulate of a discoidal 
structure of the lithosphere. 
The condition for the development of curved surfaces of foliation 
exists wherever isostatic equilibrium is imperfect. It is obviously most 
effective where erosion and sedimentation are active. It is not limited 
to any particular section of the border zone of any lighter or heavier 
mass which is not in equilibrium, but extends around each such heavier 
mass and invades the area of each such lighter mass. Since the curves 
of strain and the resultant foliation dip under the heavier masses from 
all sides, those masses have a discoidal structure. They may be 
designated as discs. They are regarded as composed of a series of 
foliae, which are convex downward and which make up the mass of the 
disc to a depth of several hundred kilometers. Their outcropping 
margins will include wider or narrower zones of the lighter structural 
masses. Between the discs may lie masses which, by reason of the 
fact that they do not pertain to the heavier disc-shaped masses, belong 
to the lighter masses. These may be called interdiscs. Their cross 
section in a vertical plane would present a more or less broadly truncated 
triangle, whose truncated upper surface would vary from sub-con- 
tinental to merely isthmian widths. 
Thus the lithosphere would superficially exhibit three structural 
types: (1) the surfaces of the heavier discs, depressed by gravitative 
adjustment and more or less deeply covered with water or continental 
sediments. The dominant characteristic of the underlying masses is 
high density. It is a permanent characteristic and its effects are per- 
manent and continuous. They should not be confused with the tem- 
porary effects of orogenic movements, which may also result in super- 
