Status of the Theory of Isostasy. 295 



responding magnitude; but it is able to flex under 

 broader loads, so that mountain systems and broad pla- 

 teaus are in greater part related to regional deficiencies 

 in subcrustal densities. The strength of the crust is be- 

 lieved to vary greatly in different regions and to shade 

 off into the weaker zone below. The span and magnitude 

 of the possible loads vary accordingly. The broader and 

 gentler flexures of this thick, strong crust involve but 

 little crustal stress and therefore but limited deforma- 

 tion within the crust itself, but imply a broad, deep, and 

 slow creep in the subcrustal zone. This creep, provided 

 the yielding zone is thick, involves but small distortion 

 of a unit mass and is thought to be coexistent with the 

 maintenance of a solid and rigid state. The probable 

 mode of yielding is by recrystallization under strain, 

 analogous to the mechanics of the flow of glacial ice. 



This combination of a strong, thick crust resting on 

 a weak but solid subcrustal shell is very different from 

 the picture set up by the extreme isostasists, on the one 

 hand, of a weak and failing earth, yielding isostatically 

 in its outer part to each minor change of load; and is 

 considerably different, on the other hand, from those 

 hypotheses of the earth's internal nature and mode of 

 deformation which have been elaborated without recog- 

 nition of isostasy. These contrasts of interpretation 

 point to the need for further discussion and investiga- 

 tion, with the object of bringing antagonistic points of 

 view into harmony. 



Definitions Involved in Isostasy. 



In order to discuss in detail the problems of isostasy, 

 the meaning of the phraseology must be held clearly in 

 mind. For this reason figure 1 is given. 



Let A be a section of the continental crust, and B a 

 section of the crust beneath the oceans. They are both 

 portions of the lithosphere or rock shell whose outer 

 part is open to geologic study. The weight of the two 

 sections of the crust is equal for equal areas. The mean 

 density of A is therefore less than the density of a column 

 whose" mean surface is at sea level. The difference is 

 known as the defect of density. The density of B is 

 greater than the sea level column. The difference is a 

 negative defect of density. The mass above sea level 



