276 B. WILLIS DISCOIDAL STRUCTURE OY THE LITHOSPHERE 



such that the thickness of the slab involved in carrying the load did not 

 in the one case exceed, say, 10 miles, or in the other approached 30 miles. 

 This is bnt a statement of the general relation between the resisting 

 shear and the resisting moment of flexure, but in the case of the earth's 

 crust the inequality between the two is exaggerated by the weight of the 

 crust itself. 



The problem may be capable of mathematical treatment, but in the 

 judgment of the writer mathematical estimates of this character are too 

 artificial to be of great value. Geologic investigations afford a safer 

 guide, and to them we turn. 



It is to be observed that all discussions of isostasy deal Mdth the earth'^ 

 surface as it is, in this viery mountainous stage of relief, when, according 

 to gravity studies, isostasy is nearly perfect. In the progress of elevation 

 from the previous condition of less perfect adjustment, which probably 

 was least perfect toward the close of the Cretaceous period, intrusions 

 of heavy igneous rocks and mechanical displacements of large masses 

 have undoubtedly introduced anomalies of gravity, details which confuse 

 the evidence. To illustrate : The Columbia basalt flows in Washington 

 and Oregon constitute a very large mass of heavy rock which has been 

 extruded from some locus below the surface. If that locus was vertically 

 below the present basalt plateaus, there has been no change in the mean 

 density of the underlying column, although the intensity of gravity at 

 the surface has been increased ; but if the original seat of the basalt was 

 to one side of the present area of occurrence, if the basalt has been moved 

 diagonally upward, then there has been a considerable addition to the 

 gravitative attraction of the column on which it now rests. It will ap- 

 pear in this discussion that there is reason to postulate the diagonal 

 movement of magmas from their deep-seated sources. Herein lies a pos- 

 sible cause of change in the isostatic relations of areas penetrated by 

 intrusive and extrusive rocks, a cause which has particularly affected the 

 margins of lighter masses. The point which it is here desired to make 

 is that gravity observations of continental areas do not in themselves, 

 without geologic correction, afford a satisfactory means of analyzing the 

 continent into its permanent heavier and lighter elements. 



If the specific gravity of a large mass, an element of the lithosphere, 

 be regarded as an original and, within narrow limits, a constant char- 

 acter, which has persisted during recorded geologic history, and if differ- 

 ences of density of large masses be regarded as a controlling condition 

 of the major features of relief of the earth's surface, then the effects of 

 the upward and downward movements of the lighter and heavier masses 

 respectively should be recognizable throughout geologic history. If the 



