REVIEWS 77 
tion to the truth. In other words the United States is not maintained in its 
position above sea-level by the rigidity of the earth, but is, in the main, buoyed 
up, floated, because it is composed of material of deficient density.* 
So far as the reviewer can see, that which is really determined does not 
extend beyond the distribution of density, and all that is added to this is 
inference or interpretation. It is therefore consistent with the very highest 
appreciation of the value of the determinations to question the validity 
of these superimposed interpretations. 
The authors are probably entirely correct in assuming that the distri- 
bution of specific gravities postulated in Solution B could not be maintained 
without great rigidity in the deeper portion of the earth. It is equally 
beyond the probabilities that such a distribution of matter could ever 
have arisen under any tenable hypothesis of the mode of the earth’s forma- 
tion. But, while the maintenance of this unrealizable condition of things 
is excluded by the investigation, it is not apparent that it excludes rigidity 
under more tenable conceptions of the formation of the earth. The exclu- 
sion of an extreme and indefensible hypothesis does not logically cover 
all other hypotheses. It is possible that the authors did not really intend 
to convey the impression that the conceptions of rigidity held by certain 
physicists and geologists were incompatible with their determinations, 
but their language seems to imply this. 
So, on the other hand, when the authors say ‘‘The United States is 
not maintained in its position above sea-level by the rigidity of the earth, 
but is, in the main, buoyed up, floated, because it is composed of material 
of deficient density,”’ their language carries the impression of a positive 
affirmation of liquidity or viscousness at the base of the crust in which the 
differentiated densities reside. Such is the usual conception that goes 
with the term “‘isostasy”’ as it has been used in geological literature. Now, 
that which is really demonstrated in this important investigation is simply 
that the compensation of densities becomes approximately complete some- 
where between 50 and 150 miles below the surface. The agencies which 
have produced this differentiation of densities and the physical conditions 
which now maintain it do not seem to be really touched by the investi- 
gation, but to be matters of inference or interpretation based upon other 
considerations. In the judgment of the reviewer, this differentiation may 
have arisen and may be now maintained without involving any nearer 
approach to fluidity than that which is manifested by bodies whose rigid- 
ities range from the best granite to the best steel and beyond. Deforma- 
tions by molecular transfers from crystal to crystal without essentially 
TERT Os 
