76 
REVIEWS 
oceanic sectors differences of specific gravity reaching to these and greater 
depths, and attributes them to differential weathering supposed to have 
A A 
A A 
B 
Cc 
FIG. I 
BC 
begun at a relatively early stage in the growth of the 
earth, and to have increased upward at more than a 
simple ratio until the surface was reached. The effects 
of the original differentiation by weathering are sup- 
posed to have been subsequently modified by vulcanism 
in such a way that the lighter portions of the differen- 
tiated material were brought to or toward the surface 
in larger percentage than the heavier material, the effect 
of which was to concentrate the differences of density 
previously developed toward the surface. The final 
differentiations of density thus postulated would there- 
fore be greatest at the surface, and would decline down- 
ward at a varying rate, whose nature may be roughly 
indicated by the curve C-C in Fig. 1, where it may be 
compared with the rectangle AAA and with the triangle 
ABB which represent the two modes of compensatory 
distribution referred to above. So far as the reviewer 
can judge from an inspection of the data furnished by 
the paper, a distribution of densities such as is repre- 
sented by the curve C—C would satisfy the requirements 
of the observations as well as either of the others. It 
would seem, therefore, to be a matter of some felicity 
that the accretion hypothesis should have assigned, on 
its own grounds and as the inevitable result of the 
processes it postulates, a specific differentiation and 
distribution of densities in fairly close accord with these 
new determinations based on wholly independent con- 
siderations. * 
The authors speak of Solution B as being based on 
the supposition that the earth is rigid, and of Solutions 
E, G, and H as though they represent isostasy. They 
say that the investigation ‘‘/eads to a definite and positive 
conclusion as to rigidity versus tsostasy.”’? They add: 
For the United States and adjacent areas, the assumption 
of extreme rigidity is far from the truth. On the contrary, the assumption that 
the earth is in the condition called isostasy is a comparatively close approxima- 
t Chamberlin and Salisbury, Geology, Vol. I], pp. 107-11. 
2 P. 10; italics theirs. 
