78 REVIEWS 
affecting the state of rigidity ought now to be regarded as at least a plausible, 
if not an established, geophysical process, as urged by Van Hise and others 
in relation to the so-called flowage of crystalline rock, and by Chamberlin 
and others in relation to the so-called flowage of glaciers. A condition 
of gravitative balance essentially equivalent to that arising from isostatic 
flotation may thus be reached in great masses of matter which are at every 
instant and in all parts affected by a high degree of rigidity. Under this 
conception the protuberant area of the United States may be supported 
by a base which is rigid in the truest sense of the term. In this case it 
could be said to float on its base in no more appropriate sense than the 
Greenland ice-fields may be said to float on their rock bottom. 
There is a specific objection to entertaining the conception of a crust 
of 70 or 100 miles floating on a mobile substratum. A crust of that thick- 
ness, if formed of the firmest granite, would still have but a limited power 
of accumulating lateral stresses, and hence must yield to such stresses as 
fast as they reach a moderate magnitude and give rise to practically con- 
tinuous folding. It is, however, quite certain that most mountain foldings 
took place in relatively short periods. We seem therefore to be shut up to 
the alternative of supposing either that the agencies which produced 
mountain foldings came into play for short periods only and then ceased, 
or that the body of the earth is capable of accumulating stresses for a 
long period until, having attained large magnitude, they reach the limits 
of resistance and deformation ensues in a comparatively short period- 
The former hypothesis does not seem to the reviewer to have been assigned 
a competent basis and a working method, while the latter appears to 
find such a basis in the pervasive rigidity of the outer half of the earth 
implied by various astronomical and physical data, provided depths of 
several hundred miles, affected by high rigidities throughout, are assumed 
to act in strict co-ordination in withstanding deformation during the 
period of stress accumulation. ‘There are, therefore, serious grounds 
for hesitating to accept conclusions involving fluidal or viscous mobility 
beneath a shallow sub-crust, unless the evidence is direct and specific. 
If that portion of the paper which relates to rigidity and isostasy be 
put into the category of inferential and interpretative matter, to which 
there are at least recognized, if not plausible, alternatives, the positive 
determinations, standing as they seem to do on a firm basis, may well 
be regarded as constituting a contribution of the first order of importance. 
Mace: 
To anticipate any misapprehension that might have crept into the foregoing 
review or that might grow out of it, the manuscript was submitted to the authors of 
