the U.S. Dept. of Commerce and Labor, 1909, p. 175.

[2] loc. cit., p. 175.



There are a number of facts which support this so-called theory
of isostasy, according to which the crust of the earth is not
capable of sustaining any very great weight, though it may be
at the outside rigid, but is itself essentially like a flexible
membrane resting on a layer of viscous fluid. However viscous
this fluid may be and rigid to transitory quickly shifting
strains like those produced by the earth's rotation, it does
NOT REMAIN AT REST in a state of strain (at any rate if this
strain passes limits which are relatively quite low). Not only
are, according to Hayford's observations, the inequalities of
the North American continent compensated for by lighter
material below, so that the plumb- bob deflections are only one
twentieth what they would be if they rested upon a rigid
substratum of uniform density, but other facts that lead to the
same conclusion are the apparent tendency of areas of
sedimentation to slowly settle under their load, the apparent
settling of the Great Lake region under a load of ice and
springing up again since the removal of the ice. But if the
theory of isostasy is true, one would at first say that there
could be no great accumulation through a geologic period of
stresses which would finally yield in the shape of folded
mountain ranges. It has, in fact, been suggested that mountain
ranges have been slowly folded and lifted as the stress which
produced them accumulated and this would seem to be true if one