REVIEWS 505 
The hypothesis appeals to the imagination, and at first thought appears 
to offer a striking explanation of both the shape and depth of the ocean 
basins. There are, however, certain assumptions involved in this restate- 
ment of it which are not clearly indicated as such, and the difficulties are 
dismissed without serious treatment. It is to call attention to these, as 
well as to give credit to the original papers on the subject, that this review 
and criticism is written. 
First, no discussion is given of the possibility that the interior density 
of the earth may be largely due to the gravitational compression of ordi- 
nary rock material, since Professor Pickering states: 
The specific gravity of the Earth as a whole is 5.6. That of the surface 
material ranges in general between 2.2 and 3.2, with an average of 2.7. The 
specific gravity of the Moon is 3.4. This indicates clearly that the Moon is 
composed of material scraped off from the outer surface of the Earth, rather 
than of matter obtained from a considerable depth. At the same time, the 
specific gravity 3.4 indicates that the layer of material removed had an appre- 
ciable thickness. (P. 24.) 
Nothing is really known as to the limits of the compressibility of rcck 
under pressures of millions of pounds per square inch, allowing the alter- 
native hypothesis to be equally assumed, that the earth may be largely 
made up of material which would not have a specific gravity of much, if 
any, over 3, if free from subcrustal pressures. Following this assumption, 
the moon may have been abstracted from any part of that mass, and at 
any time before the present relation of heavier oceanic and lighter con- 
tinental crustal segments originated. The continents may then have sub- 
sequently come into existence according to any one of a number of hypothe- 
ses, such as proposed by Chamberlin or others. As the moon’s mass is 
but one eighty-first that of the earth, its radius but a fourth, and gravity 
at its surface but one-sixth, it is seen that the material of the moon would 
suffer comparatively little compression even in its deeper parts, while its 
outer parts may consist of highly cellular rocks. 
Second, this hypothesis rests upon the doctrine of the perpetual exist- 
ence of the present forms and relations of the continental platforms. This 
is passed over with the statement that “it is the general opinion among 
geologists that the continental forms have always existed—that they are 
indestructible” (p. 28). 
From the early advocacy of J. D. Dana, this may be called an American 
view, and represented a wholesome return from previous untrammeled 
speculations regarding the interchangeability of ocean basins and conti- 
nental platforms. That Dana himself, however, came to a belief in a 
