lla et Oe i 
C.—GEOLOGY. 67 
stood, and the relationship of strata fo sea invasions has led to a fuller 
appreciation of the value of paleogeography. Grabau states that ‘in 
the future the study of lithogenesis must go hand in hand with the study 
of paleogeography. Neither science can progress without the other, and 
each is dependent on the other to a degree too little realised.’ 
So deeply are American geologists interested in the problems of sedi- 
mentation and the time factor involved, that a Committee has been 
appointed by the United States National Research Council to compile 
data bearing on the subject. A report of this Committee was presented 
on April 26, 1924. 
The Origin of the Earth and the Nature of its Interior. 
The question of the earth’s origin is evidently closely related to the 
problem of its age. Although geologists are inclined to disclaim this aspect 
of the subject, I feel that it cannot be disregarded under the title of this 
address. 
Theories of the earth were indulged in by the Greek philosophers, 
and reached a climax in the fantastic conceptions of the cosmogonists 
during the eighteenth century. The announcement of the nebular hypo- 
thesis of Kant and Laplace was an epoch-making event, and the theory 
has been generally accepted as ‘the grandest conception of the human 
intellect.’ Its influence on all cosmical philosophy has been tremendous ; 
in fact, it lay at the base of all geogeny until it was questioned by 
Chamberlin and Moulton in bringing forward their planetesimal hypothesis, 
This explanation of the origin of the earth, as due to the aggregation of 
cold discrete particles, has received much credence in America, but it has 
had a less favourable reception in Europe. 
Under the influence of the theory of Kant, Laplace, and Herschel, 
many attempts were made to explain the nature of the earth’s interior 
during the early years of the nineteenth century (Fourier, 1820; Poisson, 
1835 ; Ampére, 1833). Somewhat later, in 1871, Helmholtz published his 
classic exposition of the nature of the earth under the Laplacian hypothesis. 
In 1893 Clarence King decided that the temperature of the earth’s 
interior was originally not more than 2000° C., and that its age is about 
24,000,000 years. His determination of temperature gradients is referred 
to in another part of this paper. 
Tn more recent years a great advance has been made in the spectroscopic 
study of the sun and stars; it has been established beyond question that 
the earth, the sun, and the fixed stars are materially identical—that they 
are composed of the same chemical elements. 
The interior of the earth is beyond direct observation ; the deepest 
mines and bore-holes scarcely penetrate the outermost skin. Certain 
fundamental facts, however, may be taken as established. The interior 
is hot, rigid, and heavy (sp. gr. 5°6 as compared with 2°7 for the known 
exterior) ; the accessible exterior is composed of elements common to the 
universe. Beyond this all is vague and speculative. 
It is worthy of particular emphasis, however, that while the earth as 
a whole acts as an almost perfectly rigid body, the external envelope 
with which we are familiar is by no means rigid. Adjustments have 
taken place throughout all geological time, and I need not quote evidence 
that they are still taking place. The acquisition of perfect rigidity by 
F2 
