Contraction of the Earth. 
477 
some of which may be of equal or of greater importance than 
secular cooling, and here therefore only summarize them. 
As a result of vulcanism vast quantities of igneous rocks 
hiave been intruded within the crust of the earth, and extruded 
upon its surface. The extrusions lessen the volume of the inte¬ 
rior mass, and therefore are an important cause for nucleal con¬ 
traction. Moreover, the material intruded within the crust of 
the earth expands it, and tends to make it too large to fit the 
already proportionally smaller nucleus. Thus the transfer of 
liquid material from the interior of the earth iuto the outer 
part has a double effect in forming corrugations. The immen¬ 
sity of regional eruptions is considered later. It is only when 
the enormous volume of igneous rocks which must have been 
intruded and extruded during geological time is appreciated, 
that the importance of this cause of nucleal contraction can be 
understood. It is possible that it is of equal or greater im¬ 
portance than that due to secular cooling. 
Peirce 1 and G. H. Darwin 2 have calculated that at a time in the 
remote history of the earth, its rate of rotation was about four 
times as fast as at present. As a consequence the earth then had 
a greater oblateness. Supposing the mass not to have changed, 
its surface must have been larger than at the present time, for 
a sphere contains a greater volume with a less surface area than 
any other form of solid. The difference in superficial area as a 
result of this change in oblateness, upon the hypothesis of a 
spheroid of uniform density, as calculated by Prof. C. S. Slich- 
ter at my request, is about 200,000 square miles. 
Also at the time of more rapid rotation the pressures at any 
point within the earth were considerably less than at present, 
because of the greater centrifugal force at the time of rapid 
rotation, and the consequent lessened effectiveness of gravity, 
and therefore the earth in the past time could not have been so 
•dense as at present. As the rotation decreased in speed, the 
: The contraction of the earth, by B. Peirce: Proc. Am. Acad. Arts and 
Sci., Vol. 8, 1873, pp. 106-108. 
2 On the precession of a viscous spheroid and on the remote history of the 
earth, by G. H. Darwin: Phil. Trans. Roy. Soc., Vol. 170, Part 2, 1879, p. 
505 
