rs of great complexity, such as that of Dr. Daubeny. But more recent
consideration and wider observation have tended to lead us to the
conclusion that the ultimate cause is the most simple, the most
powerful, and the most general which can be suggested; namely, _the
contraction of the crust due to secular cooling of the more deeply
seated parts by conduction and radiation of heat into space_. Owing to
this cause, the enclosed molten matter is more or less abundantly
extruded from time to time along the lines and vents of eruption, so as
to accommodate itself to the ever-contracting crust. Nor can we doubt
that this process has been going on from the very earliest period of the
earth's history, and formerly at a greater rate than at present. When
the crust was more highly heated, the radiation and conduction must have
been proportionately more rapid. Owing to this cause also the
contraction of the crust was accelerated. To such irresistible force we
owe the wonderful flexuring, folding, and horizontal overthrusting which
the rocks have undergone in some portions of the globe--such as in the
Alps, the Highlands of Scotland and of Ireland, and the Alleghannies of
America. It is easy to show that the acceleration of the earth's
rotation must be a consequence of such contraction; but, after all, this
is but one of those compensatory forces of which we see several examples
in the world around us. It can also be confidently inferred that at an
early period of the earth's history, when the moon was nearer to our
planet than at present, the tides were far more powerful, and their
effect in retarding the earth's rotation was consequently greater.
During this period the acceleration due to contraction was also greater;
and the two forces probably very nearly balanced each other. Both these
forces (those of acceleration and retardation) have been growing weaker
down to the present day, though there appears to have been a slight
advantage on the side of the retarding force.[7]

[1] R. D. M. Verbeek, _Krakatau_, p. 105 (1886); also, J. Milne, _The
Great Earthquake of Japan_, 1891.

[2] _Bull. Mus. Comp. Zool._, vol. iii.

[3] _Proc. Roy. Soc._, No. 237 (1885); also, _Rep. Brit. Assoc._ (1881).

[4] Hopkins, _supra cit._, p. 218.

[5] C. Davison and G. H. Darwin, _Phil. Trans._, vol. 178, p; 241.

[6] Durocher, _Ann. des Mines_, vol. ii. (1857).

[7] See on this subject the author's _Textbook of Physiography_ (Deacon
and Co., 1