A brief outline is given of our present knowledge of the constitution of the interior of the earth and of the physical states of matter, which we have to consider. The insufficiency of the contraction theory to explain the surficial history of the earth, and particularly the problem of the major mountain chains, is exposed. The continental drift theories of F.B. Taylor, Alfred Wegener, and R. A. Daly are outlined in their latest aspect. Wegener's views have been most widely published and worked out in the greatest detail. A discussion follows of the main arguments which have been proposed in support of the drift theories, as well as of some of the principal objections. The most serious of the latter is the lack of sufficient explanation for the mechanism of a drift of the magnitude of the acid continental crust ("sial") over a solid, basic substratum ("sima").

A discussion of the theory presented by John Joly (1923-25), reaches the conclusion of a periodicity of fluidity and solidification of the basic (sima) substratum, caused by the generation of heat through radioactive changes in the atoms. This heat accumulates faster than it can dissipate into space, and through a period roughly estimated at 30 million years, will cause the sima sphere to become fluid under the outer sial crust. This should greatly increase the forces which tend to cause a locally differentiated westward drift of the outer crust and their effect, actual drift. In fact, if Joly's thermal theory is right, such drift seems the only means by which accumulated heat can sufficiently be relieved and dissipated into space. It is calculated that, given sufficient drift, a peri d of 5 million years would suffice to re-solidify the basic substratum.

These alternate periods of fusion and re-solidification are causally connected with the main world-wide diastrophisms, Joly's "revolutions."

If Joly's reasoning is correct, a general westward drift becomes a necessity, and a locally differentiated drift most probable. This would support the drift theories and eliminate the worst objection to them. There are, however, objections to some of Joly's views.

This is worked out and added to in greater detail; the author adds further conclusions of his own; the causes of relatively differentiated drift are discussed, not only as between the major continents, but also intra-continental, more local drift, its relations to isostasy, and the general continental deformation it must cause. Shifting of the earth's poles need only be relative, and does not necessarily imply major changes in the location of the earth's axis of rotation in space, thereby eliminating another objection against Koppen-Wegener's plausible theory of geological climates. There is no necessity for dislocating the earth's axis.

The position of the author is as follows: he considers the theory of inter-continental drift worthy of very serious consideration and gradually has come to regard it ever

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more favorably. It offers a plausible explanation for several problems, never satisfactorily explained before. The results of further thought and geological research seem increasingly to support this theory, rather than to oppose it. Serious objections become ever more weakened by further research. In this spirit the theory is offered for serious discussion here in America, where, so far, it has found but scant support. The author realizes that no such theory is ever a finished product or perfect; he approaches it with an open mind and will welcome anyone's argument in order to come nearer the truth. He offers his own thoughts and additions in the same spirit.

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