Con relación al boleto:

The theory that the terrestrial crust is swimming on magma was first offered when J. H. Pratt, in the 1850s, found that the Himalayas, the largest massif on earth, do not exert the expected gravitational pull and do not deflect a plumbline. Astronomer G. B. Airy was surprised, to the point of disbelief in fact; but then he offered a theory that the granite crust, much lighter than the magma underneath, is only sixty miles thick, and that under the mountains, on the inside of the crust, there are reversed mountains, immersed in the heavier magma, which would account for the lack of gravitational pull by mountains. 1 _{This is the theory of isostasy.}

To the study of isostasy and its anomalies (gravitation is, strangely, stronger over deep seas), F. A. Vening Meinesz, Dutch geophysicist and explorer of oceans, made many important contributions. He found in the very structure of the terrestrial crust signs of some violent displacements on a global scale. Thus it is not merely in order to explain the climates of the past that the dislocation of the crust is postulated. In 1943, Vening Meinesz analyzed "the stresses brought about by a change in position of the rigid Earth's crust with regard to the axis of rotation of the Earth." In this analysis he surmised the crust "to have the same thickness everywhere and to behave as an elastic body." He pointed out that if we assume that the crust happened to move clockwise in relation to the core by over 70° the expected effect "shows a remarkable correlation to many major topographic features and also to the shearing patterns of large parts of the Earth's surface, as, e.g., the North and South Atlantic, the Indian Ocean and the Gulf of Aden, Africa, the Pacific, etc. If the correlation is not fortuitous, and this does not appear probable, we have to suppose that the Earth's crust at some moment of its history has indeed shifted with regard to the Earth's poles and that the crust has undergone a corresponding block- shearing." 2

However, according to the theory of isostasy, the crust is not of the same thickness everywhere, the crustal protuberances are immersed in a very thick and viscous magma, and for the crust to move, even if it is only sixty miles thick, would require a greater force than is available under prevailing conditions

in the solar system or on the earth itself.

The very idea of a crust changing its position in relation to the axis of the interior, or of the globe itself, presupposes the validity of the theory of isostasy. This theory, though generally accepted, finds

difficulty in explaining the propagation of seismic waves around the globe. 3 _{If the earth's crust is not}

just sixty miles thick - which, in relation to the volume of the magma, is as the thickness of the shell to the content of an egg - but two thousand miles, as some scientists assume, then, of course, the

displacement of the crust requires forces nearly as powerful as would the displacement of the entire globe, by inclining its axis into a new position in respect to the cardinal points of the sky.

"We are fully justified in concluding that the lithosphere was displaced during the great Ice Ages, and that the displacements were the direct cause of the alterations in climates during these periods." 4 _The

author of these lines, K. A. Pauly, propagates the idea offered, or revived, by the astronomer A. S. Eddington in his paper, "The Borderland of Geology and Astronomy." According to Eddington, the ice ages were caused by the shifting of the earth's outer crust over its interior as a result of tidal friction or the inequality of lunar pull on various layers of the earth; this theory abandons every effort to find in the earth itself the force that might cause the crust in its entirety to change its position in relation to the terrestrial axis, which, in this theory, maintains its astronomical direction. In order to pull the

lithosphere, or crust, over the substratum, or core, a lesser force is required than that needed to incline the axis of the whole globe in some new direction, for the crust is but a portion of the entire mass of the earth, and the momentum is dependent on the mass. However, in order to move the crust, preserving the axis of the core as that of the entire globe, the friction between the crust and the substratum must be overcome; and because of the equatorial bulge, in order to alter the position of the crust, it must be stretched in some parts. This would require the application of a great force, which does not appear to exist in tidal friction originating in the moon.

Furthermore, the tidal force acts on the surface of the earth in an east-west direction; and a change in this direction would not disturb the position of the latitudes in relation to the pole and could not have been the cause of the ice ages. Eddington's theory requires the sliding of the crust northward and southward; to explain the origin of such sliding, he suggested that the crust, moving slowly in the east- west direction, upon meeting some excessive local friction between itself and the substratum, would change its course. But, as indicated above, the tidal friction of the moon could hardly stretch the crust over the equatorial bulge.

The theory of the sliding lithosphere shares the quantitative inadequacy of the theory of sliding continents. Some motive agent more powerful than tidal friction (Eddington), or gravitational differences at various latitudes (Wegener), or intermittent radioactivity in the earth (Du Toft), must have been at work in order to move continents or the entire lithosphere. Thus these theories meet the fate of the earlier theory that postulated the shifting of the poles because of a geological redistribution of land and sea.

Also the theory that would explain the displacement of the crust by an asymmetric growth of the polar icecaps is quantitatively indefensible; this theory uses the same phenomenon - the growing icecaps - as the cause and the effect of ice ages.

The present survey of theories, which are quantitatively inadequate yet based on the well-reasoned principle of a change of latitudes or the direction of the axis as the cause of the ice ages, was here undertaken to make clear that thoughtful researchers among geologists, climatologists, and astronomers were unsatisfied with views that would not solve the problem of the geographical distribution of the ice cover in the past, a point of which almost all other theories are strangely oblivious. It follows, then, that the clamor heard at the publication of Worlds in Collision, even from some astronomers and geologists, to the effect that the shifting axis or changing latitudes had never been heard of, is not supported by scientific literature.

W. B. Wright, of the Geological Survey of Great Britain, finds that the only way to explain ice ages is to assume that "the earth's axis of rotation has not always had the same position"; and "since it has now

become obvious that geological history has witnessed many changes in the position of the climatic zones on the surface of the earth and that at least one notable glaciation, that of the Permo-

Carboniferous [preceding the time of the large reptiles], was due to a displacement of the pole from its present position, it becomes worth while to inquire whether the Quaternary [Recent] glaciation would not have a similar cause." 5

But every inquiry in this direction, in Wright's opinion, failed to find a cause that would account for recurrent but not periodic ice ages; they did not return through geological history at measured intervals. Therefore he concluded: "Among the theories that have been brought forward to account for the

phenomena of the Ice Age, there is not a single one which meets the facts of the case in such a manner as to inspire confidence." 6

Not only must the cause have been more powerful than the agents invoked, but it must have acted with great suddenness. On this we shall dwell in the following sections.

Sudden the agent must have been, and violent; recurrent it must have been, but at highly erratic intervals; and it must have been of titanic power.

_________________________________

1 J. H. Pratt: "On the Attraction of the Himalaya Mountains ... upon the Plumbline in India," Philosophical Transactions of

the Royal Society of London, Vol. CXLV (London, 1855). G. B. Airy: "On the Computation of the Effect of the Attraction of

Mountain-Masses," Ibid.

2 F. A. Vening Meinesz: "Spanningen in de aardrost tengevolge van poolverschuivingen" in Nederlandsche Akademie van

Wetenschappen Verslagen, Vol. LI I, No. 5 (1943).

3 W. Bowie: "Isostasy," In Physics of the Earth, ed. B. Gutenberg (1939), II, 104. 4 K. A. Pauly: "The Cause of the Great Ice Ages," Scientific Monthly, August 1952. 5 Wright: The Quaternary Ice Age, p. 313.

6 Ibid,, p. 463.

Chapter 9