PRESENT PROBLEMS OF GEOPHYSICS 511 



the distribution of density in the sun, have had a restraining influence 

 on the estimates drawn from sedimentation by geologists. Many and 

 perhaps most geologists now regard something less than 100 million 

 years as sufficient for the development of geological phenomena. Yet 

 the subject cannot be regarded as settled until our knowledge of con- 

 ductivities is more complete. An iron nucleus, for example, would 

 imply greater conductivity of the interior and a higher age for the 

 earth than that computed by King, though probably well within the 

 range explicitly allowed by Lord Kelvin in view of the uncertainty 

 of this datum. 



The researches of Kelvin and Darwin, supplementing those of Kant 

 and others, have left no doubt that the moon was formerly closer to 

 the earth than it now is, and that the rotation of the latter was more 

 rapid, involving a greater ellipticity of the meridian than it now shows. 

 In a fluid or Cartesian earth the change of figure might have pro- 

 duced little effect on the structure of the planet. If the earth is chiefly 

 a mass of crystalline nickel steel, it is very possible that such a change 

 in the figure of equilibrium might rupture it. Since the epoch at which 

 the earth rotated in 5 hours 30 minutes, the polar axis must have 

 elongated by several per cent, most of it before the time of rotation 

 was reduced to 11 hours. 1 Were the earth chiefly composed of forged 

 steel, such elongation might be produced by plastic deformation; 

 but meteoric iron is rather comparable with cast-iron, or better 

 still, with relatively brittle, unforged cement steel, and might break. 



Now it is an indubitable fact that a majority of the outlines of the 

 great oceanic basins and of the chief tectonic lines of the globe, lie 

 nearly on great circles tangent to the Arctic Ocean and to the Ant- 

 arctic continent. 2 These lines, or most of them, are of extremely high 

 geological age, their main features having found expression as early 

 as the oldest known fossils and in some cases still earlier. It appears to 

 me very possible that these fundamental ruptures of the globe were 

 due to the change of figure attendant upon diminution of the earth's 

 period of rotation. Their symmetrical disposition with reference to 

 the polar axis is unquestionable, as well as the fact that they pene- 

 trate to great depths. They must be due to some tremendous force 

 acting axially, which actually altered the ellipticity of the meridian, 

 since these fissures could not have been formed without modifying 



1 Compare Thompson and Tait, Nat. Phil., 772. where the rotational period 

 and eccentricity are given for a fluid of the mass of the earth and possessing its 

 mean density. When the period is 5h. 30m., this table gives the data for comput- 

 ing that the polar axis has a length equal to 0.95 of the length which it has when 

 the period is a sidereal day. For rotation in lOh. 57m. the polar axis is 0.99 times 

 that for a day. 



2 In 1857 Professor R. Owen, of Tennessee, and, independently, Benjamin 

 Peirce, called attention to the tangency of the coast-lines to the polar circles (not 

 to the coast-lines of the arctic sea and the antarctic continent), each attributing 

 the facts to the influence of the sun. In the first Yearbook of the Carnegie Institu- 

 tion I failed to refer to these publications. 



