THE ASTRONOMICAL DEDUCTIONS. / 



from that of the earth after the condensation of the common mass had 

 reached the liquid or perhaps even incipient solid state. In what precise 

 form the separation took place is not specifically affirmed and is not material 

 here, where the only essential point is the high rotatory velocity assigned 

 the earth at the time of the moon's separation. 



Most or all of the meteoritic hypotheses of the earth's origin — using the 

 term meteoritic in the restricted sense defined in this series of papers — 

 agree essentially with the gaseous hypotheses in assigning to the earth, 

 at its earliest separate stage, a molten condition and a rate of rotation 

 either identical with or closely approximate to that of the Laplacian 

 hypothesis and of its modifications. The presumption, therefore, that the 

 rotation of the primitive earth was of a high order of velocity had the 

 sanction of these two classes of cosmogonic theories, and, as they occupied 

 the field almost exclusively during the past century, this common inference 

 from them came to have, naturally enough, a strong hold upon the beliefs 

 of astronomers and geologists. If there shall finally be found reason to 

 set these conceptions aside, it should still be recognized that they have 

 been powerful instrumentalities in advancing knowledge and in stimu- 

 lating inquiry, and that the investigations founded upon them have been 

 scarcely less than necessary steps toward a final solution. 



Besides being at one in postulating a rapid rate of primitive rotation, 

 these older hypotheses were essentially in agreement in assigning to the 

 earth a molten condition in its early stages, as already stated, and this 

 postulate has entered pervasively into the tidal and deformative theories 

 of the earth that have had currency. Until the later decades of the last 

 century, it was commonly believed that a molten condition was retained 

 by the interior of the earth, or by some notable part of it, throughout 

 the geological ages. In the latter part of the century, the conception of a 

 solid earth came to be more generally entertained, but there went with 

 this, almost universally, the postulate of such a degree of viscousness as to 

 profoundly influence conclusions relative to tidal deformation and earth- 

 movements generally. At the present time, when belief in an essentially 

 solid earth has gained a large, though not universal, adherence, the con- 

 ception that the spheroid is to be regarded as a viscous body in the treat- 

 ment of all the larger geological problems is still widely prevalent and 

 not only enters profoundly into the study of these problems but takes on 

 forms exceedingly diflScult to adjudicate. The embarrassment does not 

 arise so much from the theoretical recognition of a viscous property in 

 the substances of the lithosphere, as from the lack of firm grounds for 

 estimating its actual participation in the deformations and internal move- 

 ments of the earth. One of the most vital questions of earth-dynamics 

 relates to the respective values of viscousness and of elastic rigidity in 

 terrestrial diastrophism. 



In this discussion the elastic rigidity of the earth will be regarded as 

 the dominant factor in its morphology, and the tidal deformations of the 

 lithosphere will be regarded merely as strains in an elastic body, involving 

 viscous or liquid flowage only as an incident affecting those portions of 

 the earth's body which are in a molten, gaseous, or temporarily unattached 

 state. 



