56 THE TIDAL PROBLEM. 



briefer periods of deformation of similar prevalence. For these it does not 

 appear that there will be found a consistent explanation except in the 

 ability of the great body of the earth to accumulate stresses to a notable 

 degree during the long periods of relative quiescence necessary for the base- 

 leveling and the sea-transgression. 



It would be going beyond the proper limit of this paper to try to estab- 

 lish this thesis by the citation of evidence, for this would involve a review 

 of some large part of the great mass of stratigraphic, paleontologic, orogenic, 

 and physiographic data possessed by geology. Suffice it therefore to note 

 here that this is the one of the alternative views of the earth's deformative 

 methods that seems at present best supported by geologic evidence. It 

 is not wholly necessary to the following considerations, though it lends much 

 strength to them. 



Let it be assumed merely that the earth-body offers some appreciable 

 resistance to deformation, an assumption which can scarcely be questioned, 

 since the irregularities in the form of the geoid imply this, even when allow- 

 ances are made for differences in the distribution of density. Let a limited 

 slackening of the earth's rotation take place. This will disturb the preced- 

 ing equilibrium between the centripetal and centrifugal forces and both the 

 body of the earth and the water on its surface will experience stress-differ- 

 ences which give a tendency toward a new equilibrium. This equilibrium 

 may be established by the subtraction of matter from the equatorial regions 

 and its transfer to the polar regions internally or externally. The earth- 

 body certainly offers some resistance to this transfer while the water on its 

 surface offers practically no resistance at all because it is in circulation as 

 the result of solar influence, and to effect the new distribution it is only 

 necessary that it stop where the new demands of gravity require, and in 

 this friction will lend its aid. The water surface may therefore be supposed 

 to fall in the equatorial regions and rise in the polar regions until the new 

 water surface of the globe conforms to the new equilibrium required. 

 This must relieve, in some large part at least, the stress upon the body of 

 the earth, for if the newly developed equilibrium required more matter 

 in the polar regions the water would supply it, unless it were previously 

 exhausted. Local stresses might remain where the land was left pro- 

 tuberant, but geological evidence shows that such protuberances can be 

 maintained for long periods by the effective rigidity of the earth, if they do 

 not exceed a certain measure. Such a protuberance of the equatorial land 

 may be treated as any other local protrusion of the earth's body. When, 

 therefore, in the case in hand, an equatorial mass became protuberant 

 above the surface of the geoid sufficiently to overcome the effective rigidity 

 of the part of the earth affected, the appropriate deformation would follow. 



The determination of what mass is sufficient for such deformation is 

 qualified by the available time. Given infinite time and the requisite mass 

 would doubtless be relatively small in a body like the earth, even on the 

 hypothesis of elastic rigidity; for, even within the limit of elasticity, 

 deformations may take place by the transfer of molecules from one rigid 

 attachment to another individually. But whatever might be the results if 

 indefinite time were available, the practical case is one of limited time and. 



